Experimental and theoretical investigation of Low-Frequency vibrational modes of 4-Amino 3,5 Dinitro Pyrazole in terahertz frequency domain

Pyrazoles have recently received significant attention due to their unique and potential applications in the medical field, agriculture and are also known to be highly stable explosives. The present work describes the terahertz time-domain spectroscopy (THz-TDS) based study of 4-Amino 3,5 Dinitro Pyrazole(ADNP) in between the 0.1 and 3.0 THz ranges. A Toptica-Teraflash fibre-coupled handheld terahertz system has been employed in reflection mode configuration. We ascertained complex refractive index, absorption coefficients, and complex dielectric constants from 0.1 THz to 3.0 THz. The value of the refractive index and absorption coefficients are found to be 1.8 and 10---180 cm -1, respectively. Also, we have analyzed the structural, vibrational, and optical properties of ADNP using the plane-wave pseudopotential method based on Density Functional Theory (DFT) calculations. We have observed six low-frequency optical phonon modes, located at 0.36, 1.20, 1.52, 1.77, 2.40, and 2.75 THz, respectively, exhibiting a redshift compared to the values predicted by the DFT calculations. The possible reasoning for the above might be due to the anharmonicity that is not considered in the DFT calculations. The theoretical calculations align with the experimental results and deliver direction for further investigations and the futuristic application of ADNP.

Deep Earth Chronicles: High-Pressure Investigation of Phenakite Mineral Be2SiO4

Beryllium silicate, recognized as the mineral phenakite (Be2SiO4), is a prevalent constituent in Earth's upper mantle. This study employs density-functional theory (DFT) calculations to explore the structural, mechanical, dynamical, thermodynamic, and electronic characteristics of this compound under both ambient and high-pressure conditions. Under ideal conditions, the DFT calculations align closely with experimental findings, confirming the mechanical and dynamical stability of the crystalline structure. Phenakite is characterized as an indirect band gap insulator, possessing an estimated band gap of 7.83 eV. Remarkably, oxygen states make a substantial contribution to both the upper limit of the valence band and the lower limit of the conduction band. We delved into the thermodynamic properties of the compound, including coefficients of thermal expansion, free energy, entropy, heat capacity, and the Gruneisen parameter across different temperatures. Our findings suggest that Be2SiO4 displays an isotropic behavior based on estimated anisotropic factors. Interestingly, our investigation revealed that, under pressure, the compression of phenakite is not significantly affected by bond angle bending.

Evidences for local non-centrosymmetricity andstrong phonon anomaly in EuCu2As2:A Raman spectroscopy and lattice dynamics study

Phonon modes and their association with the electronic states have been investigated for the metallic EuCu2As2system. In this work, we present the Raman spectra of this pnictide system which clearly shows the presence of seven well defined peaks above 100 cm-1 that is consistent with the locally non-centrosymmetric P4/nmm crystal structure, contrary to that what is expected from the accepted symmorphic I4/mmm structure. Lattice dynamics calculations using the P4/nmm symmetry attest that there is a commendable agreement between the calculated phonon spectra at the Γ point and the observed Raman mode frequencies, with the most intense peak at ∼ 232 cm-1being ascribed to the A1gmode. Temperature dependent Raman measurements show that there is a significant deviation from the expected anharmonic behaviour around 165 K for the A1gmode, with anomalies being observed for several other modes as well, although to a lesser extent. Attempts are made to rationalize the observed anomalous behavior related to the hardening of the phonon modes, with parallels being drawn from metal dichalcogenide and allied systems. Similarities in the evolution of the Raman peak frequencies with temperature seem to suggest a strong signature of a subtle electronic density wave instability below 165 K in this compound.

A Chiral B-N Adduct as a New Frontier in Ferroelectrics and Piezoelectric Energy Harvesting

Within the burgeoning field of electronic materials, B-N Lewis acid-base pairs, distinguished by their partial charge distribution across boron and nitrogen centers, represent an underexplored class with significant potential. These materials exhibit inherent dipoles and are excellent candidates for ferroelectricity. However, the challenge lies in achieving the optimal combination of hard-soft acid-base pairs to yield B-N adducts with stable dipoles. Herein, we present an enantiomeric pair of B-N adducts [R/SC6H5CH(CH3)NH2BF3] (R/SMBA-BF3) crystallizing in the polar monoclinic P21 space group. The ferroelectric measurements on RMBA-BF3 gave a rectangular P-E hysteresis loop with a remnant polarization of 7.65 μC cm-2, a value that aligns with the polarization derived from the extensive density-functional theory computations. The PFM studies on the drop-casted film of RMBA-BF3 further corroborate the existence of ferroelectric domains, displaying characteristic amplitude-bias butterfly and phase-bias hysteresis loops. The piezoelectric nature of the RMBA-BF3 was confirmed by its direct piezoelectric coefficient (d33) value of 3.5 pC N-1 for its pellet. The piezoelectric energy harvesting applications on the sandwich devices fabricated from the as-made crystals of RMBA-BF3 gave an open circuit voltage (VPP) of 6.2 V. This work thus underscores the untapped potential of B-N adducts in the field of piezoelectric energy harvesting.

Ferroelectric polymorphic phenomena in the layered antiferromagnet Cu(OH)2

Ferroic orders and their associated structural phase transitions are paramount in the understanding of a multitude of unconventional condensed matter phenomena. On this note, our investigation focuses on the polymorphic ferroelectric phase transitions of Copper(II) hydroxide, Cu(OH)2, considering an antiferromagnetic ground state. By employing the first-principles studies and group theory analysis, we have provided a systematic theoretical investigation of vibrational properties in the hypothetical Cmcm high-symmetry phase to unveil the symmetry-allowed ferroic phases. We identified a non-polar to polar (Cmc21) phase transition, in which the displacive transformation is primarily responsible for the phase change induced by two B1u(i.e. Γ-2) phonon modes within the centrosymmetric phase. We also observed the existence of two polar structures with the same space group and different degrees of polarization (i.e Ps= 3.06 µC.cm-2and Ps= 42.41 µC.cm-2), emerging from the high symmetry non-polar structure. According to the structural analysis the ferroelectric order, of a geometric nature, is driven by the Γ-2mode in which the O- and H sites displacements lead the polar distortion with a minor contribution from the Cu-sites. Interestingly, the 3d9:Cu2+Jahn-Teller distortion coupled with the orientational shifts of O-H atoms enhances the polarization. .

