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We have measured the room-temperature refractive indices of GaSe throughout the 0.7–1.4 and 2.4–5μm ranges using the minimum-deviation method of light through a prism with polarization either parallel (extraordinary refractive index ne) or perpendicular to the crystal optical c axis (ordinary refractive index no). The birefringence (Δn) at room temperature has been measured directly using polarized light interference fringes obtained in the transmittance from 1.73to4.97μm (three samples with thicknesses of 1108±1, 2480±1, and 2660±1μm) and in the range from 11.85–16.37μm (sample thickness of 6000±1μm). The no, ne, and Δn values determined from the positions of fringe maxima were compared to those obtained from prism measurements in the mid-IR and with previously published results which were mainly obtained by indirect methods. It was found that the prism method and interference fringe method resulted in values of no, ne, and Δn accurate to ±0.003 and ±0.006, respectively.

The effect of an orienting electric field on the anomalies of the temperature dependence of the dielectric constant ε of the crystals TlInS2 and TlGaSe2 and of the pyroelectric current i in TlInS2 near phase transitions is investigated. It is found that the ε(T) profile of both crystals undergoes the same transformation at the point of the phase transition to the incommensurate phase under the influence of a static electric field applied in the plane of the layer. It is established that the maximum of ε(T) in TlGaSe2 and of i(T) in TlInS2 at the point of the phase transition to the commensurate polar phase is shifted in a certain range of orienting electric fields and that the sign of the temperature shift depends on the value of the external electric field. An interpretation is offered for the experimental results.