Determination of the high-pressure crystal structure ofBaWO4andPbW

Abstract

Abstract:We report the results of both angle-dispersive x -ray diffraction and x -ray absorption near-edge structure studies in BaWO 4 and PbWO 4 at pressures of up to 56 GPa and 24 GPa, respectively. BaWO 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 4 at 9 GPa. We observe a second transition to another monoclinic structure which we identify as that of the isostructural phases BaWO 4 -II and PbWO 4 -III (space group P2 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 1 /n phases were kinetically inhibited. Our experiments in BaWO 4 indicate that it becomes amorphous beyond 47 GPa.

First-order character of the displacive structural transition in BaWO 4

“…[24−27] Moreover, BaWO 4 was also suggested to experience this transition at about 7 GPa. [9,10,28] In the light of the geometrical relationship of the unit cells between the tetragonal scheelite structure (I4 1 /a) and the monoclinic fergusonite structure (I2/a), the fergusonite structure is only a slightly distorted scheelite structure with the cations deviating a little from the special Wyckoff positions and the anions dividing into two kinds of Wyckoff positions accompanied by fractional coordinate displacement, thereby inducing a lower symmetry. [14] This kind of group-subgroup relation indicates a ferroelastic characteristic phase transition.…”

“…It accords generally with K 0 = 57 GPa as K ′ 0 = 3.5 [9] and K 0 = 52(5) GPa as K ′ 0 = 5(1). [10] Intensity/arb. untis In contrast, Errandonea et al [10] reported on completely different evolutions of lattice constant and volume with pressure as scheelite-structured BaWO 4 transforms into the fergusonite structrure.…”

“…[10] Intensity/arb. untis In contrast, Errandonea et al [10] reported on completely different evolutions of lattice constant and volume with pressure as scheelite-structured BaWO 4 transforms into the fergusonite structrure. Their results show that the a s axis (= b s axis) splits continuously to form an increasing a f axis and decreasing c f axis with pressure increasing.…”

“…[8] At pressures up to about 7 GPa, BaWO 4 modified its tetragonal scheelite-type structure into a monoclinic fergusonitetype structure (space group: I2/a, No. 15, Z = 4) [9,10] and cut off the negative compression of the lowest Raman modes. [11,12] The detailed description of phase transition showed a continuous change in the axial parameters and volume with pressure, [10] despite the fact that early study showed a discontinuous fact.…”

“…15, Z = 4) [9,10] and cut off the negative compression of the lowest Raman modes. [11,12] The detailed description of phase transition showed a continuous change in the axial parameters and volume with pressure, [10] despite the fact that early study showed a discontinuous fact. [9] The mechanism of phase transition was evidenced further by the analysis of spontaneous strain and the soft mode theory, and a second-order ferroelastic nature was concluded.…”