Diffraction studies of the (222) reflection in Ge and Si: Anharmonicity and the bonding electron

Abstract

We present the results of a combined x-ray and neutron diffraction study of the temperature dependence of the "forbidden" (222) reflection in germanium. Integrated intensities were measured from room temperature to 850\ifmmode^\circ\else\textdegree\fi{}C with x rays and to 904\ifmmode^\circ\else\textdegree\fi{}C with neutrons. In addition, an earlier x-ray study of the silicon (222) reflection has been improved and extended to 800\ifmmode^\circ\else\textdegree\fi{}C and a correction has been made in the corresponding neutron results. The germanium and silicon data are interpreted in terms of anharmonic atomic vibrations and anticentrosymmetric valence-charge distributions. Contrary to our previous reports, the temperature dependence of the scattering from the bonding electrons is found to be similar to that from the core electrons.