Phonons in Copper Diphosphide (CuP2): Raman Spectroscopy and Lattice Dynamics Calculations

Copper diphosphide (CuP₂) is an emerging binary semiconductor with promising properties for energy conversion and storage applications. While functionality and possible applications of CuP₂ have been studied, there is a curious gap in the investigation of its vibrational properties. In this work, we provide a reference Raman spectrum of CuP₂, with a complete analysis of all Raman active modes from both experimental and theoretical perspectives. Raman measurements have been performed on polycrystalline CuP₂ thin films with close to stoichiometric composition. Detailed deconvolution of the Raman spectrum with Lorentzian curves has allowed identification of all theoretically predicted Raman active modes (9A_g and 9B_g), including their positions and symmetry assignment. Furthermore, calculations of the phonon density of states (PDOS), as well as the phonon dispersions, provide a microscopic understanding of the experimentally observed phonon lines, in addition to the assignment to the specific lattice eigenmodes. We further provide the theoretically predicted positions of the infrared (IR) active modes, along with the simulated IR spectrum from density functional theory (DFT). Overall good agreement is found between the experimental and DFT-calculated Raman spectra of CuP₂, providing a reference platform for future investigations on this material.

Phonon Raman spectroscopy of nanocrystalline multinary chalcogenides as a probe of complex lattice structures

Ternary (I-III-VI) and quaternary (I-II-IV-VI) metal-chalcogenides like CuInS₂or Cu₂ZnSn(S,Se)₄are among the materials currently most intensively investigated for various applications in the area of alternative energy conversion and light-emitting devices. They promise more sustainable and affordable solutions to numerous applications, compared to more developed and well understood II-VI and III-V semiconductors. Potentially superior properties are based on an unprecedented tolerance of these compounds to non-stoichiometric compositions and polymorphism. However, if not properly controlled, these merits lead to undesirable coexistence of different compounds in a single polycrystalline lattice and huge concentrations of point defects, becoming an immense hurdle on the way toward real-life applications. Raman spectroscopy of phonons has become one of the most powerful tools of structural diagnostics and probing physical properties of bulk and microcrystalline I-III-VI and I-II-IV-VI compounds. The recent explosive growth of the number of reports on fabrication and characterization of nanostructures of these compounds must be pointed out as well as the steady use of Raman spectroscopy for their characterization. Interpretation of the vibrational spectra of these compound nanocrystals (NCs) and conclusions about their structure can be complicated compared to bulk counterparts because of size and surface effects as well as emergence of new structural polymorphs that are not realizable in the bulk. This review attempts to summarize the present knowledge in the field of I-III-VI and I-II-IV-VI NCs regarding their phonon spectra and capabilities of Raman and IR spectroscopies in the structural characterizations of these promising families of compounds.

Stoichiometry modulates the optoelectronic functionality of Zinc Phosphide (Zn3-xP2+x)

Predictive synthesis–structure–property relationships are at the core of materials design for novel applications. In this regard, correlations between the compositional stoichiometry variations and functional properties are essential for enhancing the performance of devices based on these materials. In this work, we investigate the effect of stoichiometry variations and defects on the structural and optoelectronic properties of monocrystalline zinc phosphide (Zn₃P₂), a promising compound for photovoltaic applications. We use experimental methods, such as electron and X-ray diffraction and Raman spectroscopy, along with density functional theory calculations, to showcase the favorable creation of P interstitial defects over Zn vacancies in P-rich and Zn-poor compositional regions. Photoluminescence and absorption measurements show that these defects create additional energy levels at about 180 meV above the valence band. Furthermore, they lead to the narrowing of the bandgap, due to the creation of band tails in the region of around 10–20 meV above the valence and below the conduction band. The ability of zinc phosphide to form off-stoichiometric compounds provides a new promising opportunity for tunable functionality that benefits applications. In that regard, this study is crucial for the further development of zinc phosphide and its application in optoelectronic and photovoltaic devices, and should pave the way for defect engineering in this kind of material.

