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

In a recent paper, Mungan et al. [Phys. Rev. B 54, 9204 (1996)] have p roposed a model of ''universal two-state reorientational dynamics of d iatomic hydrides in fee salt crystals,'' based experimentally on an ob served disappearance of librational ir absorption around 150 K. For th e only presented example of OH- defects in RbCl hosts, we show by comp arison to our measurements that this claimed disappearance is not vali d even up to T=350 K. Moreover, we show by accurate spectral data of t wo other examples (OH- and OD- in KBr) that the claimed universal vali dity of the proposed ''thermally activated jump-rotational-diffusion m odel'' is not true for OH- and OD- fcc alkali-chloride and bromide cry stals and that there is no contrast to the OH-/OD- dynamic behavior in bcc CsCl hosts.