Crystal structure of RbBaVO4 and high-pressure modification of KCaVO4

The crystal structures of KCaVO4 and RbBaVO4, synthesized at high-pressure/high-temperature and by a conventional solid-state reaction, respectively, were determined using X-ray powder diffraction data. These compounds were found to have the β-K2SO4 structure type (space group Pnma, Z = 4) with parameters a = 7.2628(5) Å, b = 5.7258(4) Å, c = 9.6854(7) Å (KCaVO4), and a = 7.8887(1) Å, b = 5.9589(1) Å, c = 10.3958(2) Å (RbBaVO4). The unit cell volume of KCaVO4, 402.77(5) Å3, is significantly lower than for the low-temperature modification reported previously, 436.2 Å3. The difference can be explained by a pressure-induced phase transition to a more dense state resulting from the rotation of tetrahedra and the exchange of oxygen atoms from the first and second coordination spheres for potassium and calcium atoms.