UNIVERSAL 2-STATE REORIENTATIONAL DYNAMICS OF DIATOMIC HYDRIDES IN FCC SALT CRYSTALS

The reorientational spectra in the mid- and far infrared of OH-, OD-, SH-, SD-, SeH-, and TeH- in a variety of different alkali halides are studied. All of these systems have similar to 300 cm(-1) modes due to librations of the diatomics about their centers of mass. A surprising change in the dynamics of these librational modes with increasing temp erature is discovered: their absorption strengths are observed to disa ppear by similar to 150 K in the sodium-chloride-structure hosts and r eappear in, the millimeter-wave region in the forms of generalized-Deb ye spectra. This is in striking contrast to what is expected on the ba sis of the estimated orientational barrier heights using the known lib rational frequencies. It also contrasts with the cesium-chloride-struc ture salts, where the librational strengths of the molecules are conse rved up to RT. The temperature-dependent variations of the fee systems can be explained using a thermally activated jump-rotational-diffusio n model wherein each defect hops with temperature-dependent dwell time s between two distinct elastic configurations of the lattice-defect sy stem: one involving librational and the second supporting diffusive ro tational states.