Halide sodalites: macroscopic and microscopic properties, and deviations from the average structure

Abstract

Sodalites of the general formula |Na8X2|[T1T2O4]6 with X = Cl-, Br-, I- have been synthesized for Al-Si, Ga-Si, Al-Ge and Ga-Ge as T1-T2 frameworks and were examined by various methods to clarify the interactions of the atoms, especially of the Na and X ions within the sodalite cages, and the relation of these interactions to the stability and thermal behavior of the compounds. The sodalites average structure between 18 K and its decomposition has been refined using temperature-dependent X-ray diffraction experiments. The phonon structure was approximated by modelling the volume thermal expansion with Debye and Einstein contributions to the internal energy. No direct relation between the respective Debye and Einstein temperatures, decomposition temperatures from thermal analyses and the phonon densities of states from force-field calculations was found. Applying a modified form of the Lindemann criterion, the twelve compounds could be divided into three groups, indicating a different interplay between phonon systems and temperature-dependent average structures. Atomic pair distribution functions (PDF) derived from synchrotron total scattering experiments allowed the refinement of anisotropic displacement parameters (ADP) for the oxygen and sodium atoms where three distinct forms of the Na ADPs for different compounds became apparent. The PDFs of compounds with non-spherical ADPs for sodium could be modelled with structures containing one or two partly occupied Na positions, giving a total of three models for the local structure of the twelve sodalites. For the three gallogermanate compounds, the models could be refined between 100 K and 300 K using PDFs from temperature-dependent total scattering experiments. The three groups of sodalites with a differently modelled local structure have an overlap with the ordering of the compounds into the three Lindemann groups, but they are not identical.