Synthesis and characterisation of unusual sodalites

Mead, Philip John (1996) Synthesis and characterisation of unusual sodalites. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

Sodalities of the compositions, M₈[ABO₄]₆.X₂, (M = Li, Na, K; A = Al, Ga; B = Si, Ge; X = HCO₂, DCO₂, H₃C₂O₂, NO₂, BrO₃, ClO₃) have been prepared and characterised. Sodium alumino-silicate frameworks were typically synthesised from aqueous solution at 120^oC and framework substituted sodalities, with A = Ga or B = Ge, using hydrothermal methods at 180^oC. Lithium and potassium containing derivatives were obtained using ion exchange in aqueous solution at 80^oC or from nitrate melts at 270-370^oC. Thermogravimetric analyses have shown that the stabilising nature of the sodalite framework allows the intra-cage preparation of chlorite anions by partial decomposition of chlorate at 450^oC.

All sodalites crystallised in the cubic space group, P43n, consistent with partial collapse from the ideal fully expanded structure and strict framework cation ordering. Detailed structural determinations of the anion geometry and framework structure were performed from Rietveld profile of powder X-ray, time-of-flight neutron (4 K or 300 K) or constant wavelength neutron diffraction data. The anions were refined on highly disordered sites around the cage centre co-ordinated to sodium cations which are situated in the cage six rings. The central anions in the formate and nitrite sodalites were found to occupy six-fold disordered geometries in the alumino-silicates and four-fold disordered geometries in the alumino-germanates and gallo-germanates. The structure refinement of the sodalites containing entrapped halate anions showed that the halogen is apparently directed towards a cation producing an unrealistically short M-XO₃ distance; however a new structure has been proposed where one cation moves through the sodalite six ring increasing the M-X distance.

Bicchulites of the compositions Ca₈[Al₂SiO₆]₄.(OH)₈ and Ca₈[Ga₂SiO₆]₄. (OH)₈ were prepared using hydrothermal methods at 540^oC and 1 kbar. Rietveld profile refinement was performed from powder neutron diffraction data in the space group I 4 3m consistent with a disordered framework but apparently contradicting Löwenstein's rule. Structural refinement of the non-framework species showed that the sodalite cage contains two inter-penetrating tetrahedra of cations and anions respectively; the hydroxyl hydrogen was found to be three-fold disordered and weakly hydrogen bonded to one of the three close framework six ring oxygen atoms.