The structural properties of RbMnX<Subscript>3</Subscript> (X = F,Cl, Br) halides

The results of nonempirical calculation of energies of three polytypes (cubic, two-layer hexagonal, and six-layer hexagonal) are given for RbMnX₃ (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 RbMnCl₃ and RbMnBr₃ crystals have a six-layer hexagonal structure. At pressures above 11 kbar, RbMnCl₃ passes to a phase with a cubic structure; RbMnBr₃ at pressures above 90 kbar passes to a phase with a two-layer hexagonal structure. The RbMnF₃ 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.     

Lattice dynamics and hydrostatic-pressure-induced phase transitions in ScF3

New phase transitions induced by hydrostatic pressure in a cubic (under standard conditions) ScF₃ crystal are discovered by the methods of polarization microscopy and Raman scattering. The space groups $R\bar 3c$ for Z=2 and Pnma for Z=4 are proposed for the high-pressure phases. A nonempirical computation of the lattice dynamics of the crystal is carried out. It is shown that, under normal pressure, the cubic phase is stable down to T=0 K, while the application of a hydrostatic pressure gives rise to a phonon branch in the vibrational spectrum (between points R and M of the Brillouin zone) with negative values of squares of frequencies. The condensation of soft mode R₅ at the boundary point of the Brillouin zone leads to rhombohedral distortion of the cubic structure with the unit cell volume doubling. The calculated frequencies at q=0 of the ScF₃ lattice in the distorted rhombohedral phase are real-valued; the number and position of frequencies active in Raman scattering are in accord with the experimental values.     

Lattice dynamics and ferroelectric instability in ordered and disordered PbSc<Subscript>1/2</Subscript>Ta<Subscript>1/2</Subscript>O<Subscript>3</Subscript> and PbSc<Subscript>1/2</Subscript>Nb<Subscript>1/2</Subscript>O<Subscript>3</Subscript> solid solutions

The dynamic Born charges and the frequency spectra of lattice oscillations in the crystals of ordered and disordered PbSc_1/2Ta_1/2O₃ (PST) and PbSc_1/2Nb_1/2O₃ (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.     

Effect of Sr<Superscript>2+</Superscript> ion substitution with trivalent ions (Sc<Superscript>3+</Superscript>, In<Superscript>3+</Superscript>, La<Superscript>3+</Superscript>, Bi<Superscript>3+</Superscript>) on its ferroelectric instability in SrTiO<Subscript>3</Subscript>

The vibration frequencies of unstable ferroelectric and antiferrodistortion modes and the dependences of the energy on the ion displacement amplitude have been calculated within the generalized Gordon-Kim model for distortions along eigenvectors of these modes in the mixed compounds Sr_{1 − x} A _x Ti_{1 − x} /₄□ _x/4O₃ and Sr_{1 − y} A _2y /₃□ _y/3TiO₃ (A = Sc³⁺, In³⁺, La³⁺, Bi³⁺; □ is the vacancy). To compensate an excess positive charge, vacancies are introduced into the Ti⁴⁺ or Sr²⁺ site. Calculations have been performed in the “daverage” crystal approximation for impurity concentrations of 0.25 and 0.50. To this end, a set of 40 atomic superlattices with various orderings of heterovalent ions Sr²⁺ and impurity A ³⁺ has been considered. It has been found that each impurity type, independently of charge balance, induces ferroelectric instabilities in doped compounds. In the case of doping with In³⁺ and La³⁺ for concentration x = 0.25, the possibility of rotating the polarization vector has been shown.     