Zinc phosphide (Zn₃P₂) is a promising material for photovoltaic applications. Here, we investigate the effect of stoichiometry variations and defects on the structural and optoelectronic properties of monocrystalline Zn₃P₂.

Raman tensor of zinc-phosphide (Zn3P2): from polarization measurements to simulation of Raman spectra

Zinc phosphide (Zn3P2) is a II-V compound semiconductor with promising photovoltaic and thermoelectric applications. Its complex structure is susceptible to facile defect formation, which plays a key role in further optimization of the material. Raman spectroscopy can be effectively used for defect characterization. However, the Raman tensor of Zn3P2, which determines the intensity of Raman peaks and anisotropy of inelastic light scattering, is still unknown. In this paper, we use angle-resolved polarization Raman measurements on stoichiometric monocrystalline Zn3P2 thin films to obtain the Raman tensor of Zn3P2. This has allowed determination of the Raman tensor elements characteristic for the A1g, B1g and B2g vibrational modes. These results have been compared with the theoretically obtained Raman tensor elements and simulated Raman spectra from the lattice-dynamics calculations using first-principles force constants. Excellent agreement is found between the experimental and simulated Raman spectra of Zn3P2 for various polarization configurations, providing a platform for future characterization of the defects in this material.

Colloidal Cu-Zn-Sn-Te Nanocrystals: Aqueous Synthesis and Raman Spectroscopy Study

Cu-Zn-Sn-Te (CZTTe) is an inexpensive quaternary semiconductor that has not been investigated so far, unlike its intensively studied CZTS and CZTSe counterparts, although it may potentially have desirable properties for solar energy conversion, thermoelectric, and other applications. Here, we report on the synthesis of CZTTe nanocrystals (NCs) via an original low-cost, low-temperature colloidal synthesis in water, using a small-molecule stabilizer, thioglycolic acid. The absorption edge at about 0.8-0.9 eV agrees well with the value expected for Cu2ZnSnTe4, thus suggesting CZTTe to be an affordable alternative for IR photodetectors and solar cells. As the main method of structural characterization multi-wavelength resonant Raman spectroscopy was used complemented by TEM, XRD, XPS as well as UV-vis and IR absorption spectroscopy. The experimental study is supported by first principles density functional calculations of the electronic structure and phonon spectra. Even though the composition of NCs exhibits a noticeable deviation from the Cu2ZnSnTe4 stoichiometry, a common feature of multinary NCs synthesized in water, the Raman spectra reveal very small widths of the main phonon peak and also multi-phonon scattering processes up to the fourth order. These factors imply a very good crystallinity of the NCs, which is further confirmed by high-resolution TEM.

Cubic, hexagonal and tetragonal FeGe x phases (x = 1, 1.5, 2): Raman spectroscopy and magnetic properties

There is currently an emerging drive towards computational materials design and fabrication of predicted novel materials. One of the keys to developing appropriate fabrication methods is determination of the composition and phase. Here we explore the FeGe system and establish reference Raman signatures for the distinction between FeGe hexagonal and cubic structures, as well as FeGe2 and Fe2Ge3 phases. The experimental results are substantiated by first principles lattice dynamics calculations as well as by complementary structural characterization such as transmission electron microscopy and X-ray diffraction, along with magnetic measurements.

Raman and X-ray Photoelectron Spectroscopic Study of Aqueous Thiol-Capped Ag-Zn-Sn-S Nanocrystals

The synthesis of (Cu,Ag)-Zn-Sn-S (CAZTS) and Ag-Zn-Sn-S (AZTS) nanocrystals (NCs) by means of "green" chemistry in aqueous solution and their detailed characterization by Raman spectroscopy and several complementary techniques are reported. Through a systematic variation of the nominal composition and quantification of the constituent elements in CAZTS and AZTS NCs by X-ray photoemission spectroscopy (XPS), we identified the vibrational Raman and IR fingerprints of both the main AZTS phase and secondary phases of Ag-Zn-S and Ag-Sn-S compounds. The formation of the secondary phases of Ag-S and Ag-Zn-S cannot be avoided entirely for this type of synthesis. The Ag-Zn-S phase, having its bandgap in near infrared range, is the reason for the non-monotonous dependence of the absorption edge of CAZTS NCs on the Ag content, with a trend to redshift even below the bandgaps of bulk AZTS and CZTS. The work function, electron affinity, and ionization potential of the AZTS NCs are derived using photoelectron spectroscopy measurements.

