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

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 degrees 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 Pc2(1)n space group and lattice parameters: a = 9.414(1), b = 5.435(1), and c = 8.875(1) angstrom. Because of the orthohexagonal relation between b and a (a approximate to b root 3), 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 (L-T). The transformation from the lattice of the individual (L-ind) to L-T is given by: a(T) = a(ind)- b(ind), b(T) = 2b(ind), and C-T=C-ind. The refinement was initiated using the previously published atomic coordinates for RbAlSiO4. The final least-square cycles were conducted with anisotropic displacement parameters. R-1 = 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.