Structure and properties of tecto-gallosilicates II. Sodium chloride, bromide and iodide sodalites

Abstract

The crystal structures of the gallosilicate sodalites Na8[GaSiO4]6Cl2, Na8[GaSiO4]6Br2and Na8[GaSiO4]6I2were refined from X-ray powder data in the space groupP-43n:a= 894.70 pm,V= 0.7162 nm3,RWP= 0.066,RP= 0.050,RI= 0.011,a= 900.83 pm,V= 0.73102 nm3,RWP= 0.066,RP= 0.051,RI= 0.016 anda= 908.36 pm,V= 0.74951 nm3,RWP= 0.050,RP= 0.036,RI= 0.013 for 4000 data points and 14 variable positional parameters in each case for the chloride, bromide and iodide compound, respectively. The gallium and silicon atoms on the tetrahedral framework positions are totally ordered, the halide anions were found at the centre of the sodalite cage coordinated by four sodium atoms. The recorded infrared and Raman spectra show absorption bands typical for the sodalite structure, but indicate also clearly the difference between chloride, bromide and iodide incorporation in the sodalite cage by different absorption band in the FIR range. Heating X-ray investigations show the expected increases of lattice parameters for all three sodalites with increasing temperature and no phase transition was detected. But for the freshly prepared bromide sample a different nearly linear increase of the lattice parameter was observed up to 770 K and above 870 K with an invariant behaviour between these temperatures. The tilt and T—O—T angle as well as the interatomic Si—O and Ga—O distances show an expected temperature dependent behaviour as found by Rietveld calculations of the high-temperature X-ray data. The angles in the tetragonal distorted SiO4tetrahedra show values comparable to those found in alumosilicate sodalites but seem to be temperature dependent. Additionally, the corresponding GaO4angles indicate a disappearance of the tetragonal tetrahedron distortion around the temperature where the discontinuity was observed for the lattice parameter. The GaO4tetragonal tetrahedron distortion is close to zero during the formation of the framework which leads to different distortion direction. This different distortion direction could be relaxed by heating the sample or a self relaxing occurs within several months at room temperature.