The crystal-structure of CsAlSiO4: a new feldspathoid

The crystal-structure of a new synthetic compound, CsAlSiO4, was investigated by means of single-crystal X-ray diffraction. Crystals of CsAlSiO4 were synthesized hydrothermally. Intensity data were collected with a Xcalibur diffractometer equipped with CCD and monochromated MoKα radiation. 8215 reflections in the range 0<<31° were collected, giving a metrically hexagonal lattice with: a=b=10.870(1)Å, c=8.875(1)Å. However, after Lp corrections, the discrepancy factor among symmetry related reflections was Rint=42%. On the basis of the Klaska and Jarchow (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. The three individuals are rotated 120º with respect to each other around the c-axis, simulating hexagonal symmetry. The geometrical relationship between the hexagonal and orthorhombic lattices is: ao=ah+1/2bh, bo=1/2bh and co=-ch. Reflections with h+k=2n, belonging to each of the three orthorhombic individuals, are fully overlapped. On the basis of the reflection conditions, the assigned space group is Pn21c. The average Rint value, for all reflections over the three domains, is 8%. The refinement was conducted with JANA2000 software (Petricek and Dusek, 2000) starting from the atomic coordinates of Klaska and Jarchow (1975) 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 tetrahedral sheets parallel to (001), in which 6-membered rings of corner-sharing tetrahedra define channels parallel to [001] (where the extra-framework Cs-site lies). Apical oxygens of three neighboring tetrahedra in a ring point upward, whereas apical oxygens of the other three tetrahedra point down. The tetrahedral framework of CsAlSiO4 shows strong analogies with the tridymite framework, but they are not homeotypic. The Si/Al-distribution is completely ordered. The effective coordination number for the Cs-site is 11.