Molecular structure and anharmonicity in thallium iodide

Far infrared and Raman spectroscopic investigations of both the orthorhombic and cubic phases of T1I are presented for the temperature range 4–570°K and for the pressure range 0–10 kbar. The results are used to discuss the molecular structure of the orthorhombic phase and the anharmonicity associated with the q ≈ 0 transverse optic mode in the cubic phase. Six Raman active and one infrared active optical modes are observed in the orthorhombic phase whereas group theory predicts six Raman active and three infrared active modes for the generally accepted D2h17 symmetry of this phase. In the cubic phase one infrared active and one second order Raman active modes are observed. Four phonon interactions through the quartic anharmonicity are found to provide the dominant contribution to the anharmonic self-energy of the q ≈ 0 transverse optic mode of the cubic phase, but unlike for T1Cl and T1Br the magnitude of the thermal strain and anharmonic contributions to the total self-energy are approximately equal.