The anomalous behaviour of the exponential absorption edge in polar materials in the vicinity of phase transition is treated phenomenologically. It is shown, that changing of electron-phonon interaction a t phase transition results in an essential change of the absorption edge shape. The theoretical results are illustrated on a wide range of objects and types of phase transitions.

The electroreflectance (ER) of SbSI has been measured in a wide temperature range between 1.6 and 3.3 eV with linearly polarized light. The technique of transverse ER was used and the first and second harmonics of ER were measured simultaneously. The effects of domain reorientation, complementary light illumination, the specimen damping and electric field vector rotation on the sample surface are investigated. It is shown: (a) that the induced reflectivity change is not caused by purely Kern-Harbekeeffect, but depends on the piezoelectric strain; (b) that separated from the deformation component, the ER spectrum is close to the hmodulation spectrum and may be interpreted in a similar way.

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