VIBRATIONAL SPECTROSCOPY OF THE STRUCTURAL PHASE-TRANSITION AND RAMAN EVIDENCE OF LIBRATIONAL FLUCTUATIONS IN THALLIUM THIOCYANATE

A detailed analysis of Raman and IR spectra of thallium thiocyanate (TINCS) in the temperature (T) range 293 to 438 K is carried out. The orthorhombic-to-tetragonal structural phase transition (SPT) near T almost-equal-to 367 K (= T(c)) is spectroscopically characterized as an order-disorder transition exhibiting predominant second-order features. Remarkably, consistent mean field type exponents: order parameter exponent beta = 0.45 +/- 0.04 and correlation length exponent nu = 0.46 +/- 0.04, and activation energy for the reorientational motion of NCS- ions U = (0.32 +/- 0.02) eV, are extracted from characteristic spectral changes in the thermosensitive Raman modes. The appearance of side bands in each of the internal modes of the NCS- group in the Raman spectra are attributed to splittings due to an internal field in the ordered orthorhombic phase; the continuous growth in the relative intensities and decrease in splitting energies reveal the gradual diminution of the internal field as T approaches T(c).