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Low-temperature anharmonicity and symmetry breaking in the sodalite |Na8I2|[AlSiO4]6

Lars Robben, Isaac Abrahams, Michael Fischer, Stephen Hull, Martin T. Dove und Thorsten M. Gesing

Zeitschrift für Kristallographie - Crystalline Materials (2018) 234, Heft 4

https://doi.org/10.1515/zkri-2018-2122

The aluminosilicate iodide sodalite |Na8I2|[AlSiO4]6 was examined by temperature-dependent neutron time-of-flight powder diffraction from 5 K to 290 K and X-ray diffraction from 298 K to 1200 K. The temperature-dependent properties of the mean structure in space group P4̅3n were obtained by Rietveld analysis. A negative slope for the thermal expansion coefficient below 50 K could be observed, and the displacement parameters of the iodide ions indicate anharmonic effects. Local structure models (8×8×8 super cells) were refined against pair-distribution functions calculated from total scattering data collected at 5 K, 165 K and 240 K. The results indicate isotropic displacements for all atoms except for I-atoms, showing the effects of an anharmonic potential around this anion at very low temperatures.

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