Thermo Sensitive Photo-Selective Phenomenon In Single Crystal Of Ammonium Tetrabromozincate.

Raman scattering studies from single crystals of ammonium tetrabromozincate (ATBZ) are being reported for various polarization geometries for three distinct phases; phase I (450 K), phase III (300 K) and phase V (85 K). Temperature dependence of spectra under five different excitation wavelengths --514.5, 501.7, 496.5, 488.0 and 457.9 nm -- and under specific polarization geometries have revealed; (a) an hitherto unreported phase below 130 K and (b) a photo-selective phenomenon in the temperature range 440 to 325 K for the above five wavelengths. The paper describes, in addition to the vibrational analysis in three phases, the occurrence of a local irreversible change in the crystal at a temperature range which depends on the (intensity and) frequency of the laser radiation used for Raman studies. The structural information would be definitive, if combined with these observations; however the occurrence of this phenomenon, not associated with trivial heating effect, is interesting and needs explanation. The various possibilities to explain the abnormal frequency dependence of the observed changes have been discussed.