Band-Gap Energy and Electronic d-d Transitions of NiWO4 Studied under High-Pressure Conditions

We report an extensive study of the optical and structural properties of NiWO4 combining experiments and density functional theory calculations. We have obtained accurate information on the pressure effect on the crystal structure determining the equation of state and compressibility tensor. We have also determined the pressure dependence of the band gap finding that it decreases under compression because of the contribution of Ni 3d states to the top of the valence band. We report on the sub-band-gap optical spectrum of NiWO4 showing that the five bands observed at 0.95, 1.48, 1.70, 2.40, and 2.70 eV correspond to crystal-field transitions within the 3d8 (t2g6eg2) configuration of Ni2+. Their assignment, which remained controversial until now, has been resolved mainly by their pressure shifts. In addition to the transition energies, their pressure derivatives are different in each band, allowing a clear band assignment. To conclude, we report resistivity and Hall-effect measurements showing that NiWO4 is a p-type semiconductor with a resistivity that decreases as pressure increases.

Lattice dynamics across the ferroelastic phase transition in Ba2ZnTeO6: a Raman and first-principles study

Structural phase transitions drive several unconventional phenomena including some illustrious ferroic attributes which are relevant for technological advancements. On this note, we have investigated the ferroelastic structural transition of perovskite-type trigonal Ba₂ZnTeO₆ across T_c ∼ 150 K. With the help of Raman spectroscopy and density-functional theory (DFT)-based calculations, we report new intriguing observations associated with the phase transition in Ba₂ZnTeO₆. We observed the presence of a central peak (quasi-elastic Rayleigh profile), huge softening in the soft mode, hysteretic phonon behavior, and signatures of coexistent phases. The existence of a central peak in Ba₂ZnTeO₆ is manifested by a sharp rise in the intensity of the Rayleigh profile concomitant with the huge damping (or softening) of the soft mode (at ∼31 cm^-1) near T_c, shedding light on the lattice dynamics during the phase transition. This is further corroborated by our phonon calculations that show that the soft mode (E_g) in the high-symmetry structure involving TeO₆ octahedral rotation (with Ba and Zn translation) condenses into A_g and B_g modes in the C2/m low-symmetry phase. While most of the phonon bands split below T_c confirming the phase transition, we observed thermal hysteretic behavior of phonon modes, which signifies the first-order nature of the transition and the presence of coexisting phases as corroborated by our temperature-dependent X-ray diffraction and specific heat measurements.

Understanding the Re-entrant Phase Transition in a Non-magnetic Scheelite

The stereochemical activity of lone pair electrons plays a central role in determining the structural and electronic properties of both chemically simple materials such as H₂O, as well as more complex condensed phases such as photocatalysts or thermoelectrics. TlReO₄ is a rare example of a non-magnetic material exhibiting a re-entrant phase transition and emphanitic behavior in the long-range structure. Here, we describe the role of the Tl⁺ 6s² lone pair electrons in these unusual phase transitions and illustrate its tunability by chemical doping, which has broad implications for functional materials containing lone pair bearing cations. First-principles density functional calculations clearly show the contribution of the Tl⁺ 6s² in the valence band region. Local structure analysis, via neutron total scattering, revealed that changes in the long-range structure of TlReO₄ occur due to changes in the correlation length of the Tl⁺ lone pairs. This has a significant effect on the anion interactions, with long-range ordered lone pairs creating a more densely packed structure. This resulted in a trade-off between anionic repulsions and lone pair correlations that lead to symmetry lowering upon heating in the long-range structure, whereby lattice expansion was necessary for the Tl⁺ lone pairs to become highly correlated. Similarly, introducing lattice expansion through chemical pressure allowed long-range lone pair correlations to occur over a wider temperature range, demonstrating a method for tuning the energy landscape of lone pair containing functional materials.

Electronic structure, phonons and optical properties of baryte type scintillators TlXO4(XCl, Br)

This article thoroughly addresses the structural, mechanical, vibrational, electronic band structure and the optical properties of the unexplored thallous perchlorate and perbromate fromab initiocalculations. The zone centered vibrational phonon frequencies shows, there is a blue shift in the mid and high frequency range from Cl → Br due to change in mass and force constant with respect to oxygen atom. From the band structure it is clear that the top of the valence band is due to thalliumsstates, whereas the bottom of the conduction band is due to halogensand oxygenpstates, showing similar magnitude of dispersion and exhibits a charge transfer character. These characteristics and the band gap obtained are consistent with that of a favourable scintillators. Our findings deliver directions for the design of efficient TlXO₄based scintillators with high performance which are desirable for distinct applications such as medical imaging, high energy physics experiments, nuclear security.

Anisotropic transport and optical birefringence of triclinic bulk and monolayer NbX2Y2(X = S, Se and Y = Cl, Br, I)

A systematic analysis of the electronic, thermoelectric and optical properties of triclinic van der Waal's solids NbX₂Y₂(X = S, Se and Y = Cl, Br, I) is carried out within the framework of density functional theory for bulk and monolayer. The investigated compounds are semiconductors in bulk and monolayer, with band gap values ranging from 1.1 to 1.8 eV. We observed huge anisotropy in the electrical conductivity with the in-plane conductivity being 40 times higher than out-of-plane conductivity in NbS₂I₂. The observed high power factor and low thermal conductivity in NbX₂Y₂render these compounds as potential thermoelectric materials. In addition, the calculated optical properties such as refractive index and absorption coefficient reveal the optical anisotropy. We have calculated birefringence for all the studied compounds and a large value of 0.313 is observed for NbSe₂I₂. The monolayer electronic properties indicate the presence of anomalous quantum confinement. The giant birefringence along with promosing monolayer properties are the highlights of present work which might fetch future device applications in both bulk as well as monolayer.

