Loboda N. Influence of isomorphous substitution of metal ion on the dielectric dispersion, magnetoelectric interactions and optical-spectral properties of NH2(CH3)2Me(SO4)2×6H2O (Me = Al, Ga, Cr) crystals

The work is devoted to the purposeful modification of the structure, electrophysical, magnetic and optical-spectral properties of NH2(CH3)2Me(SO4)2×6H2O (DMAMeS) crystals (where Me = Al, Ga) by isomorphous substitution of Me ions with chromium in order to obtain the effective ferroelectric and magnetoelectric materials for the functional electronics and computer technique. The single crystals of NH2(CH3)2Al0,8Cr0,2(SO4)2×6H2O solid solutions were found to grow mainly in a polydomain state. It was shown that the value of chromium concentration in the two groups of ferroelastic domains with different signs of mechanical stress differs significantly – 17.6% and 20% respectively. Such are feature is caused by difference in the thermodynamic conditions of crystal growth due to the different signs of a mechanical stress in two groups of domains accompanied by different degrees of the metal ion substitution. The investigations of the nanoscale and microcrystalline structures formation on the surface of DMAAl0,8Cr0,2S single crystals kept under the conditions of high environment humidity were performed and the corresponding model was proposed. It was shown that the partial isomorphous substitution of the metal ion is followed by the nontrivial change of the ferroelectric phase transition temperature in DMAAl1-xCrxS crystals. The largest shift of this temperature was observed for the samples with a lower chromium concentration (x = 0.065). The detected effect is caused by the fact that chromium ions generate the local lattice deformations that through the piezoelectric effect affect both the spontaneous polarization and phase transition temperature as well as the size and the dynamics of the dipole clusters responsible for the dielectric dispersion. At the same time, no noticeable changes were found in the dielectric properties and the ferroelectric phase transition temperature Tc for DMAGa1-xCrxS crystals. This is explained by the closer sizes of the Cr3+ and Ga3+ ions, contrary to the case of the Cr3+ and Al3+ pair. DMAAl1-xCrxS crystals with a significant magnetoelectric interaction were created and there was demonstrated the possibility to modify the magnitude and to change the sign of the coefficient of such an interaction by variation of the Cr3+ concentration. The obtained results suggest that the magnetoelectric effect may exist in the paramagnetic compounds without the noticeable magnetic anomalies near the ferroelectric phase transition. The model explaining the nontrivial dependence of the parameters of the dielectric dispersion caused by the domain walls dynamics on the chromium concentration was proposed. Chromium ions may be considered as the sources of local lattice deformations, that lead to formation of the massive dipole clusters in vicinity of the ferroelectric phase transition at lower concentrations of chromium. Increasing of its concentration is followed by fragmentation of the clusters and corresponding decrease of the dispersion of the relaxation times distribution and Vogel-Fulcher temperature, that correlates with the Curie temperature decrease. At the same time, the aforementioned local deformations hinder reorientation of the DMA dipoles responsible for the domain walls motion. This leads to increasing of the relaxation time and activation energy with increasing of Cr3+ ions concentration. It is shown that DMAAl0,8Cr0,2S crystals possess a significant proton conductivity. It was concluded that similarly to initial DMAAlS crystals, the conductivity is realized through the Grotthus mechanism. A detailed analysis of absorption spectra of DMAMe1-xSrxS crystals (Me = Al, Ga) allowed to refine the energy diagram of Cr3+ ions in the octahedral coordination and to calculate the parameters of the crystal field and the Racah parameters. It has been shown that [Cr(H2O)6]3+ complexes in all investigated crystals possess a considerably distorted octahedral shape. The determined parameters describing the crystalline field, as well as electron-electronic repulsion, were found to be very close for all considered compounds. This allowed to conclude that [Cr(H2O)6]3+ octahedra in considered DMAMe1-xCrxS crystals are very similar by sizes (metal-distance distances) and even by a character of distortion. This conclusion is confirmed by investigations of Raman spectra. According to the performed analysis of symmetry, all metal-hydrate complexes in the investigated solid solutions and in the initial crystals possess an inversion center at room temperature. On the basis of analysis of the absorption spectra temperature evolution and the Raman spectroscopy data it was concluded that the ferroelectric phase transition is associated with a significant distortion of the [Cr(H2O)6]3+ complex and even with removing of the inversion centre.