High-pressure characterization of the optical and electronic properties of InVO4, InNbO4, and InTaO4

Electronic, Vibrational, and Structural Properties of the Natural Mineral Ferberite (FeWO4): A High-Pressure Study

This paper reports an experimental high-pressure study of natural mineral ferberite (FeWO4) up to 20 GPa using diamond-anvil cells. First-principles calculations have been used to support and complement the results of the experimental techniques. X-ray diffraction patterns show that FeWO4 crystallizes in the wolframite structure at ambient pressure and is stable over a wide pressure range, as is the case for other wolframite AWO4 (A = Mg, Mn, Co, Ni, Zn, or Cd) compounds. No structural phase transitions were observed for FeWO4, in the pressure range investigated. The bulk modulus (B0 = 136(3) GPa) obtained from the equation of state is very close to the recently reported value for CoWO4 (131(3) GPa). According to our optical absorption measurements, FeWO4 has an indirect band gap that decreases from 2.00(5) eV at ambient pressure to 1.56(5) eV at 16 GPa. First-principles simulations yield three infrared-active phonons, which soften with pressure, in contrast to the Raman-active phonons. These results agree with Raman spectroscopy experiments on FeWO4 and are similar to those previously reported for MgWO4. Our results on FeWO4 are also compared to previous results on other wolframite-type compounds.

Accurate Determination of the Bandgap Energy of the Rare-Earth Niobate Series

We report diffuse reflectivity measurements in InNbO₄, ScNbO₄, YNbO₄, and eight rare-earth niobates. A comparison with established values of the bandgap of InNbO₄ and ScNbO₄ shows that Tauc plot analysis gives erroneous estimates of the bandgap energy. Conversely, accurate results are obtained considering excitonic contributions using the Elliot-Toyozawa model. The bandgaps are 3.25 eV for CeNbO₄, 4.35 eV for LaNbO₄, 4.5 eV for YNbO₄, and 4.73-4.93 eV for SmNbO₄, EuNbO₄, GdNbO₄, DyNbO₄, HoNbO₄, and YbNbO₄. The fact that the bandgap energy is affected little by the rare-earth substitution from SmNbO₄ to YbNbO₄ and the fact that they have the largest bandgap are a consequence of the fact that the band structure near the Fermi level originates mainly from Nb 4d and O 2p orbitals. YNbO₄, CeVO₄, and LaNbO₄ have smaller bandgaps because of the contribution from rare-earth atom 4d, 5d, or 4f orbitals to the states near the Fermi level.

Formation of Novel Bimetal Oxide In2V2O7 through a Shock Compression Method

Bimetal oxides with a chemical formula of A₂B₂O₇ have received much attention from plenty of research groups owing to their outstanding properties, such as electronic, optical, and magnetic properties. Among the abundant element combinations of cations A and B, some theoretically predicted compounds have not successfully been synthesized in experiments, such as In₂Zr₂O₇, In₂V₂O₇, etc. In this study, a novel tetragonal pyrochlore-like In₂V₂O₇ nanopowder has been reported for the first time. In₂O₃ and VO₂ powders mixed through ball milling were reacted to form In₂V₂O₇ by shockwave loading. The recovered sample is investigated to be nanocrystalline In₂V₂O₇ powder through various techniques, such as X-ray diffraction, scanning electron microscopy, X-ray energy spectrum analysis, and transmission electron microscopy. The formed In₂V₂O₇ is indexed as a tetragonal cell with a = b = 0.7417 nm and c = 2.1035 nm. Moreover, the formation mechanism of In₂V₂O₇ through a shock synthesis process is carefully analyzed based on basic laws of shockwave. The experimental results also confirm that a high shock temperature and high shock pressure are the two key factors to synthesize the In₂V₂O₇ nanopowder. Our investigation demonstrates the high potential application of a shock-induced reaction on the synthesis of novel materials, including the preparation of new bimetal oxides.

