J. López-Solano

Determination of the high-pressure crystal structure of Ba W O 4 and Pb W O 4

We report the results of both angle-dispersive x-ray diffraction and x-ray absorption near-edge structure studies in BaWO{sub 4} and PbWO{sub 4} at pressures of up to 56 GPa and 24 GPa, respectively. BaWO{sub 4} is found to undergo a pressure-driven phase transition at 7.1 GPa from the tetragonal scheelite structure (which is stable under normal conditions) to the monoclinic fergusonite structure whereas the same transition takes place in PbWO{sub 4} at 9 GPa. We observe a second transition to another monoclinic structure which we identify as that of the isostructural phases BaWO{sub 4}-II and PbWO{sub 4}-III (space group P2{sub 1}/n). We have also performed ab initio total-energy calculations which support the stability of this structure at high pressures in both compounds. The theoretical calculations further find that upon increase of pressure the scheelite phases become locally unstable and transform displacively into the fergusonite structure. The fergusonite structure is, however, metastable and can only occur if the transition to the P2{sub 1}/n phases were kinetically inhibited. Our experiments in BaWO{sub 4} indicate that it becomes amorphous beyond 47 GPa.

A combined high-pressure experimental and theoretical study of the electronic band-structure of scheelite-type AWO4 (A = Ca, Sr, Ba, Pb) compounds

The optical-absorption edge of single crystals of CaWO4, SrWO4, BaWO4, and PbWO4 has been measured under high pressure up to ∼20 GPa at room temperature. From these measurements, we have obtained the evolution of the band-gap energy with pressure. We found a low-pressure range (up to 7–10 GPa) where alkaline-earth tungstates present a very small Eg pressure dependence (− 2.1 < dEg/dP < 8.9 meV/GPa). In contrast, in the same pressure range, PbWO4 has a pressure coefficient of − 62 meV/GPa. The high-pressure range is characterized in the four compounds by an abrupt decrease of Eg followed by changes in dEg/dP. The band-gap collapse is larger than 1.2 eV in BaWO4. We also calculated the electronic-band structures and their pressure evolution. The calculations allow us to interpret experiments considering the different electronic configurations of divalent metals. Changes in the pressure evolution of Eg are correlated with the occurrence of pressure-induced phase transitions. The band structures for the low- ...

High-pressure optical and vibrational properties of CdGa2Se4: Order-disorder processes in adamantine compounds

High-pressure optical absorption and Raman scattering measurements have been performed in defect chalcopyrite (DC) CdGa2Se4 up to 22 GPa during two pressure cycles to investigate the pressure-induced order-disorder phase transitions taking place in this ordered-vacancy compound. Our measurements reveal that on decreasing pressure from 22 GPa, the sample does not revert to the initial phase but likely to a disordered zinc blende (DZ) structure the direct bandgap and Raman-active modes of which have been measured during a second upstroke. Our measurements have been complemented with electronic structure and lattice dynamical ab initio calculations. Lattice dynamical calculations have helped us to discuss and assign the symmetries of the Raman modes of the DC phase. Additionally, our electronic band structure calculations have helped us in discussing the order-disorder effects taking place above 6–8 GPa during the first upstroke.

High-pressure study of the structural and elastic properties of defect-chalcopyrite HgGa2Se4

In this work, we focus on the study of the structural and elastic properties of mercury digallium selenide (HgGa2Se4) which belongs to the family of AB2X4 ordered-vacancy compounds with tetragonal defect chalcopyrite structure. We have carried out high-pressure x-ray diffraction measurements up to 13.2 GPa. Our measurements have been complemented and compared with total-energy ab initio calculations. The equation of state and the axial compressibilities for the low-pressure phase of HgGa2Se4 have been experimentally and theoretically determined and compared to other related ordered-vacancy compounds. The theoretical cation-anion and vacancy-anion distances in HgGa2Se4 have been determined. The internal distance compressibility in HgGa2Se4 has been compared with those that occur in binary HgSe and e−GaSe compounds. It has been found that the Hg-Se and Ga-Se bonds behave in a similar way in the three compounds. It has also been found that bulk compressibility of the compounds decreases following the sequenc...

