Manifestation of the Ferroelastoelectric Phase Transition in the Absorption Spectra of (NH ₄) ₂CuCl ₄×2H ₂O Crystals

This work was devoted to experimental investigations of the temperature changes 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. On the basis of the crystal field theory consideration 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 corresponding to the d-d electron transition in Cu2+ ion were found to manifest noticeable anomalies in their temperature dependences in vicinity of the ferroelastoelectric phase transition point Tc=200.5 K. The shape of these anomalies testifies to the second order phase transition. The noticeable growth of light scattering on the domains below the Curie point was considered as an indirect observation of the ferroelastoelectric domain structure. The first principles calculations in the framework of the density functional theory confirmed the experimental data and provided the additional information, important for the adequate identification of the absorption bands and corresponding electron transitions. The absorption edge at high-energy region of the calculated spectrum (above 35000 cm-1) was assigned to the electron transitions from Cu2+d orbitals to the s-p hybridized orbitals of Cl- and O2- in the bottom of the conduction band. The large blue-shift of the absorption edge with decreasing temperature qualitatively reproduces experimental data. The calculated absorption spectrum in the range of 10000-16000 cm-1 assigned to the Cu2+ ions d-d transitions fairly well correlates with the experimental data. Besides, the data of computer modeling allowed to predict the position of the fundamental absorption edge above 50000 cm -1.