Vibrational spectroscopy at high pressure: Part 53. Alkali metavanadates and copper metagermanate

Essentially complete Raman and infrared spectra at ambient and 80 K have been obtained for the isostructural metavanadates AVO₃, (A  K, Rb, Cs). An assignment is offered on the basis of line and factor group analyses. A similar study has been made on CuGeO₃. Under pressure up to 100 kbar both KVO₃ and RbVO₃ show a single phase transition, at 56 and 53 kbar. The remarkable insensitivity of the transition pressure to change of cation implies that the major effect of pressure is chain torsion and repacking, driven by the need to reduce the cation coordination number.     

Phase transitions and pseudo-spin description in the perovskite CH3NH3PbCl3

The differential scaning calorimetry study of the compound of formula CH₃NH₃PbCl₃ in the temperature range 168 K–181 K shows two phase transitions according to the phase sequence: phase III (P222₁) ← (172K) → phase II (P4/mmm) ← (179K) → phase I (Pm3m) These transitions were characterized by X-ray diffraction, DSC and Raman scattering. The III transition involves mainly the rotational and orientational motions of the organic cation, whereas the antiferroelectric transition at 172K is connected with a complex mechanism involving the freezing of the cation motions coupled with the distorsion of the PbCl₆ octahedra This sequence has been also analysed in terms of pseudo-spin theory.     

Raman spectra and the dynamic structure of the high pressure phase of NH4Br (V) and ND4Br (V)

Similar Raman spectra are observed at high pressures for phases II and V of ND₄Br and NH₄Br. Deuteration lowers the II–V phase transition from 20 to 9 kbar at 296 K. ND₄Br V and NH₄Br V are interpreted as mixed phases, and their spectra as the superposition of the spectra of two other phases, III (antiparallel arrangement of the NH₄⁺ ions) and IV (parallel arrangement). The phonons which become Raman inactive at the V–IV phase transition are assigned to clusters or domains of phase V which have antiparallel arrangement.     

Structural studies of ammonium nitrate 1. Phases III,IV and V

The laser Raman spectra of NH₄NO₃ and ND₄NO₃ have been measured between 210 and 320 K. It is shown that the phase transition V → IV is probably a λ transition which occurs gradually between 210 and 256 K with an abrupt change at 256 K. The λ transition is due to rotational disorder of ammonium ions as shown by the localised disorder mode at 172 cm^?1. The spectrum of phase IV shows clear evidence of T and L components of the nitrate ion asymmetric stretch. This is inconsistent with the assigned space group Pmmn. An explanation based on a thermally inducted IV → III transition is proposed.     

Spectroscopy at very high pressures: Part 27. Raman study of the second-order phase transitions in sodium nitrite and sodium nitrate

Raman spectra of NaNO₂ have been studied as a function of hydrostatic pressure to 40 kbar at 295 and 348 K. Slight changes in slope of mode frequency versus pressure plots support the view that a structural anomaly exists at 9 ± 1 kbar. The absence of qualitative changes in the Raman spectra allow the space group of NaNO₂ IV to be specified as one of P1, P2, B2, Pm or Bm. The Raman spectrum of NaNO₃ has been studied to 87 kbar. The changes observed are fully consistent with a second-order transition to a phase with symmetry C⁶_3v, as indicated by previous X-ray work, although the transition is sluggish.     

Infrared and Raman studies of phase transitions in metal–organic frameworks of [(CH3)2NH2][M(HCOO)3] with M=Zn,Fe

We report on temperature-dependent infrared (IR) and Raman studies of [(CH₃)₂NH₂][M(HCOO)₃] metal–organic frameworks (MOFs) with M=Zn, Fe. Based on Raman and IR data, an assignment of the observed modes to respective vibrations of atoms is proposed. Temperature-dependent studies revealed abrupt changes below 160 K that are attributed to the onset of first-order structural phase transition. The most pronounced changes are observed for the modes corresponding to the dimethylammonium cation, especially those involving motion of hydrogen atoms. This behavior proves that the phase transition has an order–disorder character and is associated with the ordering of protons. The abrupt splitting of some modes related to the formate ion indicates that this transition is also associated with significant distortion of the metal-formate framework.     

A Raman study of multiferroic LuMnO3

Magnetic and dielectric measurements confirm the multiferroic nature of LuMnO₃. Raman spectra of LuMnO₃ have been recorded in the 77–800 K range covering both the antiferromagnetic transition at 90 K and the ferroelectric–paraelectric transition at 750 K. The changes in the phonon modes frequencies and band-widths indicate the presence of phonon–spin coupling in the antiferromagnetically ordered phase. The ferroelectric–paraelectric transition is accompanied by the broadening and disappearance of many of the phonon modes. Some of the phonon modes also show anomalies at the ferroelectric transition.     

