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We studied the magnetic properties of Ni3 –xCoxB2O6 solid solutions with the kotoite structure, where x = 0; 0.19; 0.6; 0.93; 2. The compounds Ni3B2O6 and Co3B2O6 are antiferromagnets, with their easy axis of magnetization coinciding with the crystallographic directions c and b, respectively. In the Ni2.81Co0.19B2O6 solid solution, two features are found on magnetization curves, while in other solid solutions there is only one feature. With the cobalt ion concentration x > 0.9, the magnetic moments are predominantly oriented along the b axis. [ABSTRACT FROM AUTHOR]

A comparative analysis of the copper and iron ions bonds exchange energies was conducted for various variants of orderings and distributions of iron ions among crystallographic positions in ludwigite Cu2FeBO5. Analysis showed that the exchange bonds of iron ions play a key role in the formation of magnetic order. The magnetic ordering strongly depends on the distribution of iron ions among the positions. In the case when the Fe3+ is in the same position as in Fe3BO5, the most favorable magnetic structure is similar to the magnetic structure of ludwigite Fe3BO5. In other cases, the type of magnetic ordering is different. [ABSTRACT FROM AUTHOR]

Raman spectra changes in multiferroic Ho1–xNdxFe3(BO3)4(x = 0, 0.25, 0.5, 0.75) and Sm1–yLayFe3(BO3)4(y = 0, 0.75) single crystals at low temperatures have been analyzed. The anomalies corresponding to the phase transition of the second order have been detected at the temperatures of magnetic phase transitions, being consistent with previously reported results of the magnetization study. A number of anomalies in the temperature dependences of the spectral lines associated with the occurrence of magnetic order. A mode corresponding to two-magnon scattering was revealed. Comparison of the features of low-temperature low-frequency spectra of solid solutions Ho-Nd and Sm-La has been carried out. [ABSTRACT FROM PUBLISHER]

The results of structural phase transitions mechanisms study in K3WO3F3oxyfluoride are represented by different experimental and theoretical methods. The structural phase transition anomalies at T1=452K and T2=414K of Raman and IR spectra have been analyzed. Using vibrational spectroscopy methods, the NMR-experiment has been done to clarify the nature of found phase transitions: displacive types or order-disorder types. The model of "disordered" crystal was proposed, and the results of lattice dynamics calculation in frameworks of the generalized Gordon-Kim method of ordered (R3) and "disordered" crystals were compared. The high pressure phases were studied by the Raman technique too. [ABSTRACT FROM AUTHOR]

Orthorhombic Me3B2O6 (Me = Co, Ni) single crystals of the kotoite family are grown from the fluxes based on a mixture of bismuth trimolybdate and sodium oxide upon spontaneous nucleation. The field and temperature dependences of magnetization of the single crystals with the magnetic field oriented along the principal crystallographic directions are obtained. Unlike Ni3B2O6, Co3B2O6 exhibits strong anisotropy of the magnetic properties. At the temperature T = 10 K, the magnetization curves of Co3B2O6 reveal an anomaly attributed to the spin-reorientation transition. Strong anisotropy of the magnetic properties of Co3B2O6 is also confirmed by the calculation of the exchange interactions within a simple indirect coupling model. [ABSTRACT FROM AUTHOR]

Vibrational frequencies of K3WO3F3 cryolite have been calculated for Fm3m, I4 mm, Cm structures in the framework of generalized Gordon-Kim model with account of quadrupole polarizabilities. The phonon spectra have been found to have 'soft' modes with eigenvectors in agreement with experimental atomic displacements. This work was supported by Federal Special-Purpose Program 'Scientific, Academic and Teaching Staff of Innovative Russia, 2009-2013', contract No P62(NK-468P/28) [ABSTRACT FROM AUTHOR]