Crystal structure and phase transition of TlReO4: a combined experimental and theoretical study

The present work describes a density-functional theory (DFT) study of TlReO₄ in combination with powder x-ray diffraction experiments as a function of temperature and Raman measurements at ambient temperature. X-ray diffraction measurements reveal three different structures as a function of temperature. A monoclinic structure (space group P2₁/c) is observed at room temperature while two isostructural tetragonal structures (space group I4₁/a) are found at low- and high-temperature. In order to complement the experimental results first-principles DFT calculations were performed to compute the structural energy differences. From the total energies it is evident that the monoclinic structure has the lowest total energy when compared to the orthorhombic structure, which was originally proposed to be the structure at room temperature, which agrees with our experiments. The structural and vibrational properties of the low- and room-temperature phase of TlReO₄ have been calculated using DFT. Inclusion of van der Waals correction to the standard DFT exchange correlation functional is found to improve the agreement with the observed structural and vibrational properties. The Born effective charge of these phases has also been studied which shows a combination of ionic and covalent nature, resembling metavalent bonding. Calculations of zone-center phonon frequencies lead to the symmetry assignment of previously reported low-temperature Raman modes. We have determined the frequencies of the eight infrared-active, 13 Raman-active and three silent modes of low-temperature TlReO₄ along with 105 infrared-active and 108 Raman-active modes for room-temperature TlReO₄. Phonons of these two phases of TlReO₄ are mainly divided into three regions which are below 150 cm^-1 due to vibration of whole crystal, 250 to 400 cm^-1 due to wagging, scissoring, rocking and twisting and above 900 cm^-1 due to stretching in ReO₄ tetrahedron. The strongest infrared peak is associated to the internal asymmetric stretching of ReO₄ whereas the strongest Raman peak is associated to the internal symmetric stretching of ReO₄. We have also measured the room-temperature Raman spectra of monoclinic TlReO₄ identifying up to 28 modes. This Raman spectrum has been interpreted by comparison with the previously reported Raman frequencies of the low-temperature phase and our calculated Raman frequencies of low- and room-temperature phases of TlReO₄.

Comparative DFT study of vibrational, electronic, and optical properties of energetic alkali metal salts based on nitrogen-rich 5-aminotetrazole

This article presents a thorough density functional theory based comparative study on nitrogen-rich 5-aminotetrazole alkali metal salts M 5-At (M = Li, K, Rb, Cs). The calculated structural parameters using plane-wave pseudopotential method are consistent with the experimental results. The computed vibrational frequencies at ambient pressure show that vibrational modes in high energy region are due the NH bond of NH₂ group. Pressure variation IR spectra of these materials show clear frequency shifts where Li 5-At shows an overall red shift below 900 cm^-1 contrary to the blue shift seen in other materials in this range. The born effective charge values reveal the presence of strong covalency between N, H, and C atoms whereas an increased ionic nature is seen as the atomic number of metal atoms increases. Furthermore, we used full potential linear augmented plane wave (FP-LAPW) method for calculating electronic structure and optical properties with TB-mBJ potential which provides an enhanced band gap for all materials compared to standard GGA functional. Electronic structure calculation reveals that all the compounds are indirect band gap insulators with the exception of Li 5-At. The computed partial density of states show mixed ionic-covalent nature in metal-N/C bonds and covalent nature in NC bonds. In addition, we are also presenting the optical properties such as real and imaginary dielectric constant, absorption, refraction, reflection, loss spectrum as functions of photon energy. From the optical properties we can conclude that all the studied compounds are optically anisotropic in nature and are good absorbers in the ultraviolet (UV) region.

Pressure-Induced Enhancement of Thermoelectric Figure of Merit and Structural Phase Transition in TiNiSn

Half-Heusler thermoelectric materials are potential candidates for high thermoelectric efficiency. We report high-pressure thermoelectric and structural property measurements, density functional theory calculations on the half-Heusler material TiNiSn, and an increase of 15% in the relative dimensionless figure of merit, ZT, around 3 GPa. Thermal and electrical properties were measured utilizing a specialized sample cell assembly designed for the Paris-Edinburgh large-volume press to a maximum pressure of 3.5 GPa. High-pressure structural measurements performed up to 50 GPa in a diamond-anvil cell indicated the emergence of a new high-pressure phase around 20 GPa. A first-principles structure search performed using an ab initio random structure search approach identified the high-pressure phase as an orthorhombic type, in good agreement with the experimental results.

Insights into the structural variations in SmNb1-xTaxO4 and HoNb1-xTaxO4 combined experimental and computational studies

The impact of Ta doping on two orthoniobates SmNbO4 and HoNbO4 has been studied using a combination of high-resolution powder diffraction and Density-Functional Theory calculations. In both ANb1-xTaxO4 (A = Sm, Ho) series the unit cell volume decreases as the Ta content increased demonstrating that the effective ionic radii of Ta is smaller than that of Nb in this structure. The average Sm-O distance and volume of the SmO8 polyhedra were invariant of the Ta content across the SmNb1-xTaxO4 solid solution whereas the average M-O (M = Nb or Ta) distance and MO6 polyhedral volume decrease with Ta doping. The analogous Ho oxides HoNb1-xTaxO4 do not form a complete solid solution when the samples were prepared at 1400 °C, rather there is a miscibility gap around x = 0.95, with HoTaO4 exhibiting the M'-type P2/c structure rather than the M-type I2/a structure of HoNbO4. Increasing the synthesis temperature to 1450 °C eliminates the miscibility gap. The energy difference between the P2/c and I2/a structures of HoTaO4 is found to be nearly 30 meV per f.u. with the total energy of the P2/c phase of HoTaO4 being more negative. First-principles calculations, carried out using Density-Functional Theory, reveal significant covalent character in the Nb-O bonds, which is reduced in the corresponding tantalates. Anisotropy in the Born Effective Charge tensors demonstrates the impact of the long M-O bond identified in the structural studies showing that the Nb and Ta cations are effectively six-coordinate. The similarity in the frequency of the intense Raman peak near 800 cm-1 due to the symmetric stretching of the Ta-O bonds is consistent with the description of that both polymorphs of HoTaO4 contain TaO6 octahedra.

Structural, elastic, vibrational and optical properties of energetic material octanitrocubane studied from first-principles theory

We present a thorough density functional theory based computational study of crystalline properties of cubane caged potential energetic material octanitrocubane (ONC). As expected for a layered molecular solid, van der Waals corrections are inevitable and the same has been incorporated to capture the ground state properties more accurately. Study of Born effective charge and zone centered phonon frequencies using density functional perturbation theory reveals the important role of N2, N4 type nitrogen and associated oxygen atoms in contributing to the high intensity infrared modes. From the calculated electronic band structure we can conclude that ONC is an insulator with a band gap of 5.31 eV. The optical properties of ONC are found to be nearly isotropic in low energy region in spite of strong anisotropic crystal structure.

