Synthesis and crystal structure of the feldspathoid CsAlSiO4 : an open-framework silicate and potential nuclear waste disposal phase

Abstract

Crystalline CsAlSiO4 was synthesized from a stoichiometric mixture of Al2O3 + SiO2 + Cs2O (plus excess water) in Ag-capsules at hydrostatic pressure of 0.1 GPa and temperature of 695 °C. The duration of synthesis was 46 h. The crystal structure of CsAlSiO4 was investigated by single-crystal X-ray diffraction. The structure is orthorhombic with Pc21n space group and lattice parameters: a = 9.414(1), b = 5.435(1), and c = 8.875(1) Å. Because of the orthohexagonal relation between b and a (a b3), within the standard uncertainty on the lattice parameters, a hexagonal superlattice exists, which is responsible for twinning. The crystals are twinned by reflection, with twin planes (110) and (310): twinning in both cases is by reticular merohedry with twin index 2 and hexagonal twin lattice (LT). The transformation from the lattice of the individual (Lind) to LT is given by: aT = aind – bind, bT = 2bind, and cT = cind. The refinement was initiated using the previously published atomic coordinates for RbAlSiO4. The final least-square cycles were conducted with anisotropic displacement parameters. R1 = 3.04% for 66 parameters and 2531 unique reflections. For a more reliable crystallographic comparison the crystal structure of RbAlSiO4 is reinvestigated here adopting the same data collection and least-squares refinement strategy as for CsAlSiO4.

The crystal structure of the CsAlSiO4 feldspathoid is built on an ABW framework type, showing a fully ordered Si/Al-distribution in the tetrahedral framework. The only extra-framework site is occupied by Cs, lying off-center in the 8mR-channels. CsAlSiO4 is more likely to retain Cs when immersed in a fluid phase, relative to several other Cs-bearing zeolites. The topological configuration of the Cs-polyhedron (and its bonding environment), the small dimension of the pores and the high flexibility of the ABW framework type would imply a better thermal and elastic stability of CsAlSiO4 than those of the zeolitic Cs-aluminosilicates. In this light, CsAlSiO4 can be considered as a functional material potentially usable for fixation and deposition of radioactive isotopes of Cs and can also be considered as a potential solid host for a 137Cs -radiation source to be used in sterilization applications.