High-pressure behaviour of Cs2V3O8 fresnoite

Abstract Crystal structure of Cs 2 V 3 O 8 fresnoite ( P 4 bm , Z=2) has been studied using single-crystal X-ray diffraction in a diamond anvil cell to 8.6 GPa at room temperature. Cs 2 V 3 O 8 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 P 4/ 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 + , NH 4 + , Cs + ) are discussed.