Chornii Y. Influence of modification of the metal-halogen complexes and cationic substitution on the nature of phase transitions in the organic-inorganic ferroics

The work is devoted to study of the influence of inorganic anions’ modification and isomorphic substitution of alkylammonium cations on the structure, phase transitions, optical-physical and magnetic properties of the crystalline ferroics, containing complexes of the transition metal ions in their structure. X-ray diffraction study confirmed the chemical composition and symmetry of the investigated [(CH3)3NH]MnC13×2H2O (TrMAMnCl) single crystals and refined their lattice parameters. Measurements of the temperature and field dependences of the magnetic parameters have shown that TrMAMnCl behaves as a canted one-dimensional antiferromagnetic with a hidden canting of the spin along the b axis. Investigations of the temperature dependence of spontaneous polarization confirmed existence of the improper ferroelectric phase in [N(C2H5)4]2CoCl2Br2 (ТЕАССВ-2) crystal below T2=227,7 K. Presence of the anomaly of magnetic susceptibility at T2 confirms both ferromagnetic interactions around this point and the spontaneous magnetoelectric effect. A model of a capacitive magnetic field sensor based on [N(C2H5)4]2CoClBr3 crystal, which possesses a magnetodielectric effect, was proposed. Using the AFM methods it was confirmed that [(CH3)2CHNH3]4Cd3Cl10 (IPACC) crystals are characterized by the ferroelastoelectric domain structure. The observed specific etching of a sample with a different rate for the neighboring domains is caused by water condensed from atmosphere on the crystal surface. It has been found that the surface morphology of IPACC crystals kept for a long time in the open air undergoes the considerable changes in comparison with the freshly cleaved samples. Particularly, the samples are characterized by growth of the nanocrystals on the first stage and larger microcrystals on the next stages of the sample aging. Another type of the elements observed on the surface would be considered as nano- or microrods. On the basis of the investigation of vibrational spectra of IPACCC crystal, it has been found that it`s structure is very similar to the structure of initial IPACC crystal. In all cases the anionic complex possesses the same symmetry and consists of the three distorted metal-halogen octahedra with different orientation of their axes in respect to the main crystallographic directions. Cu2+ ion in IPACCC crystal statistically replaces Cd2+ ion. The analysis of frequencies and intensities of the vibration modes at different temperatures confirmed the phase transitions in ІРАССС at T1=358, T2=293 K and T3=253 K. Performed analysis of the vibrational spectra of the initial IPACC and doped with copper crystal allowed identification of the real phonons most of all involved into formation of the absorption edge. Performed spectral investigations confirmed a strong influence of the electron-phonon interaction on the position and shape of the absorption edge in (NH4)2CuCl4×2H2O crystals and existence of the ferroelastoelectric phase transition at temperature Tc=200,5 K. As a result, for the phase lying above Tc and the temperature region below 100 K the low energy tail of the edge band follows the empirical Urbach’s rule. On the basis of study of (NH4)2CuCl4×2H2O crystals’ absorption spectra the temperature changes of the shape and symmetry of the octahedron formed around Cu2+ ion were analyzed using the crystal field theory. It was shown that the ferroelastoelectric phase transition is accompanied by an anomalous change of the level of tetragonal and rhombic distortion of the octahedra. Ab initio theoretical calculations within the density functional theory using GGA+U method provided adequate identification of the absorption bands and corresponding electronic transitions. The detailed study of the size effects’ manifestation in the spectral properties of the nano- and microcrystals of [NH2(C2H5)2]2CuCl4 (DEACC) embedded into the polymer matrices were performed. It has been found that the composite with the average nanocrystals’ sizes of 160 nm undergoes a continuous thermochromic PT. It was shown that the microcomposites on the basis of DEACC crystals possess the thermochromic properties very similar to those in a bulk crystal. They undergo clear but a little diffused thermochromic PT of the first order in vicinity of 316 K (at heating) in the case of a latex matrix. The microcrystals with a polystyrene matrix show a very sharp thermochromic PT at 337 K. A method of producing of the thermochromic microcomposite based on DEACC crystals was proposed.