Crystallographic Insights into the Crystal Structure and Intrinsic Properties of Ca12Al14O33-Type Rb3LiZn2(MoO4)4 Single Crystals

Molybdate oxide materials have attracted considerable academic interest owing to their multifunctional optoelectronic properties and applications. However, to date, studies on the intrinsic properties of multiple molybdates have rarely been implemented. Herein, a prospective triple molybdate crystal, Rb₃LiZn₂(MoO₄)₄, with high crystalline quality was successfully grown using top-seeded solution growth (TSSG) approaches. Intriguingly, it affords a cage-like structure with the I4̅3d space group, analogous to that of Ca₁₂Al₁₄O₃₃ (C12A7). The Rb₃LiZn₂(MoO₄)₄ crystal exhibits excellent thermal stability up to 603 °C, accompanied by a congruent melting nature. Simultaneously, it preserves the optical merits of a large band gap of 4.10 eV and a wide transmission window of 0.29-5.4 μm, which are superior to those of most molybdate crystals. More importantly, Raman spectroscopic measurements demonstrated that the title compound possesses an intense Raman shift located at 925 cm^-1 and narrow line width, facilitating a stimulated Raman laser. In addition, first-principles calculations were also implemented to elucidate the structure-property relationships of Rb₃LiZn₂(MoO₄)₄. These observations provide an empirical platform for intuitively comprehending the underlying properties of multiple molybdates and pave the way for exploiting Raman crystals.

Pressure-Induced Structural Behavior of Orthorhombic Mn3(VO4)2: Raman Spectroscopic and X-ray Diffraction Investigations

The effect of high pressure on the structure of orthorhombic Mn₃(VO₄)₂ is investigated using in situ Raman spectroscopy and X-ray powder diffraction up to high pressures of 26.2 and 23.4 GPa, respectively. The study demonstrates a pressure-induced structural phase transition starting at 10 GPa, with the coexistence of phases in the range of 10-20 GPa. The sluggish first-order phase transition is complete by 20 GPa. Importantly, the new phase could be recovered at ambient conditions. Raman spectra of the recovered new phase indicate increased distortion and as a consequence the lowering of the local symmetry of the VO₄ tetrahedra. This behavior is different from that reported for isostructural compounds Zn₃(VO₄)₂ and Ni₃(VO₄)₂ where both show stable structures, although almost similar anisotropic compression of the unit cell is observed. The transition observed in orthorhombic Mn₃(VO₄)₂ could be due to the internal charge transfer between the cations, which favors the structural transition at lower pressures and the eventual recovery of the new phase even upon pressure release in comparison to other isostructural compounds. The experimental equation of state parameters obtained for orthorhombic Mn₃(VO₄)₂ match excellently with empirically calculated values reported earlier.

Pressure-induced phase transitions and electronic properties of Cd2V2O7

We report a density-functional theory study of the structural and electronic properties of Cd₂V₂O₇ under high-pressure conditions. The calculations have been performed by using first-principles calculations with the CRYSTAL program. The occurrence of two structural phase transitions, at 0.3 and 10.9 GPa, is proposed. The crystal structure of the different high-pressure phases is reported. Interestingly a cubic pyrochlore-type structure is predicted to stabilize under compression. The two phase transitions involve substantial changes in the coordination polyhedra of Cd and V. We have also determined the compressibility and room-temperature equation of state of the three polymorphs of Cd₂V₂O₇. According to our systematic electronic band-structure calculations, under ambient conditions Cd₂V₂O₇ is an indirect wide band-gap material with a band-gap energy of 4.39 eV. In addition, the pressure dependence of the band gap has been determined. In particular, we have found that after the second phase transition the band gap decreases abruptly to a value of 2.56 eV.

Density-functional calculations predict the existence of two structural phase transitions under high-pressure in Cd₂V₂O₇ pyrovanadate. The pressure influence on structural and electronic properties is described.

High-pressure monoclinic-monoclinic transition in fergusonite-type HoNbO4

In this paper we perform a high-pressure (HP) study of fergusonite-type HoNbO₄. Powder x-ray diffraction experiments andab initiodensity-functional theory (DFT) simulations provide evidence of a phase transition at 18.9(1.1) GPa from the monoclinic fergusonite-type structure (space group I2/a) to another monoclinic polymorph described by space group P2₁/c. The phase transition is reversible and the HP structural behavior is different than the one previously observed in related niobates. The HP phase remains stable up to 29 GPa. The observed transition involves a change in the Nb coordination number from 4 to 6, and it is driven by mechanical instabilities. We have determined the pressure dependence of unit-cell parameters of both phases and calculated their room-temperature equation of state. For the fergusonite-phase we have also obtained the isothermal compressibility tensor. In addition to the HP studies, we report ambient-pressure Raman and infrared (IR) spectroscopy measurements. We have been able to identify all the active modes of fergusonite-type HoNbO₄, which have been assigned based upon DFT calculations. These simulations also provide the elastic constants of the different structures and the pressure dependence of the Raman and IR modes of the two phases of HoNbO₄. According toab initiocalculations, the reported phase transition is related to a mechanical instability and a phonon softening.

