Elastic behavior and phase stability of pollucite, a potential host for nuclear waste

Abstract

The elastic behavior and the phase stability of natural pollucite, (Cs, Na) 16 Al 16 Si 32 O 96 · n H 2 O, were investigated at hydrostatic pressure by in situ single-crystal X-ray diffraction with a diamond-anvil cell. Pollucite experiences a P -induced phase transition, not previously reported in the literature, at P = 0.66 ± 0.12 GPa from cubic ( Ia 3 d ) to triclinic symmetry ( P 1). The phase transition is completely reversible and without any appreciable hysteresis effect. No further phase transition has been observed up to 9 GPa. Fitting the pressure-volume data of the low-pressure cubic polymorph with a second-order Birch-Murnaghan Equation-of-State (BM-EoS), we obtain V 0 = 2558.3(4) A 3 , K T0 = 41(2) GPa, and K T ′ = 4 (fixed). For the high-pressure triclinic polymorph, a third-order BM-EoS fit gives V 0 = 2577.5(40) A 3 , K T0 = 25.1(9) GPa, and K T ′ = 6.5(4). The axial bulk moduli of the high-pressure triclinic polymorph were calculated with a third-order “linearized” BM-EoS. The EoS parameters are a 0 = 13.699(12) A, K T0 ( a ) = 25.5(17) GPa, and K T ′ ( a ) = 6.8(6) for the a axis; b 0 = 13.728(12) A, K T0 ( b ) = 23.2(15)GPa, and K T ′ ( b ) = 7.7(7) for the b axis; c 0 = 13.710(7) A, K T0 ( c ) = 25.2(10) GPa, and K T ′ ( c ) = 6.8(4) for the c axis [ K T0 ( a ): K T0 ( b ): K T0 ( c ) = 1.10:1:1.09]. Brillouin light-scattering was used to investigate the single-crystal elastic properties of pollucite at ambient conditions. The aggregate adiabatic bulk modulus ( K s ) and shear modulus ( G ), calculated using the Voigt-Reuss-Hill averaging procedures, are K s = 52.1(10) GPa and G = 31.5(6) GPa. The elastic response of pollucite and other isotypic materials (e.g., analcime, leucite, and wairakite) is compared. The high thermo-elastic stability of pollucite, reflected by the preservation of crystallinity at least up to 9 GPa (at room T ) and 1470 K (at room P ) in elastic regime, the large amount of Cs hosted in this material (Cs 2 O ~ 30 wt%), the immobility of Cs at high-temperature and high-pressure conditions, and the extremely low leaching rate of Cs, make of this open-framework silicate a functional material with potential use for fixation and deposition of Cs radioisotopes in high-level nuclear waste.