Assembling Homometallic Sb6 and Heterometallic Ti4Sb2 Oxo Clusters

Isolation and structural characterization of novel organoantimony(V)-based oxo clusters are reported. (RSb)₄(OH)₄(t-BuPO₃)₆ and (RSb)₂(O)(t-BuPO₃H)₆ independently in the presence of pyridine under solvothermal conditions afford the hexanuclear organoantimonate clusters [(RSb)₆(μ₃-O)₂(μ₂-O)₆(t-BuPO₃)₄], where R = p-i-PrC₆H₄ (1), p-ClC₆H₄ (2). Further, reaction of organostibonate phosphonate with Ti(OⁱPr)₄ in the presence of pyridine under solvothermal conditions afforded the mixed-metal titanium stibonate hexanuclear clusters [(RSb)₂Ti₄(μ₃-O)₂(μ₂-O)₂(t-BuPO₃)₄(μ-OCH₃)₄(OCH₃)₄], where R = p-i-PrC₆H₄ (3), p-ClC₆H₄ (4). Band gap measurements were performed on 1-4. They reveal a remarkable reduction in the band gap on moving from the heavier main-group-based oxo cages (1 and 2) to the titanium-incorporated oxo cages (3 and 4).

Time-Domain Terahertz Spectroscopy and Density Functional Theory Studies of Nitro/Nitrogen-Rich Aryl-Tetrazole Derivatives

The paper reports the time-domain THz spectroscopy studies of noncentrosymmetric energetic nitro/nitrogen-rich aryl-tetrazole high-energy molecules. The fingerprint spectra in the THz domain reveal the role of different functional groups attached to position "1" of the tetrazole moiety, which controls the energetic properties. These responses are deliberated through density functional theory (DFT) calculations. The synthesized aryl-tetrazoles exhibit high positive heat of formation (369-744 kJ/mol), high detonation velocities, and pressures (D _v: 7734-8298 m·s^-1; D _p: 24-28 GPa) in comparison to the noncentrosymmetric 2,4,6-trinitrotoluene (TNT). These compounds exhibit variation in the refractive indices and absorption between 0.1 and 2.2 THz range. The DFT studies at the molecular and single-crystal level (using plane wave pseudo potential method) endorse in detecting these bands (with ∼1% deviation). The calculated vibrational frequencies and linear optical properties are found to have good agreement with the experimental data in UV-visible and THz regions.

Structure-property correlation studies of alkaline-earth metal-azides M(N3)2 (M = Sr, Ba)

Inorganic metal azides M(N₃)₂ (M  =  Sr, Ba) and metal nitrates M(NO₃)₂ (M  =  Sr, Ba) are interesting materials due to their wide range of industrial usefulness as gas generators, pyrotechnics, photo graphic materials and explosives. In this work, we explore the mechanical, vibrational (infrared, phonon dispersion), Born effective charge (BEC) and thermodynamic properties of these materials to understand the sensitivity correlation studies using plane wave pseudopotential method. As these materials are layered with crucial non bonding interactions, the generalized gradient approximation with Tkatchenko-Scheffler (for Sr(N₃)₂) and Ortmann-Bechstedt-Schmidt (for Ba(N₃)₂) dispersion correction methods are adopted to compute accurate ground state properties with norm-conserving pseudopotentials. The calculated lattice parameters, unit cell volumes, bond lengths are well reproduced with 1% deviation when compared to the experimental and previously reported theoretical data. The mechanical (single crystal, poly-crystalline elastic constants) property correlations corroborate with the experimental impact sensitivity trend. The vibrational, phonon dispersion spectra's, BEC's, thermodynamic properties calculated with density functional perturbative theory approach provide better conclusions about the dynamical stability and polarization (optical sensitivity) trends. From the BEC results we propose that M(NO₃)₂ (M  =  Sr, Ba) materials may find various optical applications too. Overall, we tried to explain the crucial reasons for the differences in energetic properties of the studied materials.

Ti-fraction-induced electronic and magnetic transformations in titanium oxide films

Titanium dioxide has been widely used in modern industrial applications, especially as an effective photocatalyst. Recently, freestanding TiO₂ films with a markedly reduced bandgap of ∼1.8 eV have been synthesized, indicating that the dimension has a considerable influence on the bulk band gap (>∼3 eV) and enhances the adsorption range of visible light. Titanium oxide compounds have various stoichiometries and versatile properties. Therefore, it is very necessary to explore the electronic properties and functionalities of other titanium oxide films with different stoichiometries. Here, we combined structure searches with first-principle calculations to explore candidate Ti-O films with different stoichiometries. In addition to the experimentally synthesized TiO₂ film, the structure searches identified three new energetically and dynamically stable Ti-O films with stoichiometries of Ti₃O₅, Ti₃O₂, and Ti₂O. Calculations show that the Ti-O films undergo several interesting electronic transformations as the Ti fraction increases, namely, from a wide-gap semiconductor (TiO₂, 3.2 eV) to a narrow-gap semiconductor (Ti₃O₅, 1.80 eV) and then to metals (Ti₃O₂ and Ti₂O) due to the abundance of unpaired Ti_d electrons. In addition to the electronic transformations, we observed nonmagnetic (TiO₂) to ferromagnetic (Ti₃O₅, Ti₃O₂, and Ti₂O) transformations. Notably, the Ti₃O₅ film possesses both narrow-gap semiconductive and ferromagnetic properties, with a large magnetic moment of 2.0 µ_B per unit cell; therefore, this film has high potential for use in applications such as spintronic devices. The results highlight metal fraction-induced electronic and magnetic transformations in transition metal oxide films and provide an alternative route for the design of new, functional thin-film materials.

