High-pressure structural study of the scheelite tungstatesCaWO4andSrWO4

Abstract

Angle-dispersive x -ray diffraction (ADXRD) and x -ray absorption near-edge structure (XANES) measurements have been performed on CaWO 4 and SrWO 4 up to pressures of approximately 20 GPa. Both materials display similar behavior in the range of pressures investigated in our experiments. As in the previously reported case of CaWO 4 , under hydrostatic conditions SrWO 4 undergoes a pressure-induced scheelite-to-fergusonite transition around 10 GPa. Our experimental results are compared to those found in the literature and are further supported by ab initio total energy calculations, from which we also predict the instability at larger pressures of the fergusonite phases against an orthorhombic structure with space group Cmca. Finally, a linear relationship between the charge density in the AO 8 polyhedra of ABO 4 scheelite-related structures and their bulk modulus is discussed and used to predict the bulk modulus of other materials, like hafnon.

Synthesis, characterisation and photoluminescence of nanocrystalline calcium tungstate

“…A unit cell of calcium tungstate with I4 1 /a space group [8] was simulated by CaRIne version 3.1 program [12] with its calculated lattice cell. The positions and sizes of Ca, W and O radii are shown in Table 1 [12,13]. For calcium tungstate unit cell (Figure 3), tungsten atoms are coordinated to four oxygen atoms in WO 4 tetrahedrons, and calcium atoms are coordinated to eight oxygen atoms in CaO 8 polyhedrons.…”

Investigation of the La2O3–Nb2O5–WO3 ternary phase diagram: Isolation and crystal structure determination of the original La3NbWO10 material

“…A projection of cations of an unit cell along the c axis enables the cationic defaults to be observed (Figure 5). For a two-metal compound, the Fluorite formula is transformed into ABX 4 , such as Scheelite tungstates CaWO 4 and SrWO 4 [29], which is illustrated in dashed cell. At z = 0.5c and z = c, the cations are nearly distributed over the expected positions of the Fluorite cell.…”

High pressure polymorphs and amorphization of upconversion host material NaY(WO4)2

“…The pressure dependent volume change is given in Figure 3 Unlike a previous study on CaWO 4 , we found a volume discontinuity near 10 GPa, where the T-M1 phase transition occurs. 26 For the M1-M2 phase transition, no volume discontinuity is observed. The pure M2 phase's volume is a little bit bigger than the pure M1 phase at the same pressure.…”

“…These values are very similar to those of CaWO 4 . 26 Upon compression, all diffraction peaks of the tetragonal structure shift to higher angles, indicating that both the a and c axes become shorter under pressure. Some peaks shift faster than others, which suggests that the compressibility of the a and c axes differs from each other.…”

“…A similar transition sequence from the scheelite phase to the M-fergusonite phase has also been observed in CaWO 4 , SrWO 4 , and YLiF 4 . 26,27 The diffraction peaks of the M-fergusonite-type phase shift to higher angles upon compression. At higher pressure (above 27.5 GPa), the peaks broaden and the intensity decreases as well.…”