Structure and properties of tecto-gallosilicates. I. Hydrosodalites and their phase transitions

The crystal structures of the new gallosilicate hydrosodalites Na-5[Ga0.8Si 1.2O4](6) (H2O)(8) was refined from M-ray powder data in the bodycentred sp ace group I (4) over bar 3m: a = 884.15 pm, V = 0.6912 nm(3), Z = 2, R-WP = 0.063, R-p = 0.048, R-I = 0.014 for 6000 data points and 12 variable posit ional parameters. The statistical distribution of the gallium and silicon a toms on the tetrahedral Framework positions was proved by Si-29 MAS NMR. TG /DSC experiments and in situ X-ray heating measurements indicate a phase tr ansition of this compound to the phase Na0.8Ga0.8Si1.2O4. The sodalite Na-6 [GaSiO4](H2O)(8) was reinvestigated from X-ray powder data in the space gro up P (4) over bar 3n: a = 885.92 pm, V = 0.6953 nm(3), Z = 2, R-WP = 0.047, R-p = 0.033, R-I = 0.017 for 2000 data points and 10 variables. Here the t etrahedral atoms are completely ordered as shown in the Si-29 MAS NMR spect rum. Additionally the structure was refined from single crystal diffractome ter data using a very small crystal. This refinement resulted in: a = 887.8 pm, V = 0.6998 nm(3), Z = 2, wR(2) = 0.046, R = 0.043 for 294 observed dat a and 24 variable parameters. Heating this compound to 1173 K shows a phase transition which resulted in this case in the phase NaGaSiO4-I. This phase was also refined from pow der data in the hexagonal space group P6(3): a = 872.9 pm, c = 820.8 pm, V = 0.5416 nm(3), Z = 6, R-WP = 0.069, R-P = 0.047 , R-I = 0.026 for 2000 data points and 20 variables. In comparison to simil ar structures reported before, the hexagonal space group was chosen. Becaus e two reflections could not be explained with this symmetry, it is possible that the compound crystallises in a pseudo-hexagonal orthorhombic space gr oup. Additionally, IR and Raman spectra of all compounds were recorded.