Beiträge zur Kristallchemie und zum thermischen Verhalten von wasserfreien Phosphaten, XXXVII [1]. Synthese, Kristallstruktur und kernresonanzspektroskopische Untersuchung von In2Ti6(PO4)6[Si2O(PO4)6] – Eine Hybride aus den NASICON und M4[Si2O(PO4)6] Strukturtypen / Contributions on Crystal Chemistry and Thermal Behaviour of Anhydrous Phospates, XXXVII [1]. Synthesis, Crystal Structure and Nuclear Magnetic Resonance Investigation of In2Ti6(PO4)6- [Si2O(PO4)6] – A Hybride Built from Layers with NASICON and M4[Si2O(PO4)6] Structures

Abstract

In2Ti6(PO4)6[Si2O(PO4)6] has been obtained by heating (1100 °C) stoichiometric amounts of In2O3, SiP2O7, TiP2O7, and TiO2 in air. Colourless crystals of the phosphate-silicophosphate suitable for a single crystal structure investigation have been grown by chemical vapour transport (1000°C → 900°C, mixture of 70 mg PtCl2 and 3.5 mg TiP as transport agent). In2Ti6(PO4)6[Si2O(PO4)6] adopts its own structure type (R3̅ (No. 148), Z = 3, a = 8.4380(10) Å , c = 44.295(1) Å , 1809 independent reflections, 109 variables, R1 = 0.044, wR2 = 0.112). The crystal structure represents a hybride built up from alternating layers (⟂ to the c-axis) of the NASICON structure-type and those showing the structure of silicophosphates M4[Si2O(PO4)6]. Isolated heteropolyanions [Si2O(PO4)6]12− and double-octahedra [InIIITiIVO9] occur as coordination polyhedra besides isolated octahedra [TiIVO6] and tetrahedral phosphate groups. The results of 29Si and 31P-MAS-NMR studies are in agreement with one crystallographically independent site for silicon and two sites for phosphorus. The phosphorus resonances can be related to the two sites by 2-dimensional cross-polarisation experiments, by the anisotropies of their chemical shifts, and by the observed line widths. All criteria lead to the same assignment. Substitution of In3+ by several trivalent transition metal ions leads to phosphate-silicophosphates M2Ti6(PO4)6[Si2O(PO4)6] (M = Ti3+, V3+, Cr3+, Fe3+)