Microscopic origin of pressure-induced phase-transitions in urea: a detailed investigation through first principles calculations

The potential crystal structures and properties of urea as a function of pressure were studied using ab initio based electronic structure calculations. The enthalpy-pressure behavior shows that urea undergoes a pressure induced structural phase transition from P4[combining macron]21m (phase I) → P212121 (phase III) at 0.66 GPa with a volume collapse of 4.83%, driven by softening of the acoustic mode along the Γ-X direction. Another phase transition from the P212121 → P21212 structure was identified at 3.09 GPa. The violation of Born stability criteria in the P212121 structure along with softening of the acoustic mode in the U-R direction was responsible for the pressure induced phase transition. Furthermore, the application of pressure led to the breaking and formation of N-HO bonds in the crystal structure of urea during the phase transition, i.e., the H-acceptor capacitance of the oxygen atoms was varied between phases I/IV and -III. Band structure calculations were performed using a hybrid functional (Heyd, Scuseria and Ernzerhof, HSE) which includes a part of exact Fock-exchange. The computed electronic band structure showed that the urea polymorphs are insulators with a direct band gap of 6.21, 6.85 and 6.99 eV for phase-I, -III and -IV, respectively, at selected pressures. We have also presented the dielectric functions (real (ε1(ω)) and imaginary (ε2(ω)) parts), refractive index and absorption coefficients to explore the optical characteristics of the urea phases. The geometric interpretation of intermolecular interactions were quantitatively visualized using Hirshfeld surface analysis. Our results provide a complete picture of various properties of urea polymorphs that lay the foundation for further understanding of structures and their applications.

Structure evolution of chromium-doped boron clusters: toward the formation of endohedral boron cages

The electron-deficient nature of boron endows isolated boron clusters with a variety of interesting structural and bonding properties that can be further enriched through metal doping. In the current work, we report the structural and electronic properties of a series of chromium-doped boron clusters. The global minimum structures for CrB_n clusters with an even number of n ranging from 8 to 22 are proposed through extensive first-principles swarm-intelligence structure searches. Half-sandwich structures are found to be preferred for CrB₈, CrB₁₀, CrB₁₂ and CrB₁₄ clusters and to transform to a drum-like structure at CrB₁₆ cluster. Endohedral cage structures with the Cr atom located at the center are energetically most favorable for CrB₂₀ and CrB₂₂ clusters. Notably, the endohedral CrB₂₀ cage has a high symmetry of D_2d and a large HOMO–LUMO gap of 4.38 eV, whose stability is attributed to geometric fit and formation of an 18-electron closed-shell configuration. The current results advance our understanding of the structure and bonding of metal-doped boron clusters.

The effect of chromium doping on the structure evolution of small-sized boron clusters.

Unusual optical isotropy in anisotropic alkali metal perchlorates MClO 4 (M = Li, Na, K, Rb, Cs)

We report a detailed study on structural, vibrational, born effective charge (BEC), electronic and optical properties of the alkali metal perchlorates, MClO ₄(M  =  Li, Na, K, Rb, Cs) based on Density functional theory. The ground state calculations are done using plane wave pseudopotential method by including dispersion corrected method for more accurate prediction of structural and vibrational frequencies. The calculated lattice parameters and bond lengths are consistent with the experimental values. Further, detailed interpretation of the zone centered vibrational modes yields good concurrence between the experimental and calculated values. There is a decrease in wavelength with an increase in frequency (blue shift) from Li  →  Na  →  K  →  Rb  →  Cs. The obtained BEC shows the mixed covalent-ionic character of the compounds. The electronic and optical properties are calculated using the full potential linearized augmented plane wave method by TB-mBJ potential. The TB-mBJ band structure shows indirect band gap with O-2p states dominating in the valence band. In spite of anisotropic structure, alkali metal perchlorates are found to possess optical isotropy.

Correction: Topological behaviour of ternary non-symmorphic crystals KZnX (X = P, As, Sb) under pressure and strain: a first principles study

Correction for 'Topological behaviour of ternary non-symmorphic crystals KZnX (X = P, As, Sb) under pressure and strain: a first principles study' by Atahar Parveen et al., Phys. Chem. Chem. Phys., 2018, 20, 5084-5102.

High-Pressure Studies of Hydrogen-Bonded Energetic Material 3,6-Dihydrazino-s-tetrazine Using DFT

Hydrogen bonding is an important noncovalent interaction that plays a key role in most of the CHNO-based energetic materials, which has a great impact on the structural, stability, and vibrational properties. By analyzing the structural changes, IR spectra, and the Hirshfeld surfaces, we investigated the high-pressure behavior of 3,6-dihydrazino-s-tetrazine (DHT) to provide detailed description of hydrogen bonding interactions using dispersion-corrected density functional theory. The strengthening of hydrogen bonding is observed by the pressure-induced weakening of covalent N-H bonds, which is consistent with the red shift of NH/NH₂ stretching vibrational modes. The intermolecular interactions in DHT crystals lead to more compact and stable structures that can increase the density but diminish the heat of detonation, Q. The calculated detonation properties of DHT (D = 7.62 km/s, P = 25.19 GPa) are slightly smaller than those of a similar explosive 3,6-bis-nitroguanyl-1,2,4,5-tetrazine (D = 7.9 km/s, P = 27.36 GPa). Overall, the crystallographic and spectroscopic results along with Hirshfeld surface analysis as a function of pressure reveal the presence of strong hydrogen bonding networks in the crystal structure of DHT.

Topological behaviour of ternary non-symmorphic crystals KZnX (X = P, As, Sb) under pressure and strain: a first principles study

An ab initio study on the impact of hydrostatic pressure and strain on the electronic properties of an unexplored class of ternary Zintl phases KZnX (X = P, As, Sb) is reported. Density functional theory (DFT) based studies revealed that all the three materials are direct band gap semiconductors under ambient conditions. We have theoretically demonstrated that KZnX can be driven into different metallic phases under pressure. In contrast, by applying strain the compounds can be realized as topological insulators. This is confirmed by the observed non-trivial topological character in the electronic band structure of the present ternary systems. For the precise determination of low energy band topology, the Tran Blaha modified Becke-Johnson (TBmBJ) exchange potential was used by incorporating spin-orbit coupling. The concomitant change of electronic band shapes as a function of pressure indicates a semi-metallic nature in KZnX (X = P, As, Sb) at 30 GPa, 21 GPa and 11 GPa respectively. Based on an analysis of the parity eigenvalues, we anticipate that a band inversion occurs between the Zn-s and X-p states, thus demonstrating a weak topological behaviour in semi-metallic states. Also, a weak non-trivial topologically insulating phase is realized in strained KZnAs (18%) and KZnSb (10%) which appears to be due to overlapping of the Zn-s and X-p orbitals. The calculated surface spectral functions further validate the non-triviality of strained KZnX (X = As, Sb), whereas strained KZnP is found to be a trivial insulator. We confirm the topological behaviour of these materials by calculating topological surface states and defining a Z₂ topological invariant. Our work based on sophisticated first-principles calculations highlights that both pressure and strain can trigger topological phases in non-symmorphic trivial band insulators even with a weak spin orbit interaction. This study paves the way for realizing semi-metallic and topological insulating states in non-symmorphic ternary semiconductors, which have not been experimentally demonstrated so far.

