A raman study of hydrostatic pressure induced phase transitions in Rb2KInF6 crystals

The Raman spectra of the elpasolite (Rb₂KInF₆) crystal have been studied in the pressure range from 0 to 5.3 GPa at a temperature of 295 K. A phase transition at a pressure of approximately 0.9 GPa has been found. An analysis of the variations in the spectral parameters has led to the conclusion that the phase transition to a distorted phase is accompanied by the doubling of the volume of the primitive cell of the initial cubic phase. Numerical calculations of the lattice dynamics in the Rb₂KInF₆ crystal have been performed. The numerical simulation has established that the phase transition at a pressure of 0.9 GPa is associated with condensation of the F _lg mode. A probable high-pressure phase is the phase with space group C2/m.     

Study of temperature fields in a rectangular plate with a temperature-dependent internal source with the aid of fast expansions

The approximate analytic solution of the problem of temperature field in a rectangular plate with an internal temperature- dependent source is obtained by the method of fast expansions. The critical value of a parameter characterizing heat release, which fundamentally affects the analytic solution form, is found. The maximum solution error is shown to amount to 0.02 at the consideration of the first three terms of the Fourier series in fast expansion. Temperature fields are presented, and an analysis of the influence of the plate sizes and the heat release magnitude on their formation is given. Recommendations on the plate shape choice are given.     

Powder diffraction and Raman spectroscopic study of lawsonite at high water pressure

The structural behavior of natural lawsonite CaAl₂Si₂O₇(OH)₂ · H₂O in aqueous and nonaqueous media (pressure up to 9.5 GPa) has been studied by synchrotron powder diffraction and Raman spectroscopy. The volume compressibility of lawsonite is found to be similar in both aqueous and nonaqueous media, while irreversible amorphization is observed only under compression in the aqueous medium to a pressure of about 6 GPa. Along with the observed increase in framework vibration frequencies, this reveals that the lawsonite structure is unstable when hydrostatic conditions of compression differ from those provided by crystallization of ice VII in an aqueous medium.     

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.     

Phase transition in the layered perovskite CsScF4 induced by hydrostatic pressure

Using polarization microscopy and Raman scattering, we have discovered a new phase transition in the layered perovskite crystal CsScF₄ under a hydrostatic pressure around 6 GPa. The proposed space group of the high-pressure phase is C _2h ² , Z=4. Fiz. Tverd. Tela (St. Petersburg) 41, 1683–1685 (September 1999)     

Raman Scattering in Hexagonal Diamond

Polycrystalline powder of a diamond–lonsdaleite system from the Popigai impact crater (Siberia) is studied via UV micro-Raman scattering (RS) and X-ray diffraction. A spectrum of pure lonsdaleite is obtained by subtracting two experimental Raman spectra of diamond–lonsdaleite samples with close ratios of diamond and lonsdaleite. The maximum contribution to the difference spectrum is due to the difference between the contribution from lonsdaleite, and not to the change in the bandwidth of the diamond. Deconvolution of the lonsdaleite spectrum allows isolation of all three vibrational modes (E_2g, A_1g, and E_1g) active in the first-order RS, the positions of which agree well with the results from first-principles calculations.     

Raman Scattering in Lonsdaleite

We present the results of our study of a polycrystalline diamond–lonsdaleite powder from the Popigai crater (Siberia) using UV micro-Raman scattering and synchrotron X-ray diffraction. By subtracting two experimental Raman spectra of diamond–lonsdaleite samples with a close ratio of the diamond and lonsdaleite fractions, when the maximum contribution to the difference spectrum is related to the difference of the lonsdaleite contributions and not to the change in the diamond band width, we have managed to obtain the spectrum of “pure” lonsdaleite. Its deconvolution has allowed us to identify all three Raman-active vibrational modes E_2g, A_1g, and E_1g whose positions agree well with the results of ab initio calculations.