The work is devoted to study of impact of the cation-anion substitution, spatial modulation and size effects on the crystalline structure, dielectric dispersion, magnetoelectric and optical-spectral properties of the crystals with alkylammonium cations and complexes of transition metal ions in order to obtain effective ferroelectric and magnetoelectric materials for the functional electronics and computer technique.
The investigations of NH2(CH3)2Ме1-xCrx(SO4)2×6H2O (DMAМе1-xCrxS, Me= Al, Ga) solid solutions revealed influence of substitution of the metal ion on the phase transitions and dielectric properties of the crystals. In particular, on the basis of the calorimetric studies there was confirmed the nonlinear dependence of the phase transition temperature on the chromium concentration x in DMAAl1-xCrxS crystals, simultaneously with practical absence of such an influence of the gallium-chromium substitution in DMAGa1-xCrxS solid solutions.
DMAGa1-xCrxS crystals are characterized by two types of dispersion in the ferroelectric phase associated – the fundamental ferroelectric one and the dispersion caused by motion of the domain walls. Substitution of gallium with chromium substantially affects the parameters of the domain wall dynamics. In particular, this leads to an increase in the relaxation time, the activation energy and the half-width of the Gaussian describing the relaxation times distribution, and to a significant decrease of the domain walls freezing temperature.
The significant influence of the magnetic field on the temperature dependences of the spontaneous polarization of DMAAl1-xCrxS crystals has been found, that allows to create the material with controlled magnetoelectric properties.
The analysis of the temperature and the field dependences of ЕАСС crystal magnetization clearly showed the key role of the ferromagnetic interactions in the magnetic phase transition at Тс = 10,2 К. It was found that (C2H5NH3)2CuCl4 (ЕАСС) undergoes the transition into the ferroelectric phase at Т5 = 247 К. The ferroelectric properties are observed at cooling down to the temperature of 5 K. So, it was shown that EACC crystal is a magnetic multiferroic with coexisting of spontaneous polarization and dominant ferromagnetic interactions. The magnetodielectric effect was observed in [N(C2H5)4]2CoClBr3 (ТЕАССВ-3) solid solution.
It was created a piezoelectric transducer in a gigahertz frequency region with a TEACCB-2 + PMMA nanocomposite as a piezoelectric element. The operating frequency of the device can be controlled by changing the size of the nanocrystals during the manufacturing of the piezoelectric element.
It was found that the potential multiferroic [NH2(C2H5)2]2CоCl4 (DEACC) crystal at room temperature belongs to P2/n space group with the lattice parameters: a = 10,667(1), b = 11,501(2), c = 7,4053(8) Å, β = 90,939(3)°; V = 908,33 Å3; Z = 2.
The optical absorption spectra of DEACC and ТЕАССВ-2 crystals in the region of the intraion absorption of the cobalt ion and optical absorption edge were investigated. It was found that the symmetry of metal-halogen complex is lower than tetrahedral, that is confirmed by removal of the certain energy levels degeneration.
The thermochromic nature of the ferroelastic phase transition at Т1 = 326 К has been revealed. The low temperature anomalies at Т2 = 255 К, Т3 = 216 К, Т4 = 165 К, Т5 = 110 К and Т6 = 45 К were observed in the temperature dependences of thermal dilatation and optical-spectral parameters of DEACC crystal. They were related to the previously unknown phase transitions, which can be connected with ordering of diethylammonium cations and reorientation of the metal-halogen complexes, that are accompanied by a different degree of distortion of the latter and corresponding changes in the metal-ligand distances.
The analysis of the temperature evolution of the absorption spectra of TEACCB crystal confirmed existence of the first-order phase transitions at the temperatures Т1 = 244 К and Т2 = 228 К (in a cooling run). Besides, the additional anomaly corresponding to the early unknown phase transition, probably of the second order, was observed at Т3 = 200 K. The detected anomalies of the absorption spectra parameters are related to the change in the degree of [CoCl2Br2]2- complex deformation.
Performed spectral investigations confirmed a strong influence of the electron- phonon interaction (EPI) on the position and shape of the absorption edge both in DEACC and ТЕАССВ-2 crystals. As a result, in two high-temperature phases (Т > T2) for both crystals the low energy tails of the edge bands are described by the empirical Urbach’s rule. The values of σ0 constant testify to a quite small strength of the EPI in the investigated crystals that is characteristic of the charge transfer edge bands. Nonfulfillment of the Urbach’s rule in the low temperature phases would be related to overlapping of the edge bands of different nature.
