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

Abstract

The crystal structures of the new gallosilicate hydrosodalites Na5[Ga0.8Si1.2O4]6 (H2O)8 was refined from X-ray powder data in the bodycentred space group I-43m: a = 884.15 pm, V = 0.6912 nm3, Z = 2, RWP = 0.063, RP = 0.048, RI = 0.014 for 6000 data points and 12 variable positional parameters. The statistical distribution of the gallium and silicon atoms on the tetrahedral framework positions was proved by 29Si MAS NMR. TG/DSC experiments and in situ X-ray heating measurements indicate a phase transition of this compound to the phase Na0.8Ga0.8Si1.2O4. The sodalite Na6[GaSiO4]6(H2O)8 was reinvestigated from X-ray powder data in the space group P-43n: a = 885.92 pm, V = 0.6953 nm3, Z = 2, RWP = 0.047, RP = 0.033, RI = 0.017 for 2000 data points and 10 variables. Here the tetrahedral atoms are completely ordered as shown in the 29Si MAS NMR spectrum. Additionally, the structure was refined from single crystal diffractometer data using a very small crystal. This refinement resulted in: a = 887.8 pm, V = 0.6998 nm3, Z = 2, wR2 = 0.046, R = 0.043 for 294 observed data 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 powder data in the hexagonal space group P63: a = 872.9 pm, c = 820.8 pm V = 0.5416 nm3, Z = 6, RWP = 0.069, RP = 0.047, RI = 0.026 for 2000 data points and 20 variables. In comparison to similar structures reported before, the hexagonal space group was chosen. Because two reflections could not be explained with this symmetry, it is possible that the compound crystallises in a pseudo-hexagonal orthorhombic space group. Additionally, IR and Raman spectra of all compounds were recorded.