Manifestation of the ferroelastoelectric phase transition in the absorption spectra of (NH4)2CuCl4×2Н2О crystals

This work was devoted to the experimental investigations of the absorption spectra of (NH4)2CuCl4 × 2H2O crystals in order to obtain information about the variation of the chromophore group and related structural changes around the ferroelastoelectric phase transition at Tc = 200.5 K. It was shown that the anionic complex around Cu2+ ion possesses the shape of a flattened octahedron with the shortest distance O–Cu–O. The parameters of the elementary contours were found to manifest the noticeable anomalies in their temperature dependences at the phase transition point. The first principles calculations in the framework of the density functional theory confirmed the experimental data. The absorption edge in the UV–visible range of the spectrum was assigned to the charge transfer bands. The parameters of the absorption bands in the calculated spectrum within the range of 10000–16000 cm−1 corresponding to the Cu2+ ions d-d transitions fairly well correlate with the data of experimental spectra.