A comparative study of the structure, stability and energetic performance of 5,5′-bitetrazole-1,1′-diolate based energetic ionic salts: future high energy density materials

The structure–property–performance interrelationship of energetic ionic salts based on 5,5′-bitetrazole-1,1′-diolate was thoroughly investigated using ab initio calculations.

Role of spin-orbit interaction on the nonlinear optical response of CsPbCO3F using DFT

We explore the effect of spin-orbit interaction (SOI) on the electronic and optical properties of CsPbCO₃F using the full potential linear augmented plane wave method with the density functional theory (DFT) approach. CsPbCO₃F is known for its high powder second harmonic generation (SHG) coefficient (13.4 times (d₃₆ = 0.39 pm V^-1) that of KH₂PO₄ (KDP)). Calculations are done for many exchange correlation (XC) potentials. After the inclusion of SOI, the calculated Tran-Blaha modified Becke-Johnson (TB-mBJ) band gap of 5.58 eV reduces to 4.45 eV in agreement with the experimental value. This is due to the splitting of Pb p-states. Importantly, the occurrence of a band gap along the H-A direction (indirect) transforms to the H-H (direct) high symmetry points/direction in the first Brillouin zone. We noticed a large anisotropy in the calculated complex dielectric function, absorption, and refractive index spectra. The calculated static birefringence of 0.1049 and 0.1057 (with SOI) is found to be higher than that of the other carbonate fluorides. From the Born effective charge (BEC) analysis we notice that the Cs atom shows a negative contribution to birefringence whereas Pb, C, and F atoms show a positive contribution. In addition, we have also calculated the nonlinear optical χ(-2ω;ω,ω) dispersion of a CsPbCO₃F single crystal. We found that d₁₁ = d₁₂ = 4.35 pm V^-1 at 1064 nm, which is 11.2 times higher than d₃₆ of KDP. The origin of the highly nonlinear optical susceptibility dispersion of CsPbCO₃F is explained. Overall, our results are in agreement with experiments and it is obvious from the present study that CsPbCO₃F is a direct band gap, large second harmonic generation, and good phase matchable NLO crystal in the ultraviolet region.

Evidence for the antiferromagnetic ground state of Zr2TiAl: a first-principles study

A detailed study on the ternary Zr-based intermetallic compound Zr₂TiAl has been carried out using first-principles electronic structure calculations. From the total energy calculations, we find an antiferromagnetic L1₁-like (AFM) phase with alternating (1 1 1) spin-up and spin-down layers to be a stable phase among some others with magnetic moment on Ti being 1.22 [Formula: see text]. The calculated magnetic exchange interaction parameters of the Heisenberg Hamiltonian and subsequent Heisenberg Monte Carlo simulations confirm that this phase is the magnetic ground structure with Néel temperature between 30 and 100 K. The phonon dispersion relations further confirm the stability of the magnetic phase while the non-magnetic phase is found to have imaginary phonon modes and the same is also found from the calculated elastic constants. The magnetic moment of Ti is found to decrease under pressure eventually driving the system to the non-magnetic phase at around 46 GPa, where the phonon modes are found to be positive indicating stability of the non-magnetic phase. A continuous change in the band structure under compression leads to the corresponding change of the Fermi surface topology and electronic topological transitions (ETT) in both majority and minority spin cases, which are also evident from the calculated elastic constants and density of state calculations for the material under compression.

Two-dimensional silicon and carbon monochalcogenides with the structure of phosphorene

Phosphorene has recently attracted significant interest for applications in electronics and optoelectronics. Inspired by this material an ab initio study was carried out on new two-dimensional binary materials with a structure analogous to phosphorene. Specifically, carbon and silicon monochalcogenides have been considered. After structural optimization, a series of binary compounds were found to be dynamically stable in a phosphorene-like geometry: CS, CSe, CTe, SiO, SiS, SiSe, and SiTe. The electronic properties of these monolayers were determined using density functional theory. By using accurate hybrid functionals it was found that these materials are semiconductors and span a broad range of bandgap values and types. Similarly to phosphorene, the computed effective masses point to a strong in-plane anisotropy of carrier mobilities. The variety of electronic properties carried by these compounds have the potential to broaden the technological applicability of two-dimensional materials.

ZnGeSb2: a promising thermoelectric material with tunable ultra-high conductivity

First principles calculations predict the promising thermoelectric material ZnGeSb₂ with a huge power factor (S²σ/τ) on the order of 3 × 10¹⁷ W m^-1 K^-2 s^-1, due to the ultra-high electrical conductivity scaled by a relaxation time of around 8.5 × 10²⁵ Ω^-1 m^-1 s^-1, observed in its massive Dirac state. The observed electrical conductivity is higher than the well-established Dirac materials, and is almost carrier concentration independent with similar behaviour of both n and p type carriers, which may certainly attract device applications. The low range of thermal conductivity is also evident from the phonon dispersion. Our present study further reports the gradual phase change of ZnGeSb₂ from a normal semiconducting state, through massive Dirac states, to a topological semi-metal. The maximum power factor is observed in the massive Dirac states compared to the other two states.

Calculated high-pressure structural properties, lattice dynamics and quasi particle band structures of perovskite fluorides KZnF3, CsCaF3 and BaLiF3

A detailed study of the high-pressure structural properties, lattice dynamics and band structures of perovskite structured fluorides KZnF3, CsCaF3 and BaLiF3 has been carried out by means of density functional theory. The calculated structural properties including elastic constants and equation of state agree well with available experimental information. The phonon dispersion curves are in good agreement with available experimental inelastic neutron scattering data. The electronic structures of these fluorides have been calculated using the quasi particle self-consistent [Formula: see text] approximation. The [Formula: see text] calculations reveal that all the fluorides studied are wide band gap insulators, and the band gaps are significantly larger than those obtained by the standard local density approximation, thus emphasizing the importance of quasi particle corrections in perovskite fluorides.

