The polarization properties of bands in Raman scattering and IR reflection spectra are studied experimentally in TlGaS2, TlGaSe2, and TlInS2 layer single crystals and in solid solutions based on them. Additionally the angular dependence of IR‐active modes on the angle between the phonon wave vector and the direction of the optical axis c of single crystals is investigated. It is shown that the observed phonon spectra can be interpreted on the basis of a primitive cell which contains two tetragonal‐symmetry layers bonded to each other by an inversion operation. Bands corresponding to cells with a mixed composition are observed in the Raman spectra of the solid solutions.

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Polarized IR‐reflection and Raman scattering spectra are obtained for TlGaTe2, TlInTe2, and TlInSe2 crystals (space group D 4h18). The IR‐reflection spectra of all the crystals show the presence of two (E ‖ c) and three (E ⟂ c) modes, which is in full agreement with the prediction of a group‐theoretical analysis. All seven Raman active modes are detected in TlInSe2. In TlGaTe2 and in TlInTe2 only three of seven Raman active modes can be observed. Normal coordinates, effective force constants, and effective charges are determined for the infra‐red active lattice vibrations.

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