Raman and infrared spectra of thallous nitrate have been studied in the region 30 to 2000 cm-' for the temperature interval 80 to 460 K. The bands recorded in TlN03 (111) correspond to (v, +Oj) composite transitions where vi refers to internal and Oj to lattice vibrations. There are two sets of nitrate ions in the unit cell because of the observed splitting of the internal bands of the NO; group and also the doubling of some bands due to lattice vibrations.In the vibrational spectra of a crystal with poly-atomic ions not only the internal modes vi and the lattice vibrations Oj but also the bands due to composite transitions (vi +aj) are observed. Detailed study of such composite bands can provide information on anharmonicity and the nature of vibration interaction in a crystal, changes in ionic configuration, and internal crystal field symmetry.The present work is aimed at the study of internal and lattice vibration interaction in a thallous nitrate crystal with three phase transitions in a relatively small temperature interval [12]: 75°C145T

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