Raman spectroscopy and lattice dynamics calculations of tetragonally-structured single crystal zinc phosphide (Zn3P2) nanowires

Earth-abundant and low-cost semiconductors, such as zinc phosphide (Zn₃P₂), are promising candidates for the next generation photovoltaic applications. However, synthesis on commercially available substrates, which favors the formation of defects, and controllable doping are challenging drawbacks that restrain device performance. Better assessment of relevant properties such as structure, crystal quality and defects will allow faster advancement of Zn₃P₂, and in this sense, Raman spectroscopy can play an invaluable role. In order to provide a complete Raman spectrum reference of Zn₃P₂, this work presents a comprehensive analysis of vibrational properties of tetragonally-structured Zn₃P₂ (space group P4₂/nmc) nanowires, from both experimental and theoretical perspectives. Low-temperature, high-resolution Raman polarization measurements have been performed on single-crystalline nanowires. Different polarization configurations have allowed selective enhancement of A_1g, B_1g and E_g Raman modes, while B_2g modes were identified from complementary unpolarized Raman measurements. Simultaneous deconvolution of all Raman spectra with Lorentzian curves has allowed identification of 33 peaks which have been assigned to 34 (8 A_1g + 9 B_1g + 3 B_2g + 14 E_g) out of the 39 theoretically predicted eigenmodes. The experimental results are in good agreement with the vibrational frequencies that have been computed by first-principles calculations based on density functional theory. Three separate regions were observed in the phonon dispersion diagram: (i) low-frequency region (<210 cm^-1) which is dominated by Zn-related vibrations, (ii) intermediate region (210-225 cm^-1) which represents a true phonon gap with no observed vibrations, and (iii) high-frequency region (>225 cm^-1) which is attributed to primarily P-related vibrations. The analysis of vibrational patterns has shown that non-degenerate modes involve mostly atomic motion along the long crystal axis (c-axis), while degenerate modes correspond primarily to in-plane vibrations, perpendicular to the long c-axis. These results provide a detailed reference for identification of the tetragonal Zn₃P₂ phase and can be used for building Raman based methodologies for effective defect screening of bulk materials and films, which might contain structural inhomogeneities.

Raman and infrared phonons in tetragonal ZnP2and CdP2crystals: a density functional study

Lattice dynamic properties of the tetragonal modification of ZnP2and CdP2crystals (space group P41212, no 92) are calculated within the density functional theory. Theoretical results are shown to compare favorably with available Raman scattering and infrared reflection/transmission experimental data, which allows assignment of Raman-and infrared-active modes to the specific lattice eigenmodes. It is confirmed that several distinct features of vibrational spectra of these compounds steam from the presence of four phosphorous spiraling chains within crystallographic unit cell.

Optical properties and lattice dynamics of a novel allotrope of orthorhombic elemental germanium

Optical and vibrational properties of a novel allotrope of elemental germanium Ge(oP32), which crystallizes in the structure corresponding to the orthorhombic space group Pbcm, are studied experimentally by means of absorption and polarized Raman scattering measurements and theoretically using the first principles density functional theory. Material is found to be a direct band gap semiconductor with E _g   =  0.33 eV. Out of theoretically predicted 48 Raman-active modes, 27 are observed in the spectra and assigned to the specific lattice eigenmodes of the crystal based on their symmetry and a comparison with the results of first principles lattice dynamics calculations. Remarkably, the highest frequency vibration is observed at 316 cm^-1, that is higher than the cubic crystalline [Formula: see text]-Ge mode at 300 cm^-1. Exceptional sharpness of observed phonon lines (between 0.8 and 2.5 cm^-1 at T  =  10 K) implies excellent crystallinity of Ge(oP32) crystals.