On [Na(POCl3)4]+ Complex Ion Formation: Conductivity,Raman, and NMR studies in the system phosphoryl chloride–sodium tetrachloro aluminate and related compounds

The system POCl₃–NaAlCl₄ was investigated by measuring the conductivity and the Raman and NMR spectra (²⁷Al, ²³Na and ³¹P) as a function of the mol fraction x of NaAlCl₄ in POCl₃. Additionally, Raman spectra of POCl₃ solutions of NaFeCl₄, LiAlCl₄, LiFeCl₄, and KAlCl₄ were recorded. In solutions containing Li⁺ or Na⁺ ions a liquid to solid (or jelly) phase transition was observed under certain conditions, dependent on salt concentration and temperature. Observed changes in the Raman spectra of the electrolyte solutions in comparison to the pure solvent POCl₃ demonstrate the existence of interactions. Clearly, the POCl₃ eigenfrequencies and hence the molecules are pertubed. The formation of [M(POCl₃)₄]⁺ complexes (M = Li, Na) can be deduced from the Raman measurements. NMR investigations support this conclusion. For assigning of Raman spectra, (Li⁺, K⁺) cation and ([FeCl₄]^?, [SbCl₆]^?) anion substitutions were employed.     

Observation of a phase transition in ThBr4 and ThCl4 single crystals by far-infrared and Raman spectroscopy study

At 4 K the visible and infrared absorption and emission spectra of U⁴⁺ in ThBr₄ and ThCl₄ single crystals are not very consistent with what is predicted by the selection rules for the room temperature structure. Thus we investigated Raman scattering in the temperature range 10–300 K to look for a structure change and obtain a better understanding of the spectroscopy of U⁴⁺ in ThBr₄ and ThCl₄. At room temperature, the observed Raman lines have been assigned on the basis of a D_4h factor group analysis. The study of the temperature dependence of the Raman spectra permitted us to discover phase transitions of ThBr₄ and ThCl₄ at 95 and 70 K, respectively. The splitting observed for the strongest E_g symmetry mode shows a lowering of the symmetry below the transition point. Powder X-ray diffraction at 77 K of hygroscopic ThBr₄ is being carried out to determine the low-temperature structure.     

The vibrational spectra and crystallographic properties of CsPF6

Indications are that CsPF₆(I) at ambient conditions is cubic with a possible space group of Fm3m-O⁵_h. A slight distortion of the unit cell cannot, however, be ruled out. Assuming Fm3m symmetry the Raman spectra of CsPF₆(I) are consistent with a disordered model in which the PF^?₆ ions are tilted away from the crystallographic axes. The phase transition which occurs below 90 K in CsPF₆ is reflected in the vibrational spectra and further significant changes occur below 60 K particularly in the Raman bands. It is not yet clear whether these changes represent the establishment of long-range order or whether a further phase of CsPF₆ exists below 60 K. A possible structure for CsPF₆ at very low temperatures is discussed.     

High-pressure Raman scattering of MgMoO4

In this paper, we present results of high-pressure Raman scattering studies in β-MgMoO₄ from atmospheric to 8.5 GPa. The experiments were carried out using methanol–ethanol as pressure medium. By analyzing the pressure dependence of the Raman data (change in the number of lattice modes, splitting of bands and wavenumber discontinuities) we were able to observe a phase transition undergone by the β-MgMoO₄ at 1.4 GPa, which is only completed at ∼5 GPa. The transition was observed to be irreversible and the modifications in the Raman spectra were attributed to the changes in coordination of Mo ions from tetrahedral to octahedral. The transition possibly changes the original C2/m symmetry to C2/m or to P2/c. Implication on the phase transition for similar molybdate structures, such as α-MnMoO₄, is also highlighted.     

Vibrational spectra and rotational barriers of methyl titanium halides CH3TiX3 and CD3TiX3 (X = Cl,Br, I)

The IR and Raman spectra of gaseous and solid CH₃TiX₃ and CD₃TiX₃ species (X = Cl, Br, I) are reported. The gas phase spectra have been recorded between 4000 and 20 cm^?1 at pressures of 1 atm and 4 atm at 350 K and the Raman spectra of the solid phase recorded at 4.2 K. Internal rotation barriers and thermodynamic functions have been calculated.     

Pressure induced phase transformation in U2O(PO4)2

The high pressure behavior of U₂O(PO₄)₂ has been investigated with the help of Raman scattering and X-ray diffraction measurements up to ∼14 and 6.5 GPa, respectively. The observed changes in the Raman spectra as well as the X-ray diffraction patterns suggest that U₂O(PO₄)₂ undergoes a phase transition at ∼6 GPa to a mixture of a disordered ambient pressure phase and a new high pressure phase. The new phase resembles the triclinic mixed-valence phase of uranium orthophosphate (U(UO₂)(PO₄)₂). On release of pressure the initial phase is not retrieved.     

Pressure-induced phase transitions in multiferroic RbFe(MoO4)2—Raman scattering study

High pressure Raman scattering experiments were performed on RbFe(MoO₄)₂. These experiments revealed that two phase transitions take place in RbFe(MoO₄)₂ at very low pressures, i.e. between ambient pressure and 0.2 GPa and between 0.4 and 0.7 GPa. Raman results showed that at the first phase transition the room temperature P3?m1 phase transforms into the P3? phase, which is also observed at ambient pressure below 190 K. The second pressure-induced phase transition occurs into a low symmetry phase of unknown symmetry. The performed lattice dynamics calculations for the P3?m1 phase and ab initio calculation of the structural changes under hydrostatic pressure helped us to get better insights into the mechanism of the observed phase transitions.     

