Infrared absorption investigation of the role of octahedral groups upon the phase transition in the Rb2KMoO3F3 crystal

The infrared absorption spectra of the oxyfluoride Rb₂KMoO₃F₃ have been measured in the frequency range corresponding to stretching vibrations of the Mo-O anion octahedron with the purpose of clarifying their role in the phase transition. A semi-empirical calculation of two possible configurations of quasioctahedral MoO₃F₃ groups has been performed. The results of the investigations have demonstrated that some octahedra in the crystal structure change the local symmetry from C _3v to C _2v due to the phase transition (T = 197 K).     

Thermodynamic properties and structure of oxyfluorides Rb<Subscript>2</Subscript>KMoO<Subscript>3</Subscript>F<Subscript>3</Subscript> and K<Subscript>2</Subscript>NaMoO<Subscript>3</Subscript>F<Subscript>3</Subscript>

According to the results of calorimetric and structural studies, the Fm{ie1202-1}m phase in K₂NaMoO₃F₃ remains stable at least to 100 K. No ferroelectric transformation assumed earlier has been revealed in a series of Rb₂KMoO₃F₃ samples prepared using various technologies. Only a phase transition of nonferroelectric origin has been observed near 195 K, and its thermodynamic characteristics have been determined. An analysis of the stability of the cubic structure of molybdenum fluorine-oxygen elpasolites-cryolites has been performed in the framework of the hypothesis on strengths of interatomic bonds. The barocaloric effect in Rb₂KMoO₃F₃ has been estimated.     

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.     

A Raman scattering study of the phase transition in the (NH4)3WO3F3 oxyfluoride

The Raman scattering spectra of polycrystalline samples of the (NH₄)₃WO₃F₃ perovskite-like oxyfluoride were measured at frequencies of 70–3600 cm^?1 in the temperature range 93–323 K, including the transition from the orientationally disordered cubic phase to the low-symmetry phase. Transitional anomalies in the spectral parameters were revealed in the frequency ranges of internal vibrations of the ammonium ions and WO₃F₃ octahedral groups. An analysis of the experimental results allowed the conclusion that the phase transition under study is associated primarily with the ordering of the octahedral groups.     

Etude des transitions de phases des composes Rb2KMO3F3, Cs2KMO3F3 et Cs2RbMO3F3 (M = Mo,W)

Crystallographic, micro-D.T.A., dielectric and pyroelectric measurements have been performed for the Rb₂KMO₃F₃, Cs₂KMO₃F₃ and Cs₂RbMO₃F₃ (M = Mo, W) compounds. Two types of transitions have been detected : the first one, for Rb₂KMoO₃F₃ at low temperature, is due to the disappearance of the superstructure detected for Rb₃MoO₃F₃α, the second one is of ferroelectric - paraelectric type (Rb₂KMO₃F₃ and Cs₂RbMO₃F₃ ; M = Mo, W). The transition temperatures have been compared vs the size of the alcaline ions and the nature of the M-O bonds. The increase of the transition temperatures from the A₂A'MO₃F₃ to the A₃MO₃F₃ compounds is likely due to the simultaneous presence of the A⁺ cations in the former ones in 6 and 12 coordination sites.     

Structural transformations in a single‐crystal Rb2NaYF6: Raman scattering study

This paper reports Raman spectroscopy investigation of phase transitions in Rb₂NaYF₆ crystal. The experimental spectra were compared with the calculated one. The spectra were obtained in temperature range from 8 to 300 K. The Raman spectra shows anomalous temperature‐dependent behavior at T₁ = 154 and T₂ = 122 K. Soft mode restoration has been found, which allows us to attribute first transition at 154 K to displacive type. Detailed analysis temperature dependencies of the line positions and widths have been performed. We found no effects of possible lattice disorder anywhere, except narrow (about 20 K) range above the T₁ temperature. The Raman spectra of Rb₂NaYF₆ crystal have been obtained and analyzed under hydrostatic pressure up to 4.33 GPa (at T = 295 K). The high pressure experiment up to 4.33 GPa did not disclose any effects associated with phase transitions. The lattice vibration spectra were calculated up to 10 GPa. The calculation has been demonstrated that the Rb₂NaYF₆ does not undergo high pressure phase transition. Copyright © 2013 John Wiley & Sons, Ltd.     

Vibrational spectra of disordered oxyfluoride (NH<Subscript>4</Subscript>)<Subscript>3</Subscript>WO<Subscript>3</Subscript>F<Subscript>3</Subscript>

Comparative analysis of the IR absorption and Raman scattering spectra of a polycrystalline sample of perovskite-type oxyfluoride (NH₄)₃WO₃F₃ has been performed in the frequency range 370–4000 cm^?1 at temperatures from 92 to 303 K, including the transition between the orientationally disordered cubic and low-symmetry phases. The conformation of WO₃F₃ octahedral groups is established and transitional anomalies of the internal modes of these groups and ammonium ions are revealed. Comparative analysis of the IR and Raman spectra suggests that the phase transition under study is mainly related to the ordering of octahedral groups and formation of W-O…H-N hydrogen bonds.     

