REORIENTATIONAL DYNAMICS OF AMIDE IONS IN ISOTYPIC PHASES OF STRONTIUM AND CALCIUM AMIDE - 1 - NEUTRON-DIFFRACTION EXPERIMENTS

Strontium and calcium amide are ionic compounds crystallizing in a tet ragonally distorted anatase structure type. They include asymmetricall y charged amide ions (NH2-/ND2-) which resemble water molecules in the ir structure and in their charge distribution. Bq means of temperature -dependent X-ray scattering measurements an anomalous thermal expansio n was observed for both compounds (295 K for Sr(ND2)(2), 370 K for Ca( ND2)(2)). Fourier analyses based on high-resolution neutron powder dif fraction experiments carried out in a large temperature range show esp ecially for the deuterium atoms a remarkably large, anisotropic spatia l distribution of the probability density function. It is caused by st rongly anisotropic librations of the amide ions. The amplitudes of the ir normal librations were analyzed using a rigid-body model to paramet rize the atomic mean-square displacements of the atoms belonging to th e amide ions. They can be described as composed of strong rocking libr ations with weak temperature dependence and a weak wagging libration w ith strong temperature dependence.