Predicted superconductivity of Ni2VAl and pressure dependence of superconductivity in Ni2NbX (X = Al, Ga and Sn) and Ni2VAl

A first-principles study of the electronic and superconducting properties of the Ni2VAl Heusler compound is presented. The electron-phonon coupling constant of λ(ep)=0.68 is obtained, which leads to a superconducting transition temperature of Tc = ~ 4K (assuming a Coulomb pseudopotential μ(*)=0.13), which is a relatively high transition temperature for Ni based Heusler alloys. The electronic density of states reveals a significant hybridization between Ni-eg and V-t(2g) states around the Fermi level. The Fermi surface, consisting of two electron pockets around the X-points of the Brillouin zone, exhibits nesting and leads to a Kohn anomaly of the phonon dispersion relation for the transverse acoustic mode TA2 along the (1, 1, 0) direction, which is furthermore found to soften with pressure. As a consequence, T(c) and λ(ep) vary non-monotonically under pressure. The calculations are compared to similar calculations performed for the Ni2NbX (X  =  Al, Ga and Sn) Heusler alloys, which experimentally have been identified as superconductors. The experimental trend in T(c) is well reproduced, and reasonable quantitative agreement is obtained. The calculated T(c) of Ni2VAl is larger than either calculated or observed T(c)s of any of the Nb compounds. The Fermi surfaces of Ni2NbAl and Ni2NbGa consist of only a single electron pocket around the X point, however under compression a second electron pocket similar to that of Ni2VAl emerges Ni2NbAl and the T(c) increases non-monotonically in all the compounds. Fermi surface nesting and associated Kohn anomalies are a common feature of all four compounds, albeit weakest in Ni2VAl.

Predicted thermoelectric properties of olivine-type Fe2GeCh4 (Ch = S, Se and Te)

We present here the thermoelectric properties of olivine-type Fe2GeCh4 (Ch  =  S, Se and Te) using the linear augmented plane wave method based on first principles density functional calculations. The calculated transport properties using the semi-local Boltzmann transport equation reveal very high thermopower for both S and Se-based compounds compared to their Te counterparts. The main reason for this high thermopower is the quasi-flat nature of the bands at the valence and conduction band edges. The calculated thermopower of Fe2GeS4 is in good agreement with the experimental reports at room temperature, with the carrier concentration around 10(18)-10(19)cm(-3). All the investigated systems show an anisotropic nature in their electrical conductivity, resulting in a value less than the order of 10(2) along the a-axis compared to the b- and c-axes. Among the studied compounds, Fe2GeS4 and Fe2GeSe4 emerge as promising candidates with good thermoelectric performance.

Effect of lead and caesium on the mechanical, vibrational and thermodynamic properties of hexagonal fluorocarbonates: a comparative first principles study

The experimental crystal structure of CsPbCO₃F consists of alternate CsF and PbCO₃ layers.

Structural, electronic and optical properties of well-known primary explosive: Mercury fulminate

Mercury Fulminate (MF) is one of the well-known primary explosives since 17th century and it has rendered invaluable service over many years. However, the correct molecular and crystal structures are determined recently after 300 years of its discovery. In the present study, we report pressure dependent structural, elastic, electronic and optical properties of MF. Non-local correction methods have been employed to capture the weak van der Waals interactions in layered and molecular energetic MF. Among the non-local correction methods tested, optB88-vdW method works well for the investigated compound. The obtained equilibrium bulk modulus reveals that MF is softer than the well known primary explosives Silver Fulminate (SF), silver azide and lead azide. MF exhibits anisotropic compressibility (b > a > c) under pressure, consequently the corresponding elastic moduli decrease in the following order: C22 > C11 > C33. The structural and mechanical properties suggest that MF is more sensitive to detonate along c-axis (similar to RDX) due to high compressibility of Hg⋯O non-bonded interactions along that axis. Electronic structure and optical properties were calculated including spin-orbit (SO) interactions using full potential linearized augmented plane wave method within recently developed Tran-Blaha modified Becke-Johnson (TB-mBJ) potential. The calculated TB-mBJ electronic structures of SF and MF show that these compounds are indirect bandgap insulators. Also, SO coupling is found to be more pronounced for 4d and 5d-states of Ag and Hg atoms of SF and MF, respectively. Partial density of states and electron charge density maps were used to describe the nature of chemical bonding. Ag-C bond is more directional than Hg-C bond which makes SF to be more unstable than MF. The effect of SO coupling on optical properties has also been studied and found to be significant for both (SF and MF) of the compounds.

Effect of Pressure on Valence and Structural Properties of YbFe2Ge2 Heavy Fermion Compound--A Combined Inelastic X-ray Spectroscopy, X-ray Diffraction, and Theoretical Investigation

The crystal structure and the Yb valence of the YbFe2Ge2 heavy fermion compound was measured at room temperature and under high pressures using high-pressure powder X-ray diffraction and X-ray absorption spectroscopy via both partial fluorescence yield and resonant inelastic X-ray emission techniques. The measurements are complemented by first-principles density functional theoretical calculations using the self-interaction corrected local spin density approximation investigating in particular the magnetic structure and the Yb valence. While the ThCr2Si2-type tetragonal (I4/mmm) structure is stable up to 53 GPa, the X-ray emission results show an increase of the Yb valence from v = 2.72(2) at ambient pressure to v = 2.93(3) at ∼9 GPa, where at low temperature a pressure-induced quantum critical state was reported.

