Layered character of dielectric function in TlGaSe2 at phase transition from exciton spectroscopy

It is shown that within the temperature region corresponding to paraphasia--ferroelectric phase transition dielectric constant of layered crystals TlGaSe2 and TlInS2 can be considered as consisting of two slabs with different dielectric constants 1, 2 and thickness d1 and d2 (d1+d2=c, c is the lattice vector projection in the direction normal to layer). So, the dielectric anomaly and spontaneous polarization occurring at phase transition takes place only in one of the slabs. This model is confirmed by some experimental results, such as dielectric function anisotropy and spectroscopy of excitons at phase transition temperatures. 15 refs.