Density-functional study of pressure-induced phase transitions and electronic properties of Zn2V2O7

We report a study of the high-pressure behavior of the structural and electronic properties of Zn₂V₂O₇ by means of first-principle calculations using the CRYSTAL code. Three different approaches have been used, finding that the Becke-Lee-Yang-Parr functional is the one that best describes Zn₂V₂O₇. The reported calculations contribute to the understanding of previous published experiments. They support the existence of three phase transitions for pressures smaller than 6 GPa. The crystal structure of the different high-pressure phases is reported. We have also made a systematic study of the electronic band-structure, determining the band-gap and its pressure dependence for the different polymorphs. The reported results are compared to previous experimental studies. All the polymorphs of Zn₂V₂O₇ have been found to have a wide band gap, with band-gap energies in the near-ultraviolet region of the electromagnetic spectrum.

Pressure-dependent modifications in the optical and electronic properties of Fe(IO3)3: the role of Fe 3d and I 5p lone–pair electrons

The electronic and transport properties of Fe(IO3)3 have been characterized under compression. A nice correlation of bandgaps of iodates to orbital configuration is proposed giving an explanation for the 2.1 eV bandgap of Fe(IO3)3.

PrVO4 under High Pressure: Effects on Structural, Optical, and Electrical Properties

In the pursuit of a systematic characterization of rare-earth vanadates under compression, in this work we present a multifaceted study of the phase behavior of zircon-type orthovanadate PrVO₄ under high-pressure conditions, up to 24 GPa. We have found that PrVO₄ undergoes a zircon to monazite transition at around 6 GPa, confirming previous results found by Raman experiments. A second transition takes place above 14 GPa, to a BaWO₄-II type structure. The zircon to monazite structural sequence is an irreversible first-order transition, accompanied by a volume collapse of about 9.6%. The monazite phase is thus a metastable polymorph of PrVO₄. The monazite-BaWO₄-II transition is found instead to be reversible and occurs with a similar volume change. Here we report and discuss the axial and bulk compressibility of all phases. We also compare our results with those for other rare-earth orthovanadates. Finally, by means of optical-absorption experiments and resistivity measurements, we determined the effect of pressure on the electronic properties of PrVO₄. We found that the zircon-monazite transition produces a collapse of the band gap and an abrupt decrease in the resistivity. The physical reasons for this behavior are discussed. Density functional theory simulations support our conclusions.

Pressure-Induced Phase Transition in Mn(Ta,Nb)2O6: An Experimental Investigation and First-Principle Study

The high-pressure and high-temperature behaviors of manganotantalite Mn(Ta,Nb)₂O₆ have been investigated by single-crystal X-ray diffraction and Raman spectroscopy combined with diamond anvil cell technique, as well as first-principle calculations. A pressure-induced reversible phase transition of manganotantalite occurs at 9.5 GPa and room temperature, accompanied by a large volume collapse (∼7.0%) and drastic color change from brownish-yellow to red. The space groups of low-pressure (LP) and high-pressure (HP) phases are the same (Pbcn), but the coordination numbers of Mn increase from six to eight and Ta increase from six to seven, respectively. The band gap becomes narrow from 2.37 to 1.59 eV. We determined the P-T phase diagram of manganotantalite with a positive Clapeyron slope of dP/dT = 0.0073 GPa/K. The P-V data were fitted to a second-order Birch-Murnaghan equation of state with B₀ = 149(4) GPa for the LP phase and B₀ = 188(3) GPa for the HP phase. The isothermal Grüneisen parameters were determined to be 0.23∼2.03 of the LP phase and 0.59∼0.86 of the HP phase for Raman modes. High-pressure behaviors of Mn(Ta,Nb)₂O₆ indicate that this kind of material is a potential effective pressure sensor to monitor pressure change or warn pressure abnormality.

