Synthesis and characterisation of some novel sodalite phases

Brenchley, Matthew Edward (1994) Synthesis and characterisation of some novel sodalite phases. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

Aluminate sodalites of general composition M₈[A10₂]₁₂.X₂, where M = Ca²⁺, Sr²⁺, Cd²⁺; and X = SO₄^2-, SeO₃^2-, WO₄^2-, MoO₄^2-, CrO₄^2-, have been prepared by a solid state sintering method at 1000-1200 ^oC. The sulphate and selenite bearing sodalites have been reduced to 100% H₂ at 600-800 ^oC to yield a new range of S^2- and Se^2- bearing sodalites. Sodalites crystallising in the simple cubic space group 143m were structurally characterised by Rietveld refinement of powder X-ray or time-of-flight neutron diffraction data. The aluminate framework is capable of incorporating this variety of different sized metals and anions by a partial collapse of the framework. This is achieved by cooperative rotations of the A10₄ tetrahedra and by changing the A1-O-A1 angle and deformation of the A10₄ tetrahedra. For sodalites containing XO_n^2-_,the anions were found to have their oxygens disordered over twelve one-third occupied sites.

Vibrational spectra of the sodalities showed smooth transitions of the frequencies of the framework modes as a function of both cell parameter and A1-O-A1 angle. The observed bands were related to the complex vibrational modes giving rise to them. Modes associated with the anions were found to be very similar in frequency to the free ions. ²⁷A1 MAS NMR spectroscopy showed a linear relationship between the isotropic chemical shift and the A1-O-A1 angle. The extent of A10₄ tetrahedral distortion as measured by structure refinement is directly related to the quadrupolar coupling constant. Optical spectroscopy of the S^2-bearing sodalites showed that each M₄X cluster is electronically isolated by the framework and undergoes a charge transfer process.

Alumino-silicate sodalites, of general composition M₈[A1SiO₄]_6.X₂, were synthesised by hydrothermal methods with the incorporation of NaC10₄ and NaMnO₄. These sodium sodalites were ion exchanged with Li⁺, K⁺ and Ag⁺. Structural characterisation of these materials was performed in the space group P43n by Rietveld refinement of powder X-ray or time-of-flight neutron diffraction data. The framework was shown to be able to incorporate metals of different sizes by a partial collapse of the framework facilitated by cooperative rotations of the ordered A10₄ and SiO₄ tetrahedra accompanied by a decrease in the A1-O-Si angle. The XO₄^- anions were found to be disordered over a twelve fold site and to possess a large amount of vibrational freedom in the expanded K⁺ bearing sodalites.