Synthesis and crystal structure of CsAlSiO4: a new open-framework silicate usable as a potential nuclear waste disposal phase.

Cs-bearing minerals are extremely rare. Only a few structures of natural and synthetic Cs-aluminosilicates are known, pollucite (CsAlSi2O6), Cs-4A zeolite, and Cs-bikitaite (CsAlSi5O12). All of them form open-framework structures, like feldspathoids or zeolites. There has been no determination of the structure of CsAlSiO4 in the literature. Cesium aluminosilicate, CsAlSiO4, grew in several hydrothermal experiments aimed at the synthesis of a Cs-mica that was being prepared in search of a suitable mineral phase potentially usable for fixation and deposition of radioactive Cs-isotopes. The sample of CsAlSiO4 presently investigated was synthesized from a stoichiometric proportion of natural kaolinite Bohemia and Cs2O (plus excess water) in Ag-capsules at P=0.1 GPa and T=413-430°C. The crystal structure of CsAlSiO4 was investigated by means of single-crystal X-ray diffraction. The diffraction pattern gave a metrically hexagonal lattice with: a=b=10.870(1), c=8.875(1)Å. However, the discrepancy factor among symmetry related reflections was Rint=42%. On the basis of Klaska and Jarchow’s (1975) experience with RbAlSiO4, we have treated the diffraction pattern as due to a triple twin. Each individual has an orthorhombic lattice with a=9.414(1), b=5.435(1), c=8.875(1)Å, with twinning planes (110) and (130), simulating hexagonal symmetry. The geometrical relationship between the hexagonal and orthorhombic lattices is: ao=ah+1/2bh, bo=1/2bh and co=-ch. On the basis of the reflection conditions, the assigned space group is Pc21n. The refinement was conducted starting from the atomic coordinates for RbAlSiO4. The final least-square cycles were conducted with the anisotropic thermal parameters and the agreement index was 3.04% for 66 parameters and 2531 unique reflections. The structure of this feldspathoid consists of two-dimensional tetrahedral sheets, parallel to (001), in which the corner-sharing tetrahedra form an infinite mesh of six-membered rings (6mRs). Every tetrahedral sheet is connected to a sheet below and to a sheet above, which results in a three-dimensional tetrahedral framework. Si-occupied and Al-occupied tetrahedra alternate along the 6mRs, matching the 1:1 stoichiometry. Apical oxygens of three neighbouring tetrahedra in a ring point upwards whereas apical oxygens of the other three point down. Neighbouring pairs of sheets are held together by sharing triplets of apical oxygens, such that if there is an Al atom in the tetrahedron in the lower sheet, there is an Si atom in the tetrahedron above it and vice versa. The tetrahedral framework of CsAlSiO4 shows strong analogies with that of the tridymite, but they are not homeotypic. In tridymite (or stuffed-tridymite structures), the orientation of tetrahedra belonging to the six-membered rings (…upward-downward-upward…) gives rise to a hexagonal topological symmetry. In contrast, in CsAlSiO4 the aforementioned orientation of tetrahedra (upward-upward-upward-downward-downward-downward) gives rise to a reduction of the topological symmetry to Icmm. The arrangement of pairs of tetrahedral sheets in CsAlSiO4 described above leaves void spaces between them in which large univalent cations reside. The nominal coordination number of Cs is eleven (assuming Cs-Omax=3.65Å). The univalent cations are large, but the void spaces are larger, and the cations lie off-center, away from the apical oxygens. With respect to other Cs-aluminosilicates characterised by zeolite-like structures with large pores, CsAlSiO4 should be able to retain Cs, when immersed in a fluid phase, better than the Cs-bearing zeolites. The small dimension of the nanopores 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 new functional material potentially usable for fixation and deposition of radioactive isotopes of Cs.