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

G. Diego Gatta^1,2,*, N. Rotiroti^1,2, P.F. Zanazzi³, M. Rieder⁴, M. Drabek⁵, Z. Weiss^4, and R. Klaska⁶

¹ Dipartimento di Scienze della Terra, Università degli Studi di Milano, Italy
² CNR-Istituto per la Dinamica dei Processi Ambientali, Milano, Italy
³ Dipartimento di Scienze della Terra, Università degli Studi di Perugia, Italy
⁴ Institute of Materials Chemistry, VSB, Technical University of Ostrava, Czech Republic
⁵ Czech Geological Survey, Prague, Czech Republic
⁶ HeidelbergCement AG, Ennigerloh, Germany

Correspondence: ^* E-mail: diego.gatta{at}unimi.it

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 (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. 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.

Key Words: CsAlSiO₄ • RbAlSiO₄ • ABW framework type • feldspathoid • crystal structure • nuclear waste disposal phase