Raman spectroscopic study of the phase transitions in the Cs2NH4WO3F3 oxyfluoride

The Raman spectra of Cs₂NH₄WO₃F₃ elpasolite crystals are studied in the temperature range 93–373 K at pressures of up to 6.3 GPa. No indication of a phase transition is revealed from the Raman spectra as the temperature decreases to 93 K. An analysis of the Raman spectra measured under pressure demonstrates that the Cs₂NH₄WO₃F₃ elpasolite crystals undergo a phase transition at a pressure of 2.58 GPa. Judging from the behavior of the pressure dependences of the vibrational frequencies, the revealed phase transition is associated with the lowering of the symmetry of the WO₃F₃ octahedra.     

Decoupling of the order-disorder and displacive contributions to the phase transition in NH4H(ClH2CCOO)2

The effects of hydrostatic pressure and substitution of Rb⁺for the ammonium cations on the ferroelectric phase transition temperature in NH₄H(ClH₂CCOO)₂ have been studied by electric permittivity measurements. The transition temperature (T_c) decreases with increasing pressure up to 800 MPa and the pressure coefficient dT_c/dp=−1.4×10⁻² [K/MPa] has been experimentally determined. The substitution of Rb⁺ for the ammonium cations has been shown to considerably lower the ferroelectric phase transition temperature T_c. In mixed crystals, additional electric permittivity anomaly has been clearly evidenced. The results are discussed assuming a model, which combines polarizability effects, related to the heavy ion units, with the pseudo-spin tunnelling.     

Central peak in the strontium titanate crystal near the tetragonal-to-cubic phase transition

Raman spectra of the SrTiO3 crystal have been measured in wide temperature (22?C316 K) and frequency (2?C1020 cm^?1) ranges. It has been shown that a central peak appears in low-frequency Raman spectra at temperatures above 70 K. In the spectral geometry with polarization rotation near the temperature T _c = 106 K of the cubic-to-tetragonal phase transition, the central peak exhibits properties of the order-disorder phase transition. Such a behavior of the central peak has been explained by the interaction of the low-frequency soft mode E _g with the relaxation mode near T _c .     

A comment on the phase transition in RbCaF3

The structural phase transition in RbCaF₃ has been examined by observing super lattice points by means of neutron diffraction. There is one structural phase transition with the R₂₅^x soft mode phonon condensation up to 4.2 K. The rotation angle of CaF₆ octahedra, Ø, is calculated as 8.64 degrees at 60 K. The temperature dependence of Ø is also given.     

Variable-temperature Raman scattering and X-ray diffraction studies of Bi3.25Nd0.75Ti3O12 ceramics

Neodymium-substituted bismuth titanate (Bi_3.25Nd_0.75Ti₃O₁₂, BNT_0.75) ceramics was prepared by chemical co-precipitation along with calcinations. The lattice instability has been investigated by variable-temperature Raman scattering and X-ray diffraction. The results showed that there was an orthorhombic to pseudo-tetragonal phase transition at about 695 K, in terms of the evolution of temperature dependence of Raman scattering frequencies. Some changes at about 695 K in the XRD lines, the lattice parameters (a, b, and c) as well as the orthorhombic distortion b/a have been detected in the high temperature X-ray diffraction, which confirmed the conclusion that the BNT_0.75 ceramics undergoes a ferroelectric to paraelectric phase transition at about 695 K.     

A raman scattering study of the phase transition in the ammonium oxyfluoride (NH4)3TiOF5

Raman spectra of a polycrystalline sample of the perovskite-like oxyfluoride (NH₄)₃TiOF₅ are measured in the frequency region 100–3600 cm^?1 at temperatures ranging from 91 to 370 K under hydrostatic pressures of up to 9 GPa, which include the range of the phase transition from the orientationally disordered cubic phase to the low-symmetry phase. Anomalies in the spectral parameters due to the phase transition are revealed in the range of vibrations of TiOF₅ octahedral groups.     

Heat capacity,structure, and <Emphasis Type="Italic">p-T</Emphasis> phase diagram of elpasolite (NH<Subscript>4</Subscript>)<Subscript>2</Subscript>KMoO<Subscript>3</Subscript>F<Subscript>3</Subscript>

Thermophysical and structural studies of an (NH₄)₂KMoO₃F₃ crystal show that this crystal belongs to the family of elpasolites (space group \(Fm\bar 3m\)) and undergoes an order-disorder phase transition at T ₀ = 241.5 K. Under hydrostatic pressure, this phase transition splits into two consecutive transitions at the tricritical point with parameters T _tr = 232.5 K and p _tr=0.21 GPa. It was found that anomalous hysteresis and relaxation phenomena accompany the transitions from the cubic to both distorted phases. The results are analyzed taking into account the data on the phase transition in the related elpasolite (NH₄)₂KWO₃F₃.     