Structural stability, vibrational, and bonding properties of potassium 1, 1'-dinitroamino-5, 5'-bistetrazolate: An emerging green primary explosive

Potassium 1,1'-dinitroamino-5,5'-bistetrazolate (K2DNABT) is a nitrogen rich (50.3% by weight, K2C2N12O4) green primary explosive with high performance characteristics, namely, velocity of detonation (D = 8.33 km/s), detonation pressure (P = 31.7 GPa), and fast initiating power to replace existing toxic primaries. In the present work, we report density functional theory (DFT) calculations on structural, equation of state, vibrational spectra, electronic structure, and absorption spectra of K2DNABT. We have discussed the influence of weak dispersive interactions on structural and vibrational properties through the DFT-D2 method. We find anisotropic compressibility behavior (b<a<c) from pressure dependent structural properties. The predicted equilibrium bulk modulus reveals that K2DNABT is softer than toxic lead azide and harder than the most sensitive cyanuric triazide. A complete assignment of all the vibrational modes has been made and compared with the available experimental results. The calculated zone center IR and Raman frequencies show a blue-shift which leads to a hardening of the lattice upon compression. In addition, we have also calculated the electronic structure and absorption spectra using recently developed Tran Blaha-modified Becke Johnson potential. It is found that K2DNABT is a direct band gap insulator with a band gap of 3.87 eV and the top of the valence band is mainly dominated by 2p-states of oxygen and nitrogen atoms. K2DNABT exhibits mixed ionic (between potassium and tetrazolate ions) and covalent character within tetrazolate molecule. The presence of ionic bonding suggests that the investigated compound is relatively stable and insensitive than covalent primaries. From the calculated absorption spectra, the material is found to decompose under ultra-violet light irradiation.

Phase stability and lattice dynamics of ammonium azide under hydrostatic compression

We report the phase stability of hydro-nitrogen solids and mechanical and dynamical stability of the thermodynamic ground state of N₄H₄ compounds (AA).

Structural, vibrational, and quasiparticle band structure of 1,1-diamino-2,2-dinitroethelene from ab initio calculations

The effects of pressure on the structural and vibrational properties of the layered molecular crystal 1,1-diamino-2,2-dinitroethelene (FOX-7) are explored by first principles calculations. We observe significant changes in the calculated structural properties with different corrections for treating van der Waals interactions to Density Functional Theory (DFT), as compared with standard DFT functionals. In particular, the calculated ground state lattice parameters, volume and bulk modulus obtained with Grimme's scheme, are found to agree well with experiments. The calculated vibrational frequencies demonstrate the dependence of the intra and inter-molecular interactions on FOX-7 under pressure. In addition, we also found a significant increment in the N-H...O hydrogen bond strength under compression. This is explained by the change in bond lengths between nitrogen, hydrogen, and oxygen atoms, as well as calculated IR spectra under pressure. Finally, the computed band gap is about 2.3 eV with generalized gradient approximation, and is enhanced to 5.1 eV with the GW approximation, which reveals the importance of performing quasiparticle calculations in high energy density materials.

Phase transitions in rare earth tellurides under pressure

Using first-principles calculations we have studied the valence and structural transitions of the rare earth monotellurides RTe (R = Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb) under pressure. The self-interaction corrected local spin-density approximation is used to establish the ground state valence configuration as a function of volume for the RTe in both the NaCl (B1) and CsCl (B2) structures. We find that in ambient conditions all the RTe are stabilized in the B1 structure. A trivalent (R(3+)) rare earth ground state is predicted for the majority of the RTe, with the exception of SmTe, EuTe, DyTe, TmTe and YbTe, where the fully localized divalent (R(2+)) rare earth configuration is found to be energetically most favourable. Under pressure, the trivalent RTe undergo structural transitions to the B2 structure without associated valence transition. The divalent RTe on the other hand are characterized by a competition between the structural and electronic degrees of freedom, and it is the degree of f-electron delocalization that determines the sequence of phase transitions. In EuTe and YbTe, where respectively the half-filled and filled shells result in a very stable divalent configuration, we find that it is the structural B1 → B2 transition that occurs first, followed by the R(2+) → R(3+) valence transition at even higher pressures. In SmTe, DyTe and TmTe, the electronic transition occurs prior to the structural transition. With the exception of YbTe, the calculated transition pressures are found to be in good agreement with experiment.

Polymorphism and thermodynamic ground state of silver fulminate studied from van der Waals density functional calculations

Silver fulminate (AgCNO) is a primary explosive, which exists in two polymorphic phases, namely, orthorhombic (Cmcm) and trigonal (R3) forms at ambient conditions. In the present study, we have investigated the effect of pressure and temperature on relative phase stability of the polymorphs using planewave pseudopotential approaches based on Density Functional Theory (DFT). van der Waals interactions play a significant role in predicting the phase stability and they can be effectively captured by semi-empirical dispersion correction methods in contrast to standard DFT functionals. Based on our total energy calculations using DFT-D2 method, the Cmcm structure is found to be the preferred thermodynamic equilibrium phase under studied pressure and temperature range. Hitherto Cmcm and R3 phases denoted as α- and β-forms of AgCNO, respectively. Also a pressure induced polymorphic phase transition is seen using DFT functionals and the same was not observed with DFT-D2 method. The equation of state and compressibility of both polymorphic phases were investigated. Electronic structure and optical properties were calculated using full potential linearized augmented plane wave method within the Tran-Blaha modified Becke-Johnson potential. The calculated electronic structure shows that α, β phases are indirect bandgap insulators with a bandgap values of 3.51 and 4.43 eV, respectively. The nature of chemical bonding is analyzed through the charge density plots and partial density of states. Optical anisotropy, electric-dipole transitions, and photo sensitivity to light of the polymorphs are analyzed from the calculated optical spectra. Overall, the present study provides an early indication to experimentalists to avoid the formation of unstable β-form of AgCNO.

Density functional study of electronic structure, elastic and optical properties of MNH2 (M=Li, Na, K, Rb)

We report a systematic first principles density functional study on the electronic structure, elastic and optical properties of nitrogen based solid hydrogen storage materials LiNH2, NaNH2, KNH2, and RbNH2. The ground state structural properties are calculated by using standard density functional theory, and also dispersion corrected density functional theory. We find that van der Waals interactions are dominant in LiNH2 whereas they are relatively weak in other alkali metal amides. The calculated elastic constants show that all the compounds are mechanically stable and LiNH2 is found to be a stiffer material among the alkali metal amides. The melting temperatures are calculated and follow the order RbNH2 < KNH2 < NaNH2 < LiNH2. The electronic band structure is calculated by using the Tran–Blaha modified Becke–Johnson potential and found that all the compounds are insulators, with a considerable band gap. The [NH2]− derived states completely dominate in the entire valence band region while the metal atom states occupy the conduction band. The calculated band structure is used to analyze the different interband optical transitions occurring between valence and conduction bands. Our calculations show that these materials have considerable optical anisotropy.