Yonak P. Modification of structure and optical-physical properties of crystals with metal-halogen complexes by substitution of cations and anions

The work is devoted to study of influence of the isomorphous substitution of organic cations and inorganic anions on the structure, electrophysical, magnetic and optical-spectral properties of crystals with transition metal ions complexes and elaboration of the program package for interpretation of their absorption spectra. It was found that the structure of [(CH3)2CHNH3]4Cd3Cl10:Cu (IPACCC) is close to the structure of the initial [(CH3)2CHNH3]4Cd3Cl10 (IPACC) crystal without copper adding. The anionic complex was found to possess a similar symmetry and consists of the three metal-halogen octahedra with different orientation of their axes relatively to the main crystallographic directions. As it follows from the analysis of the crystal field spectra, the intermediate phase II (T2-T3) is characterised by a less pronounced tetragonal distortion of the metal-halogen octahedra of type І, whereas the octahedra of type ІІ are characterised by increasing of all metal-halogen distances in comparison with other phases. The values of the metal-ligand distances, obtained in our calculations, were found to be 1-5 % higher in comparison with the X-ray diffraction data for initial IPACC crystal. The reason for this difference consists in the statistical substitution of cadmium ions with copper in the investigated crystals. On the basis of the performed investigations of the optical birefringence temperature increments at heating we detected the jump-like anomaly characteristic of the first order phase transitions in IPACCC crystals at Т1 = 358 К and the second anomaly at Т2 =293 К. On the basis of the Landau theory analysis the phase transition at T2=293 K was related to the first order close to the second order one. Using the atomic force microscopy existence of the ferroelastoelectric phase in these crystals was revealed and the corresponding domain structure was visualized for the first time. On the basis of the performed dielectric investigations of [N(C2H5)4]2CoClBr3 (TEACCB) crystals the anomaly of dielectric permittivity characteristic of the proper ferroelectric phase transition was observed at Tc= 258 K. The dielectric dispersion observed in TEACCB solid solution first of all is connected with ordering of the distorted metal-halogen complexes. It has been found that TEACCB crystals are characterized by magnetic ordering below 8 K coexisting with the spontaneous polarization. This allows to relate these crystals to the multiferroics. It was shown that the absorption spectra of IPACCC crystal obtained at room temperature for different polarizations of incident light are characterized by clear pleochroism of the absorption edge caused by distortion of Cu2+ ion coordination. At lower energies the broad intense bands, connected with the internal transitions in Cu2+ ion, in the field of low symmetry were observed. There was elaborated the program package, which allows to analyze on the basis of the optical spectroscopy data the important physical and chemical parameters of the crystals with transition metal ions complexes and various electronic configuration of the valence shell at the crystalline field symmetries Td, Oh, D4h, C4v, D2d, C2v. Performed spectral investigations ascertained a noticeable influence of the doping with Cu2+ ions on the electron-phonon interaction (EPI) and respectively on the temperature evolution of the optical absorption edge of IPACC crystals. Due to this interaction, for the phases II and III of both IPACC and IPACCC crystals the low energy tails of the edge bands are described by the empirical Urbach’s rule. The only exception concerns the phase II in IPACCC for the light polarised along a axis, when lnα = f(E) lines were found do not cross in a single point. Such a behaviour is explained by overlapping of the mentioned absorption edge with the charge transfer (CT) bands. It has been found that parameters of Urbach’s rule show anomalous behaviour at the phase transition point T2. The anomalies are much more pronounced for IPACCC than in the case of IPACC crystal that implies more considerable corresponding change of EPI character for the crystals doped with copper. It has been also found that the low energy tail of the CT band in IPACCC is formed with participation of the internal vibrations of the metal-halogen complex. On the other hand, the low energy tail of the fundamental edge for both crystals is formed with participation of the internal vibrations of the metal-halogen complex or skeletal (or translational) vibration of IPA complex.It was also shown that partial metal ion substitution is followed by increasing of the EPI strength that is inversely proportional to o constant. It has been found that the Racah’s and crystal field parameters considerably depend on the isomorphous cation substitution. Indeed, increasing of the organic cation mass in A2CoCl4 crystals is followed by diminishing of the crystal field.