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

Abstract

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 0.0001GPa) and up to 10GPa (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 [100] and [010] at P>4GPa. 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+α0·T+α1·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 [100] (i.e. α0(a)=−9.97(1)×10−6°C−1), almost no expansion along [010] (i.e. α0(b)=0.36(1)×10−6°C−1) and a pronounced positive expansion along [001] (i.e. α0(c)=47.46(6)×10−6°C−1). The main P/T-induced structure deformation mechanisms, at the atomic level, are discussed.