High-pressure characterization of multifunctional CrVO4

The structural stability and physical properties of CrVO4under compression were studied by x-ray diffraction, Raman spectroscopy, optical absorption, resistivity measurements, andab initiocalculations up to 10 GPa. High-pressure x-ray diffraction and Raman measurements show that CrVO4undergoes a phase transition from the ambient pressure orthorhombic CrVO4-type structure (Cmcm space group, phase III) to the high-pressure monoclinic CrVO4-V phase, which is proposed to be isomorphic to the wolframite structure. Such a phase transition (CrVO4-type → wolframite), driven by pressure, also was previously observed in indium vanadate. The crystal structure of both phases and the pressure dependence in unit-cell parameters, Raman-active modes, resistivity, and electronic band gap, are reported. Vanadium atoms are sixth-fold coordinated in the wolframite phase, which is related to the collapse in the volume at the phase transition. Besides, we also observed drastic changes in the phonon spectrum, a drop of the band-gap, and a sharp decrease of resistivity. All the observed phenomena are explained with the help of first-principles calculations.

Precise Characterization of the Rich Structural Landscape Induced by Pressure in Multifunctional FeVO4

We have studied the high-pressure behavior of FeVO₄ by means of single-crystal X-ray diffraction (XRD) and density functional theory (DFT) calculations. We have found that the structural sequence of FeVO₄ is different from that previously assumed. In particular, we have discovered a new high-pressure phase at 2.11(4) GPa (FeVO₄-I'), which was not detected by previous powder XRD studies. We have determined that FeVO₄, under compression (at room temperature), first transforms at 2.11(4) GPa from the ambient-pressure triclinic structure (FeVO₄-I) to a second previously unknown triclinic structure (FeVO₄-I'), which experiences a subsequent phase transition at 4.80(4) GPa to a monoclinic structure (FeVO₄-II'), which was also previously detected in powder XRD experiments. Single-crystal XRD has enabled these novel findings as well as an accurate determination of the crystal structure of FeVO₄ polymorphs under high-pressure conditions. The crystal structure of all polymorphs has been accurately solved at all measured pressures. The pressure dependence of the unit-cell parameters and polyhedral coordination have been obtained and are discussed. The room-temperature equation of state and the principal axes of the isothermal compressibility tensor of FeVO₄-I and FeVO₄-I' have also been determined. The structural phase transition observed here between these two triclinic structures at 2.11(4) GPa implies abrupt coordination polyhedra modifications, including coordination number changes. DFT calculations support the conclusions extracted from our experiments.

Investigation on the Luminescence Properties of InMO4 (M = V5+, Nb5+, Ta5+) Crystals Doped with Tb3+ or Yb3+ Rare Earth Ions

We explore the potential of Tb- and Yb-doped InVO4, InTaO4, and InNbO4 for applications as phosphors for light-emitting sources. Doping below 0.2% barely change the crystal structure and Raman spectrum but provide optical excitation and emission properties in the visible and near-infrared (NIR) spectral regions. From optical measurements, the energy of the first/second direct band gaps was determined to be 3.7/4.1 eV in InVO4, 4.7/5.3 in InNbO4, and 5.6/6.1 eV in InTaO4. In the last two cases, these band gaps are larger than the fundamental band gap (being indirect gap materials), while for InVO4, a direct band gap semiconductor, the fundamental band gap is at 3.7 eV. As a consequence, this material shows a strong self-activated photoluminescence centered at 2.2 eV. The other two materials have a weak self-activated signal at 2.2 and 2.9 eV. We provide an explanation for the origin of these signals taking into account the analysis of the polyhedral coordination around the pentavalent cations (V, Nb, and Ta). Finally, the characteristic green (5D4 → 7F J ) and NIR (2F5/2 → 2F7/2) emissions of Tb3+ and Yb3+ have been analyzed and explained.

Pressure Effects on the Optical Properties of NdVO4

We report on optical spectroscopic measurements in pure NdVO4 crystals at pressures up to 12 GPa. The influence of pressure on the fundamental absorption band gap and Nd3+ absorption bands has been correlated with structural changes in the crystal. The experiments indicate that a phase transition takes place between 4.7 and 5.4 GPa. We have also determined the pressure dependence of the band-gap and discussed the behavior of the Nd3+ absorption lines under compression. Important changes in the optical properties of NdVO4 occur at the phase transition, which, according to Raman measurements, corresponds to a zircon to monazite phase change. In particular, in these conditions a collapse of the band gap occurs, changing the color of the crystal. The changes are not reversible. The results are analyzed in comparison with those deriving from previous studies on NdVO4 and related vanadates.