Ce. Mungan et al., UNIVERSAL 2-STATE REORIENTATIONAL DYNAMICS OF DIATOMIC HYDRIDES IN FCC SALT CRYSTALS, Physical review. B, Condensed matter, 54(13), 1996, pp. 9204-9212
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.