High-pressure behaviour of Cs2V3O8 fresnoite

Crystal structure of Cs2V3O8 fresnoite (P4bm, Z=2) has been studied using single-crystal X-ray diffraction in a diamond anvil cell to 8.6 GPa at room temperature. Cs2V3O8 undergoes a reversible first-order phase transition at about 4 GPa associated with anomalies in the pressure dependencies of the lattice parameters and unit-cell volume but without any symmetry change. Both structures consist of layers of corner-sharing V-5+O-4 tetrahedra and V-4+O-5 tetragonal pyramids separated by the Cs+ cations located between the layers. At low pressures, the compression has little effect on the polarity of the structure. Above 4 GPa, the pseudosymmetry with respect to the corresponding centrosymmetric space group P4/mbm abruptly increases. The effects of external pressure and of the A(+) cation substitution in the vanadate fresnoites A(2)V(3)O(8) (A(+): K+, Rb+, NH4+, Cs+) are discussed. (C) 2016 Elsevier Inc. All rights reserved.