Far infrared and Raman spectroscopic investigations of both the orthorhombic and cubic phases of T1I are presented for the temperature range 4ndash;570deg;K and for the pressure range 0ndash;10 kbar. The results are used to discuss the molecular structure of the orthorhombic phase and the anharmonicity associated with theq ap; 0transverse 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 acceptedD2h17symmetry 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 selfhyphen;energy of theq ap; 0transverse optic mode of the cubic phase, but unlike for T1Cl and T1Br the magnitude of the thermal strain and anharmonic contributions to the total selfhyphen;energy are approximately equal.
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