Lattice dynamics and the ferroelectric phase transition in ordered Pb2B′B″O6 (B′ = Ga, In, Lu; B″ = Nb, Ta) solid solutions

The lattice vibration spectrum, rf permittivity, and dynamic Born charges have been calculated for ordered Pb₂ B′B″O₃ (B′=Ga, In, Lu; B″=Nb, Ta) solid solutions in terms of the generalized Gordon—Kim model. It has been shown that all compounds exhibit a ferroelectric instability and that the frequencies of “soft” ferroelectric modes are close in magnitude. The ferroelectric phase-transition temperatures and the spontaneous polarization in the ferroelectric phase of the solid solutions under consideration have been calculated by the Monte Carlo method using the model Hamiltonian in the local mode approximation. The transition temperature is found to increase with increasing atomic number of the B′ ion.     

Study of phase transitions and the incommensurate phase in ACBX 4 crystals by the Monte Carlo method

The statistical model of successive phase transitions in the family of crystals described by the general formula ACBX ₄ has been studied. It is assumed that tetrahedral BX ₄ groups in the disordered tetragonal phase can be located in four equally probable equilibrium positions. Five representatives with different sequences of phase transitions in this family of crystals are considered. The effective equilibrium constants of interactions between tetrahedral groups are calculated within the framework of the electrostatic model. The corresponding phase diagrams and the thermodynamic characteristics of the phase transitions are studied by the Monte Carlo method. The experimental data provided the establishment of the sequence of phase transitions in Rb₂ZnCl₄ and K₂SeO₄ crystals with the formation of an intermediate incommensurate phase. The character of the structure modulation in this phase is established. The calculated thermodynamic characteristics of the phase transitions agree quite well with the experimental data for all the crystals under consideration.     

Thermodynamics of the two-dimensional superexchange model of Mitsui

L. V. Kirenski Institute of Physics, Siberian Branch of the USSR Academy of Sciences. Translated from Teoreticheskaya i Matematicheskaya Fizika, Vol. 79, No. 2, pp. 272–281, May, 1989.     

Mass spectrometric study of the stereoisomers of 2-furyl- and 2-phenyldecahydro-4-quinolinone and their tertiary alcohols

A study was carried out on the major pathways for the decomposition of stereoisomers of 2-furyl- and 2-phenyldecahydro-4-quinolinone and their tertiary alcohol derivatives upon electron impact. The elemental composition of the characteristic ions was determined by high-resolution mass spectrometry. The introduction of a heavy substituent at C₍₂₎ leads to marked changes in the directions of the major fragmentation pathways of decahydroquinoline. The dependence of the probability for the formation of characteristic ions on the molecular geometry in the stereoisomer series was demonstrated.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 242–247, February, 1986.     

Hydrostatic pressure-induced phase transitions in RbMnCl<Subscript>3</Subscript>: Raman spectra and lattice dynamics

Raman scattering spectra of RbMnCl₃ are measured at room temperature under high hydrostatic pressure. The results are interpreted based on first principles lattice dynamics calculations. The experimental data obtained correlate with the calculations in the low frequency domain but disagree slightly in the region of high-frequency vibrations. The transition from the hexagonal to the cubic perovskite phase observed earlier (near 0.7 GPa) was confirmed, and new transitions to lower symmetry distorted phases were discovered (at 1.1 and 5 GPa).     

Lattice dynamics and Raman scattering spectrum of elpasolite Rb<Subscript>2</Subscript>KScF<Subscript>6</Subscript>: Comparative analysis

Raman scattering spectra of elpasolite Rb₂KScF₆ are studied in a wide temperature range including two phase transitions: from the cubic to the tetragonal phase and then to the monoclinic phase. The experimental Raman scattering spectrum is compared with the lattice vibration spectra of these phases calculated using an ab initio approach. A number of anomalies (caused by structural rearrangement during the phase transitions) are revealed and quantitatively analyzed in the ranges of both the intramolecular vibrations of the octahedron molecular ScF₆ ions and low-frequency intermolecular lattice vibrations. The interaction between low-frequency intramolecular vibrations and the intermolecular modes is found to be significant, and strong resonance interaction of the rotational soft modes (which are recovered below the phase transition points) with hard low-frequency vibrations of the rubidium ion sublattice is detected. These interactions are shown to substantially complicate the spectra.