Structural transitions under high-pressure in a langasite-type multiferroic Ba3TaFe3Si2O14

The iron containing langasite family compound Ba₃Ta⁵⁷Fe₃Si₂O₁₄ was studied at high pressure up to 30 GPa at room temperature by means of in situ X-ray diffraction, Raman and Mössbauer spectroscopies in diamond anvil cell. Two structural transitions at pressures ∼5 and ∼20 GPa are observed. At ∼5 GPa, the low-pressure trigonal P321 phase undergoes phase transition to the most likely P3 structure as manifested by slight increase in the c/a ratio and by anomalies of the Mössbauer and Raman spectra parameters. At ∼20 GPa, the first order phase transition to monoclinic structure occurred with a drop of unit cell volume by 9%. The appearance of the ferroelectric state at such transitions is discussed in connection with the multiferroic properties.     

Vibrational spectrum of crystalline cyclopentanone

Infrared and Raman spectra of liquid and solid cyclopentanone have been studied. Spectroscopic evidence shows that the molecule undergoes a solid state phase transition at 168°K. The crystalline form stable below this temperature is ordered and close-packed; its infrared and Raman spectra have been interpreted assuming that cyclopentanone crystallizes in the C⁵_2h or C¹₁ centrosymmetric space groups.     

Spectroscopic investigations of phase transition in 5-chloro-2-pyridone

Spectroscopic investigation of 5-chloro-2-pyridone has been carried out in the temperature range 77–300 K. At room temperature the ³⁵Cl NQR spectrum shows a single line at 35.618 MHz, but at 250.7 K two lines appear at 35.850 MHz and 35.840 MHz respectively indicating the presence of a phase transition. IR, far-IR, laser Raman and dielectric measurements have been carried out to investigate the phase transition further. Low temperature IR studies show splitting of ν(CCl), β(NH) and ν(CO) bands at T_c. Dielectric measurements show a small, but finite, change in the value of the dielectric constant around T_c. Raman spectra at different temperatures support the existence of a new phase, as shown by the appearance of a new band at 81 cm⁻¹, the frequency of which changes slowly as T_c is approached and which disappears at T_c. The temperature dependence of the NQR frequencies has been analysed using Bayer Kushida and Brown equations toevaluate the torsional frequencies.     

Molecular motions and λ phase transition: Raman and far-ir studies of neat and isotopic mixed hexamethylbenzene crystal

Raman and far-IR studies of hexamethylbenzene (HMB) low temperature crystalline phases (neat and isotopic mixed) are presented. The Raman phonon spectrum changes drastically during the λ-phase transition. Site splittings on two intramolecular modes of HMB-h₁₈ (but on only one corresponding HMB-d₁₈ mode) are observed in the room temperature phase (phase II) spectrum. These splittings disappear in the lower temperature phase (phase III). The methyl torsional bands are identified and a significant shift is observed for them during the phase transition. Also, while the λ-phase transition takes place at 113°K (± 2°K) for HMB-h₁₈ crystal, the transition temperature is 133°K (± 2°K) for HMB-d₁₈. Our results suggest that (for phonon interactions) the symmetry in phase Ill is close to D_3d and reduces to C_i in phase II. Furthermore, the results support the mechanism of phase transition which involves a tilting of the methyl groups out of the benzene ring.     

Features of the ion mobility and phase transitions in hexafluoro complex compounds of tantalum(V), niobium(V), and titanium(IV) with the tetramethylammonium cation

¹⁹F, ¹H NMR and DSC methods are used to study the ion mobility and phase transitions in hexafluoro complex compounds of tantalum(V), niobium(V), and titanium(IV) with the tetramethylammonium cation. A transformation of the NMR spectra observed in a temperature range 77(130)-450 K is found to be associated with a change in the type of ion motions in the anion and cation sublattices of the studied compounds. In a temperature range 170-450 K the main types of ion motions are isotropic reorientations of TaF₆, NbF₆, TiF₆ octahedra and tetramethylammonium ions. Two endothermic effects with the maxima at 232.5 K and 256.5 K for [N(CH₃)₄]TaF₆ and 235 K and 250 K for [N(CH₃)₄]NbF₆, which are observed in the DSC curve in the range 170-400 K correspond to phase transitions that have almost no effect on the NMR spectral parameters the a temperature range 230-260 K. For the [N(CH₃)₄]₂TiF₆ compound, an endothermic effect at 422 K is observed, corresponding to the phase transition from the rhombohedral to the cubic modification.     

Detection of an ordering of lithium sublattice in LiKSO4 in low-temperature phases via raman scattering

The polarized Raman spectra of LiKSO₄ in low-temperature phases have been investigated and the group-theoretical analysis of normal vibrations in all possibly realizing phases has been carried out. The mechanism of phase transition at 38 K, consisting in the gradual ordering of Li⁺ sublattice, is proposed. The dependence of some physical characteristics on the rate of the cooling-heating cycle is discussed.