OPTICAL PHONONS AND PHASE TRANSITION OF THALLOUS IODIDE.

This work was supported by NASA grant NGR 22-011-051.Author Institution: Physics Department, Northeastern UniversityFar infrared transmittance and reflectance and Raman spectroscopic measurements have been made on polycrstalline thallous iodide in the temperature range $5^{\circ}-550^{\circ}K$. In particular, the spectroscopic investigations have been used to study the 4 molecule/unit cell $D^{17}_{2h}$ (Cmem) orthorhombic structure transition to the $O^{1}_{b}$ (Pm3m) CsCl structure which occurs near $445^{\circ}K$. In the orthorhombic structure one infrared active transverse optic mode and six Raman active modes were observed; group theoretical predictions indicate that there should be three infrared active and six Raman active modes. In the CsCl structure a single infrared transverse optic mode is still found but displaced to much lower frequencies than the mode observed below the transition temperature. The Raman spectra in the CsCl structure is purely second order. A significant hysteresis is observed in traversing the orthorhombic-CsCl phase transition