Temperature dependent structural and spectroscopic studies of sodium gallosilicate nitrite sodalite

We report on temperature dependent crystal structures of sodium gallosilicate nitrite sodalite Na8[GaSiO4]6(NO2)2 between 293 and 973 K. The crystal structures were refined from X-ray powder data Rietveld refinements in the space group P4¯3n. The gallium and silicon atoms on the tetrahedral framework positions are totally ordered. The nitrite anion was found slightly away from the center of the sodalite cage, which is tetrahedrally surrounded by four sodium atoms. The linear thermal expansion coefficient was calculated from the lattice expansion data. The change of the geometries of the framework has been observed on heating the polycrystalline sample, that is, T–O bond lengths slightly decreased, T–O–T angle non-linearly increased, tilt of the TO4 tetrahedra decreased (framework untwist) and their tetragonal tetrahedral distortion decreased with some scatterings. Of particular note, the tetragonal tetrahedra distortion of GaO4 tetrahedra approached close to zero at higher temperatures, which points to a distortion direction to a relaxed state of GaO4 tetrahedra on heating. The mobility of sodium atoms above 600 K leads to different sodium content in some sodalite cages and results in the appearance of different domains. The consequence of this domain formation to the strain of the crystalline system and to the average crystals size have been explained. Both Fourier transform infrared and micro-Raman spectra showed typical absorption bands of nitrite sodalite. The change of the frequency shift and full-width at half-maximum of some selected bands has been studied as function of temperature.