The dynamic Born charges and the frequency spectra of lattice oscillations in the crystals of ordered and disordered PbSc1/2Ta1/2O3 (PST) and PbSc1/2Nb1/2O3 (PSN) solid solutions have been calculated within the framework of the generalized Gordon-Kim model with allowance for the dipole and quadrupole polarizabilities. The phonon spectra of both compounds contain ferroelectric soft modes. The influence of various interactions on the magnitude of dynamic charges and ferroelectric instability in PSN and PST solid solutions has been studied and it is shown that both these charges and the ferroelectric instability are determined by the competition between long-range dipole-dipole interactions and short-range dipole-charge interactions, the determining role played by the interaction of Nb (Ta) cations and oxygen anions in the Nb-O (Ta-O) bond direction. [ABSTRACT FROM AUTHOR]

Ferroelectric phase transitions and thermodynamic properties of disordered PbSc1/2Ta1/2O3 and PbSc1/2Nb1/2O3 solid solutions have been investigated by the Monte Carlo method. The parameters of the effective Hamiltonian were calculated within the generalized Gordon-Kim model. The obtained values of the phase-transition temperatures and spontaneous polarization are in satisfactory agreement with the experimental data. [ABSTRACT FROM AUTHOR]

The effective Hamiltonian was obtained and thermodynamic properties of B-cation ordering in solid solution of PZT were studied by Monte-Carlo method. The temperatures of B-cation ordering phase transitions Tc ∼⃒ 180 K and Tc∼⃒ 250 K were determined for concentrations x = 1/3 and x = 1/2, respectively. This temperatures are essentially lower than melting temperature (Tm ∼⃒ 1200 K) of this compound and because of diffusion character of ordering process in alloy kinetics of such ordering at room temperatures is “frozen” and there is no the real phase transition to ordered phase that was observed experimentally. The lattice parameters, phonon dispersions, dielectric constant, Born effective charges for some ordered and fully disordered PbZr1-xTixO3 for 0 ≤ x ≤ 1 were calculated. The calculation was performed using the nonparametric generalized Gordon-Kim model with regard to the deformability, dipole and quadropole polarizabilities of ions, for disordered solid solution in framework of the used virtual crystal approximation. [ABSTRACT FROM AUTHOR]

The results of nonempirical calculation of energies of three polytypes (cubic, two-layer hexagonal, (and six-layer hexagonal) are given for RbMnX[SUB3](X = F, Cl, Br) crystals. The calculation is performed using an ionic crystal model with regard for the deformability and the dipole and quadrupole polarizabilities of ions. The behavior of these crystals under the action of hydrostatic pressure is studied. It is demonstrated that, at normal (pressure, the RbMnC[SUB3] and RbMnBr[SUB3] crystals have a six-layer hexagonal structure. At pressures above 11 kbar, RbMnCl[SUB3] passes to a phase with a cubic structure; RbMnBr[SUB3] at pressures above 90 kbar passes to a phase with a two-layer hexagonal structure. The RbMnF[SUB3] crystal under normal conditions has a cubic structure and experiences no phase transformations under the effect of pressure. The obtained results are in satisfactory agreement with the known experimental data. [ABSTRACT FROM AUTHOR]

This paper presents the results of a nonempirical calculation of the static and dynamic properties of K[sub 2]NaAlF[sub 6], K[sub 3]AlF[sub 6], and Na[sub 3]AlF[sub 6] crystals with the elpasolite structure. The calculation is based on a microscopic model of an ionic crystal that allows for the deformability and polarizability of the ions. The deformability parameters of the ions are determined by minimizing the total energy of the crystal. The total energy is regarded as a functional of the electron density, using the local Thomas-Fermi approximation and taking into account exchange (correlation) effects. The results of the calculations of the equilibrium lattice parameters and of the permittivities are in good agreement with the experimental data. Unstable vibrational modes are found in the spectrum of the lattice vibrations, with these modes occupying the phase space in the entire Brillouin zone. [ABSTRACT FROM AUTHOR]