Stability at high-pressure, elastic behaviour and pressure-induced structural evolution of CsAlSi5O12, a potential host for nuclear waste

The elastic and structural behaviour of the synthetic zeolite CsAlSi5O12 (a = 16.753(4), b = 13.797(3)and c = 5.0235(17) A ° , space group Ama2, Z = 2) were investigated up to 8.5 GPa by in situ single-crystal X-ray diffraction with a diamond anvil cell under hydrostatic conditions. No phase-transition occurs within the P-range investigated. Fitting the volume data with a third-order Birch–Murnaghan equation-of-state gives: V0 = 1,155(4)A°^3, KT0 = 20(1) GPa and K0 = 6.5(7). The ‘‘axial moduli’’were calculated with a third-order ‘‘linearized’’ BM-EoS, substituting the cube of the individual lattice parameter (a^3,b^3, c^3) for the volume. The refined axial-EoS parameters are: a0 = 16.701(44) A° , KT0a = 14(2) GPa (beta_a = 0.024(3)GPa-1), K0a = 6.2(8) for the a-axis; b0 = 13.778(20)A° , KT0b = 21(3) GPa (beta_b = 0.016(2) GPa-1), K0b = 10(2) for the b-axis; c0 = 5.018(7) A°, KT0c = 33(3) GPa (beta_c = 0.010(1) GPa-1), K0c = 3.2(8) for the c-axis (KT0a:KT0b:KT0c = 1:1.50:2.36). The HP-crystal structure evolution was studied on the basis of several structural refinements at different pressures: 0.0001 GPa (with crystal in DAC without any pressure medium), 1.58(3), 1.75(4),1.94(6), 3.25(4), 4.69(5), 7.36(6), 8.45(5) and 0.0001 GPa (after decompression). The main deformation mechanisms at high-pressure are basically driven by tetrahedral tilting, the tetrahedra behaving as rigid-units. A change in the compressional mechanisms was observed at P = 2 GPa. The P-induced structural rearrangement up to 8.5 GPa is completely reversible. The high thermo-elastic stability of CsAlSi5O12, the immobility of Cs at HT/HP-conditions, the preservation of crystallinity at least up to 8.5 GPa and 1,000C in elastic regime and the extremely low leaching rate of Cs from CsAlSi5O12 allow to consider this openframework silicate as functional material potentially usable for fixation and deposition of Cs radioisotopes.