Lattice dynamics study of scheelite tungstates under high pressure I.BaWO4

Effect of pressure onLa2(WO4)3with a modulated scheelite-type structure

“…In particular, we must note that there are three groups of bands with negative pressure coefficient: the lowest-frequency lattice modes; a group of lattice modes between 165 and 190 cm −1 ; and the group of the highest bending modes near 540 cm −1 . We think that this last group could correspond to the highest bending modes of the C2/c structure, because similar bands above 400 cm −1 were observed for tungstates of the ABO 4 family with a monoclinic structure [37,38].…”

“…However, besides taking into account the crossing of enthalpy curves which provide the theoretical coexistence pressures for pressure-driven phase transitions, it is also important to realize that the existence of kinetic barriers does hinder (quite severely in many cases) a massively firstorder solid-solid phase transition, which involves large atomic rearrangements. This is the case, for example, of the BaWO 4 and PbWO 4 ternary oxides, where a barrierless second-order scheelite-to-fergusonite transition takes place instead of the ab initio predicted transition from the scheelite-type phase to the BaWO 4 -II-type (PbWO 4 -III-type) phase, the latter structure being only observed in x-ray experiments performed at rather higher pressures [37,38,42]. Although the Bi 2 (MoO 4 ) 3 -type phase is more stable than the α phase at pressures above 3.8 GPa, both structures have a completely different ordering of cations and vacancies [3], and thus the α-to-Bi 2 (MO 4 ) 3 -type transition should be kinetically hindered and rather unlikely at the predicted coexistence pressure.…”

Raman scattering study of bulk and nanocrystalline PbMoO4 at high pressures

“…However, it is noteworthy that the Raman spectra of the high-pressure phase of bulk PbMoO 4 are characterized by a gap between 500 and 650 cm À1 without first-order Raman bands. This feature has been already observed in the fergusonite phase of tungstates 7,9 and, together with the similar phase transition arrows indicate the first-order (secondorder) Raman modes of the fergusonite phase. Black (red) asterisks correspond to first-order (second-order) Raman modes of the remaining scheelite phase.…”

“…7,8 These phase transitions are common to AWO 4 compounds with A ¼ Ca, Sr, and Ba. [8][9][10] Since AWO 4 and AMoO 4 compounds are very similar from crystal chemical considerations, 6 we expect that PbMoO 4 undergoes pressure-induced phase transitions similar to those of PbWO 4 and related to those that have been already found in CaMoO 4 , SrMoO 4 , CdMoO 4 , EuMoO 4 , and BaMoO 4 . [11][12][13][14][15][16][17] Wulfenite was studied at high pressures by Raman scattering up to 3.5 GPa in the late 1970s by Ganguly and Nicol using NaCl as pressure-transmitting medium, and no phase transition was observed.…”

“…For the study of the dynamical properties, we perform the calculations by the direct method in order to obtain the dynamical matrix and the phonons at the C point, as described in Ref. 9. In this study we only present DFT studies for PbMoO 4 bulk material.…”