Narrow Gap Semiconducting Germanium Allotrope from the Oxidation of a Layered Zintl Phase in Ionic Liquids

A metastable germanium allotrope, Ge(oP32), was synthesized as polycrystalline powders and single crystals from the mild-oxidation/delithiation of Li₇Ge₁₂ in ionic liquids. Its crystal structure, from single crystal X-ray diffraction ( Pbcm, a = 8.1527(4) Å, b = 11.7572(5) Å, c = 7.7617(4) Å), features a complex covalent network of 4-bonded Ge, resulting from a well-ordered topotactic oxidative condensation of [Ge₁₂]^7- layers. It is a diamagnetic semiconductor ( E_g = 0.33 eV), and transforms exothermically and irreversibly to α-Ge at 363 °C. This demonstrates the potential of ionic liquids as reactive media in the mild oxidation of Zintl phases to new highly crystallized modifications of elements and simple compounds.

Pressure-Temperature Phase Diagram Reveals Spin-Lattice Interactions in Co[N(CN)2]2

Diamond anvil cell techniques, synchrotron-based infrared and Raman spectroscopies, and lattice dynamics calculations are combined with prior magnetic property work to reveal the pressure-temperature phase diagram of Co[N(CN)₂]₂. The second-order structural boundaries converge on key areas of activity involving the spin state exposing how the pressure-induced local lattice distortions trigger the ferromagnetic → antiferromagnetic transition in this quantum material.

Fermi resonance in the phonon spectra of quaternary chalcogenides of the type Cu2ZnGeS4

The experimental resonant and non-resonant Raman scattering spectra of the kesterite structural modification of Cu2ZnGeS4 single crystals are reported. The results are compared with those calculated theoretically within the density functional perturbation theory. For the majority of lines a good agreement (within 2-5 cm(-1)) is established between experimental and calculated mode frequencies. However, several dominant spectral lines, in particular the two intense fully symmetric modes, are found to deviate from the calculated values by as much as 20 cm(-1). A possible reason for this discrepancy is found to be associated with the Fermi resonant interaction between one and two-phonon vibrational excitations. The modelling of spectra, which takes into account the symmetry of interacting states, allows a qualitative description of the observed experimental findings. Due to the similarity of the vibrational spectra of Cu2A (II) B (IV) S4 (A  =  Zn, Mn, Cd; B  =  Sn, Ge, Si) chalcogenides, Fermi resonance is argued to be a general phenomenon for this class of compounds.

Local Lattice Distortions in Mn[N(CN)2]2 under Pressure

We combined synchrotron-based infrared spectroscopy, Raman scattering, and diamond anvil cell techniques with complementary lattice dynamics calculations to reveal local lattice distortions in Mn[N(CN)2]2 under compression. Strikingly, we found a series of transitions involving octahedral counter-rotations, changes in the local Mn environment, and deformations of the superexchange pathway. In addition to reinforcing magnetic property trends, these pressure-induced local lattice distortions may provide an avenue for the development of new functionalities.

Pressure-induced local lattice distortions in α-Co[N(CN)2]2

This work brings together diamond anvil cell techniques, vibrational spectroscopies, and complementary lattice dynamics calculations to investigate pressure-induced local lattice distortions in α-Co[N(CN)2]2. Analysis of mode behavior and displacement patterns reveals a series of pressure-driven transitions that modify the CoN6 counter-rotations, distort the octahedra, and flatten the C-N(ax)-C linkages. These local lattice distortions may be responsible for the low temperature magnetic crossover. We also discuss prospects for negative thermal expansion and show that there is not a straightforward low pressure pathway between the pink α and blue β ambient pressure phases of Co[N(CN)2]2.

Electron-phonon and magnetoelastic interactions in ferromagnetic Co[N(CN)2]2

We combined Raman and infrared vibrational spectroscopies with complementary lattice dynamics calculations and magnetization measurements to reveal the dynamic aspects of charge-lattice-spin coupling in Co[N(CN)2]2. Our work uncovers electron-phonon coupling as a magnetic field-driven avoided crossing of the low-lying Co2+ electronic excitation with two ligand phonons and a magnetoelastic effect that signals a flexible local CoN6 environment. Their simultaneous presence indicates the ease with which energy is transferred over multiple length and time scales in this system.

