Anharmonic thermal vibrations in Mg₂Si

The recent X-ray diffraction data front Mg₂Si, obtained by Panke & Wölfel (Z. Kristallogr. (1969), 129, 9) show systematic differences in the structure factors from those predicted by a model which assumes harmonic thermal vibration of the atoms. These differences indicate an anharmonic component in the thermal vibration of the magnesium atoms, consistent with their tetrahedral site symmetry. An analysis of these data has therefore been carried out in terms of a model which includes anharmonic thermal vibration and a least-squares refinement gave a value of the anharmonicity parameter (β_Mg = − 2.39 × 10⁻¹² erg.Å⁻³) which is similar in magnitude to those found for other systems with the fluorite structure.