Equation of state of zircon- and scheelite-type dysprosium orthovanadates: a combined experimental and theoretical study

Dysprosium orthovanadate, DyVO4, belongs to a family of zircon-type orthovanadates showing a phase transition to scheelite-type structures at moderate pressures below 10 GPa. In the present study, the equations of state (EOSs) for both these phases were determined for the first time using high-pressure x-ray diffraction experiments and ab initio calculations based on the density functional theory. Structural parameters for scheelite-type DyVO4 were calculated from x-ray powder diffraction data as well. The high-pressure experiments were performed under pseudo-hydrostatic conditions at pressures up to 8.44 GPa and 5.5 GPa for the stable zircon-type and metastable (quenched) scheelite-type samples, respectively. Assuming as a compression model the Birch-Murnaghan EOS, we obtained the EOS parameters for both phases. The experimental bulk moduli (K0) for zircon-type and scheelite-type DyVO4 are 118(4) GPa and 153(6) GPa, respectively. Theoretical equations of state were determined by ab initio calculations using the PBE exchange-correlation energy functional of Perdew, Burke, and Ernzerhof. These calculations provide K0 values of 126.1 GPa and 142.9 GPa for zircon-type and scheelite-type DyVO4, respectively. The reliability of the present experimental and theoretical results is supported by (i) the consistency between the values yielded by the two methods (the discrepancy in K0 is as low as about 7% for each of the studied polymorphs) and (ii) their similarity to results obtained under similar compression conditions (hydrostatic or pseudo-hydrostatic) for other rare-earth orthovanadates, such as YVO4 and TbVO4.

Negative pressures in CaWO4 nanocrystals

Tetragonal scheelite-type CaWO4 nanocrystals recently prepared by a hydrothermal method show an enhancement of its structural symmetry with the decrease in nanocrystal size. The analysis of the volume dependence of the structural parameters in CaWO4 nanocrystals with the help of ab initio total-energy calculations shows that the enhancement of the symmetry in the scheelite-type nanocrystals is a consequence of the negative pressure exerted on the nanocrystals; i.e., the nanocrystals are under tension. Besides, the behavior of the structural parameters in CaWO4 nanocrystals for sizes below 10 nm suggests an onset of a scheelite-to-zircon phase transformation in good agreement with the predictions from our ab initio calculations. CaWO4 nanocrystals exhibit a reconstructive-type mechanism for the scheelite-to-zircon phase transition that seems to follow the tetragonal path that links both structures. This result is in contrast with the mechanism recently proposed for this transition in bulk ZrSiO4 where the ...

A combined study of the equation of state of monazite-type lanthanum orthovanadate using in situ high-pressure diffraction and ab initio calculations.

Lanthanum orthovanadate (LaVO4) is the only stable monazite-type rare-earth orthovanadate. In the present paper the equation of state of LaVO4 is studied using in situ high-pressure powder diffraction at room temperature, and ab initio calculations within the framework of the density functional theory. The parameters of a second-order Birch-Murnaghan equation of state, i.e. those fitted to the experimental and theoretical data, are found to be in perfect agreement - in particular, the bulk moduli are almost identical, with values of 106 (1) and 105.8 (5) GPa, respectively. In agreement with recent reported experimental data, the compression is shown to be anisotropic. Its nature is comparable to that of some other monazite-type compounds. The softest compression direction is determined.

Experimental and theoretical study of α-Eu2(MoO4)3 under compression.

The compression process in the α-phase of europium trimolybdate was revised employing several experimental techniques. X-ray diffraction (using synchrotron and laboratory radiation sources), Raman scattering and photoluminescence experiments were performed up to a maximum pressure of 21 GPa. In addition, the crystal structure and Raman mode frequencies have been studied by means of first-principles density functional based methods. Results suggest that the compression process of α-Eu2(MoO4)3 can be described by three stages. Below 8 GPa, the α-phase suffers an isotropic contraction of the crystal structure. Between 8 and 12 GPa, the compound undergoes an anisotropic compression due to distortion and rotation of the MoO4 tetrahedra. At pressures above 12 GPa, the amorphization process starts without any previous occurrence of a crystalline-crystalline phase transition in the whole range of pressure. This behavior clearly differs from the process of compression and amorphization in trimolybdates with [Formula: see text]-phase and tritungstates with α-phase.

Ab initio study of high-pressure structural properties of the LuVO4 and ScVO4 zircon-type orthovanadates

We use ab initio theoretical methods to study the relative stability under pressure of the zircon, scheelite and fergusonite structures in the LuVO4 and ScVO4 orthovanadates. In LuVO4, our study confirms the zircon→scheelite→fergusonite sequence of stable phases proposed in previous experimental works. In ScVO4, we find a zircon-to-scheelite phase transition in agreement with experiments. At higher pressures, our calculations show that the scheelite phase becomes unstable with respect to the fergusonite phase, although no second phase transition has been observed in experiments.

High-pressure theoretical and experimental study of HgWO4

HgWO4 at ambient pressure is characterized using a combination of ab initio calculations, X-ray diffraction and Raman scattering measurements. The effect of low pressure and temperature on the structural stability is analysed. Extending our ab initio study to the range of higher pressures, a sequence of stable phases up to 30 GPa is proposed.