STRUCTURAL RELATIONS AND TETRAHEDRAL ORDERING PATTERN OF SYNTHETIC ORTHORHOMBIC CS2CDSI5O12 LEUCITE - A COMBINED SYNCHROTRON X-RAY-POWDER DIFFRACTION AND MULTINUCLEAR MAS NMR-STUDY

A previously unknown leucite-related structure has been determined for synthetic Cs2CdSi5O12. NMR spectroscopy shows that there are five distinct tetrahedral sites (T-sites) occupied by Si and one T-site occupied by Cd in the framework structure, while analysis of the synchrotron X-ray powder diffraction pattern establishes that this material is orthorhombic, Pbca [R(I) = 13.1%, R(wp) = 16.1%, R(exp) = 13.1%; eight formula units per unit cell; unit-cell parameters a = 13.6714 (1), b = 13.8240 (1), c = 13.8939 (1) Angstrom, V = 2625.83 (6) Angstrom(3)]. Tetrahedral cation ordering rates for Si and Cd are sufficiently high for both hydrothermally and dry-synthesized samples to be fully ordered. The symmetry relations between leucites with P2(1)/c and Pbca structures are discussed and it is shown that such materials are related by a displacive phase transition, in which the number of T-sites is reduced from 12 in P2(1)/c to 6 in Pbca. The Si-29 MAS NMR data are, at present, less useful than the X-ray results for providing absolute T-O distances and T-O-T bond angles.