Phase stability and thermo-elastic behavior of CsAlSiO4 (ABW): A potential nuclear waste disposal material

The thermo-elastic behavior and the P/T-induced structure evolution of a synthetic CsAlSiO4 [ABW framework type, with Pc21n space group and lattice parameters: a = 9.414(1), b = 5.435(1), c = 8.875(1) Å at room conditions] have been investigated up to 1000 °C (at 0.0001 GPa) and up to 10 GPa (at 20 °C) by means of in-situ synchrotron powder diffraction. No phase transition has been observed within the temperature- and pressure-range investigated. P-V data were fitted with a third-order Birch-Murnaghan Equation of State (BM-EoS), giving: V0 = 457.9(4) Å3, KT0 = 42(1) GPa and K? = 3.9(3) (with a second-order Birch-Murnaghan Equation of State: V0 = 458.1(2) Å3, KT0 = 41.3(3) GPa). The evolution of the "Eulerian finite strain" vs. "normalized stress" yields Fe(0) = 41.9(5)(1) GPa as intercept values, with an almost horizontal slope of the regression line. The evolution of the lattice parameters with pressure shows a remarkably anisotropic compressional pattern, along with subtle change in the axial elastic behavior along [1 0 0] and [0 1 0] at P > 4 GPa. The elastic parameters calculated with a "linearized" BM-EoS are: KT0(a) = 244(11) GPa for the a-axis (K(a)? = 4); KT0(b) = 181(3) GPa for the b-axis (K(b)? = 4), and KT0(c) = 14.5(5) GPa and K(c)? = 2.6(1) for the c-axis. The volume thermal expansion with T was described by the polynomial function: V(T)/V0 = 1 + ?0·T + ?1·T2 = 1 + 3.63(1) × 10-5·T - 3.8(1) × 10-9·T2. The structure reacts, in response to the applied T, by a negative thermal expansion along [1 0 0] (i.e. ?0(a) = -9.97(1) × 10-6 °C-1), almost no expansion along [0 1 0] (i.e. ?0(b) = 0.36(1) × 10-6 °C-1) and a pronounced positive expansion along [0 0 1] (i.e. ?0(c) = 47.46(6) × 10-6 °C-1). The main P/T-induced structure deformation mechanisms, at the atomic level, are discussed.