Compressibilities of disordered fluoride pyrochlores NaCdZn2F7 and NaCaMg2F7

Abstract

The compressibilities of disordered pyrochlores NaCaMg2F7 and NaCdZn2F7 (both Fd (3) over barm Z = 8) have been studied with X-ray single-crystal and powder diffraction using diamond anvil cells to 6.5 and 9.0 GPa at room temperature, respectively. The compressibility data are fitted with the Murnaghan equations of state. The zero-pressure bulk modulus B-0 and the unit-cell volume at ambient pressure V-0 (for the fixed first pressure derivative of the bulk modulus B' = 4.00) are equal to 83(2) GPa and 1107.12(1.33) angstrom(3) for NaCdZn2F7 and to 83(5) GPa and 1079.29(2.62) angstrom(3) for NaCaMg2F7. Upon decreasing the unit-cell volume, the positional x parameter of the F(2) atom increases in NaCdZn2F7 but is constant in NaCaMg2F7. In both cases, the (Na,Cd)F-8 and (Na,Ca)F8 cubes become more regular and are softer than the ZnF6 and MgF6 octahedra, respectively. Both materials are structurally stable at least to the respective highest pressures reached in this study. These observations are compared to the high-pressure behavior of oxide pyrochlores. (c) 2007 Elsevier Inc. All rights reserved.