Quantum critical transition amplifies magnetoelastic coupling in Mn[N(CN)2]2

We report the discovery of a magnetic quantum critical transition in Mn[N(CN)(2)](2) that drives the system from a canted antiferromagnetic state to the fully polarized state with amplified magnetoelastic coupling as an intrinsic part of the process. The local lattice distortions, revealed through systematic phonon frequency shifts, suggest a combined MnN(6) octahedra distortion+counterrotation mechanism that reduces antiferromagnetic interactions and acts to accommodate the field-induced state. These findings deepen our understanding of magnetoelastic coupling near a magnetic quantum critical point and away from the static limit.

Second-order Raman scattering in CuO

Polarized second-order Raman scattering spectra of CuO single crystals are reported. It is shown that for some scattering geometries the second-order processes dominate the inelastic light scattering spectra. Group-theoretical symmetry analysis of the selection rules for the first- and second-order scattering processes is performed and phonon dispersion relations are calculated within density functional theory. The main spectral features of the two-phonon spectra are assigned to overtones of the vibrational branches at various special points across the Brillouin zone.

Electronic excitations and lattice dynamics of coordinatively "unsaturated" complex transition metal compounds

Single crystal polarized Raman and infrared spectra of the series Na(5)[MO(2)][X] with M = Co(I), Ni(I), and Cu(I) and X = S(2-) and CO(3)(2-), are reported. All phonon modes are assigned to the lattice eigenmodes based on the group theory analysis and first principles lattice dynamics calculations. The energies of the fundamental symmetric and asymmetric vibrations of the [MO(2)](3-) complex are discussed on the basis of their electronic structure and variation in M-O interatomic distances. Electronic Raman scattering and luminescence are observed for the magnetic members of the series (Co(I), d(8), and Ni(I), d(9)). Ligand field theory is employed to account for the electronic effects which originate from states split by spin-orbit coupling.

Electronic structure, optical properties and lattice dynamics of MgSO3·6H2O

The electronic band structure, optical properties and lattice vibrations of MgSO(3)·6H(2)O were studied within density functional theory and compared to the experimental optical data and polarized Raman spectra. Due to the 'molecular' nature of the MgSO(3)·6H(2)O crystal all Γ-point phonon modes could be separated into groups belonging to specific structural blocks: Mg(H(2)O)(6) octahedra, SO(3) units and H(2)O molecules. All Raman lines in the experimental spectra are assigned to definite vibrations of the structure and reasonable agreement is found between theoretical and experimental mode frequencies. The temperature-dependent Raman spectra reveal at 60 °C a sharp transition from MgSO(3)·6H(2)O to anhydrous amorphous MgSO(3) without the noticeable presence of intermediate lower hydrates, such as MgSO(3)·3H(2)O.

Spin-lattice interactions mediated by magnetic field

Application of a magnetic field offers an incisive opportunity to tune competing interactions in complex materials. Here we probe field-induced changes in the local structure of DyMn2O5 by using magnetoinfrared spectroscopy. The high tunability of the dielectric constant and ferroelectric polarization with field is well documented in the literature, but the lattice response on the microscopic level remains unknown. In this work, we reveal the dynamic nature of the local structural response to field and analyze it in terms of calculated mode displacements and local lattice distortions.

Field-induced reentrant novel phase and a ferroelectric-magnetic order coupling in HoMnO3

A reentrant novel phase is observed in the hexagonal ferroelectric HoMnO3 in the presence of magnetic fields in the temperature range defined by a plateau of the dielectric constant anomaly. The plateau evolves with fields from a narrow dielectric peak at the Mn-spin rotation transition at 32.8 K in zero field. The anomaly appears both as a function of temperature and as a function of magnetic field without detectable hysteresis. This is attributed to the indirect coupling between the ferroelectric (FE) and antiferromagnetic (AFM) orders, arising from an FE-AFM domain wall effect.