Changes in the structure of the Rb2NaYF6: Yb3+ crystal upon transition from the cubic to tetragonal phase

The pattern of lattice distortions occurring in the vicinity of Yb³⁺ ions during the transition of the Rb₂NaYF₆: Yb³⁺ crystal from the cubic to tetragonal phase has been revealed using all the parameters of the empirically found crystal fields for paramagnetic centers of the Yb³⁺ ions with cubic and tetragonal symmetry. It has been shown that the YbF₆ octahedra are rotated about the fourfold axis through an angle approximately equal to 1.2°. Moreover, the octahedra themselves are deformed so that the F^? ions symmetrically located in the plane perpendicular to the axis of rotation come close to the impurity ion at a distance of 0.0004 nm. The fluoride ions located on the axis of rotation, conversely, move away from the Yb³⁺ ion at a distance of 0.0005 nm. Based on the obtained results, it has been concluded that the total condensate of order parameters of the studied phase transition involves not only the critical rotations of octahedral groups but also the noncritical displacements of atoms in the rotated octahedra.     

Temperature dependence of the lattice parameters in the S1G and S2G phases of HxBPA

The temperature dependence of the monoclinic lattice constants in the two smectic phases, S¹_G and S²_G, of HxBPA, has been obtained from X-ray diffraction data, in the temperature range 300 K < T < 240 K. The variation of b (unique axis) is consistent with the chain ordering in the S²_G phase indicated by Raman and NMR measurements.     

Optical studies of phase transitions in the (NH<Subscript>4</Subscript>)<Subscript>3</Subscript>Ti(O<Subscript>2</Subscript>)F<Subscript>5</Subscript> crystal

Polarization-optical study of twinning and measurements of the Raman spectra and birefringence in oxyfluoride (NH₄)₃Ti(O₂)F₅ were carried out over the temperature range 90–350 K. Phase transitions were detected at temperatures T ₀₁ = 266 K (second-order transition) and T ₀₂ = 225 K (first order). It is assumed that the crystal symmetry is changed as follows: Fm3m ? I4/mmm ? I4/m. Anomalies of the spectral parameters are established in the frequency range of internal vibrations of ammonium ions and Ti(O₂)F₅ complexes. An analysis of the results shows that the transition at T ₀₁ is likely due to small shifts of the tetrahedral groups from their position on the triad axis and that the transition at T ₀₂ is due to fluorine-oxygen ordering of Ti(O₂)F₅ complexes.     

Raman scattering study of temperature and hydrostatic pressure phase transitions in Rb2KTiOF5 crystal

Raman spectra of Rb₂KTiOF₅ crystal were obtained and analyzed in the temperature range from 77 to 297 K and under hydrostatic pressure up to 4.2 GPa (at T = 295 K). The experimental results were compared with quantum‐chemical simulation of TiOF₅ pseudo‐octahedron. To interpret effects of lattice ordering, phonon spectra of several ordered phases of Rb₂KTiOF₅ were calculated within ab initio generalized Gordon–Kim model, and ordering of TiOF₅ molecular groups were simulated within Monte Carlo approach. The spectra exhibited orientation disordering in the cubic phase under ambient conditions. Cooling below the phase transition temperature (215 K) leads to partial ordering of the structure. The isotropic perovskite‐like phase was found to undergo first‐order transition into a low‐symmetry anisotropic phase at about 1 GPa. Further compression up to 4.1 GPa did not show any effects associated with phase transitions. Copyright © 2011 John Wiley & Sons, Ltd.     

Br NQR relaxation and successive phase transitions of CH3NH3HgBr3

The measurement of ⁸¹Br NQR in CH₃NH₃HgBr₃ has been carried out in the temperature range between 80 and 300 K using a pulse NQR method. The temperature dependence of ⁸¹Br NQR frequencies in CH₃NH₃HgBr₃ has revealed that it undergoes three characteristic successive phase transitions at T?=?123, 184 and 239 K. The phase transition temperature at T?=?239 K is the second-order type, whereas those at T?=?184 and 123 K are the first-order nature of the phase transitions. Each phase transition seems to be closely related to the motions of methyl ammonium cation as a partial or whole. The enhancement of 1/T ₁ at T?=?239 K indicates the onset of the molecular motion of the cation as a whole with increasing temperatures.     

Phase transitions in RbCaF3. I: Optical studies

Phase transitions in RbCaF₃ have been investigated by optical birefringence measurements and Raman scattering experiments. A near discontinuity in the onset of spontaneous birefringence at 196 K shows that the cubic to tetragonal phase change is “slightly” first order. The Raman spectra of the tetragonal phase support a D¹⁸_4h structure in which two phonons of A_1g and E_g symmetry soften as the 196 K transition is approached from lower temperatures. A very slow transition to a lower symmetry structure was observed at about 42 K in the Raman measurements.