Pressure-induced structural evolution and elastic behaviour of Na6Cs2Ga6Ge6O24•Ge(OH)6 variant of cancrinite: A synchrotron powder diffraction study

The elastic behaviour and the pressure (P) induced structural evolution of Na6Cs2Ga6Ge6O24 center dot Ge(OH)(6), a synthetic compound isotypic with cancrinite (CAN topology), have been investigated up to 5.01(5) GPa by means of in situ X-ray synchrotron powder diffraction with a diamond anvil cell and using a nominally penetrating hydrous P-transmitting medium (methanol:ethanol: water = 16:3:1). No evidence of phase-transition was observed within the P-range investigated. The P-V data were fitted with a Murnaghan equation-of-state (M-EoS). The elastic parameters obtained, using the data weighted by the uncertainties in P-V, are: V-0 = 757.16(7)angstrom(3), K-T0 = 36(2) GPa [K-T0 = 1/beta = -V-0(partial derivative P/partial derivative V)(p-0), where beta is the volume compressibility coefficient] and K' = (partial derivative K-T0/partial derivative P) = 9(1). The elastic behaviour along the a and c axis was described with a "linearised" M-EoS. The refined parameters of the linearised M-EoS are: a(0) = 13.0314(4) A, Kr-0(a) = 40(2) [beta(j) = -1/(3K(T0)(j)) = (1/l(0j))(partial derivative l(j)/partial derivative P), where beta(j) is the axial compressibility coefficient] and K'(a) = 11(1) for the a-axis: c(0) = 5.1485(4) angstrom, K-T0(c) = 31(1) GPa and K'(c) = 7.4(8) for the c-axis. The elastic anisotropy of Na6Cs2Ga6Ge6O24 center dot Ge(OH)(6) is only marginal, being K-T0(a): K-T0(c)=1.29:1. The configuration of the extra-framework content is basically maintained at high-pressure, without any major changes. The Ge(OH)(6)-polyhedron is strongly compressed in response to the applied pressure, but there is no evidence of further deformation. On the other hand the Na(O3OH3)-polyhedron and the large Cs-polyhedron appear to be compressed in a way to lessen the deformation with increasing pressures. Despite the use of a hydrous P-transmitting fluid for the HP-experiment and the large "free-diameters" of the channels in CAN framework, no evidence of superhydration effect was observed within the P-range investigated. (C) 2008 Elsevier Inc. All rights reserved.