Anharmonicity and the Temperature Dependence of the Forbidden (222) Reflection in Silicon

We have measured the "forbidden" (222) reflection of neutrons from silicon between 15 and 1353^∘{}C. We have used these measurements to correct the x-ray (222) structure factor for the anharmonic motion of the atoms. The scattering from the anticentrosymmetric valence charge density obtained in this way is found to be less temperature dependent than that from the core. We take this as evidence of the failure of the rigid-ion model.