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 CsAlSiO 4 [ABW framework type, with Pc2 1n 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: V 0 = 457.9(4) Å 3, K T0 = 42(1) GPa and K′ = 3.9(3) (with a second-order Birch-Murnaghan Equation of State: V 0 = 458.1(2) Å 3, K T0 = 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: K T0(a) = 244(11) GPa for the a-axis (K(a)′ = 4); K T0(b) = 181(3) GPa for the b-axis (K(b)′ = 4), and K T0(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)/V 0 = 1 + α 0·T + α 1·T 2 = 1 + 3.63(1) × 10 -5·T - 3.8(1) × 10 -9·T 2. The structure reacts, in response to the applied T, by a negative thermal expansion along [1 0 0] (i.e. α 0(a) = -9.97(1) × 10 -6 °C -1), almost no expansion along [0 1 0] (i.e. α 0(b) = 0.36(1) × 10 -6 °C -1) and a pronounced positive expansion along [0 0 1] (i.e. α 0(c) = 47.46(6) × 10 -6 °C -1). The main P/T-induced structure deformation mechanisms, at the atomic level, are discussed.