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.     

Raman intensities of the lattice modes of KH1−xDxF2 as the order parameter for phase transition

In this work, the Raman intensities of the librational mode of the bifluoride ion and the translational mode of the K⁺ ion are considered as the order parameters for the phase transition of KH_1−xD_xF₂ since group theoretical consideration shows that the Raman active translational and librational modes in the low temperature phase will be inactive in the high temperature phase if the latter is of symmetry group O or higher. The measured Raman intensities are correlated with the temperature from 163°C to 196°C to elucidate the exponent β which relates the Raman intensity to the temperature close to T_c through (T^c-T)^β for various deuteration compositions. The β value of KHF₂ is very close to that predicted by the Ising model of three-dimensional space with a one-dimensional order parameter, suggesting that the nearest-neighbour interaction of the translational modes can play a very important role for the phase transition. Experiments also show that the phase transition temperature is slightly isotope composition dependent.     

Continuous and discontinuous phase transitions in ammonium halides

Raman spectra of NH₄Cl and NH₄Br have been recorded as functions of temperature and pressure. The λ-type phase transition in NH₄Cl has been studied as (i) a weakly first order. (ii) a tricritical and (iii) a second order transition. A strongly first order transition has been studied in NH₄Br. The analysis of the data has concentrated on the correlation of frequency shift with volume change across the phase change regions. This correlation has been established for the frequencies of the ν₂ and ν₅ Raman modes of NH₄Cl at zero pressure (1st order) 1.6 kbar (tricritical) and 2.8 kbar (2nd order), and the frequencies of the ν₅ Raman mode of NH₄Br at zero pressure (1st order). A single Y (mode Grünelsen parameter) has been shown to describe each frequency shift right through the phase change region once an order-disorder contribution has been introduced at and below the transition temperatures.     

High Performance Full Sodium‐Ion Cell Based on a Nanostructured Transition Metal Oxide as Negative Electrode

A novel design of a sodium‐ion cell is proposed based on the use of nanocrystalline thin films composed of transition metal oxides. X‐ray diffraction, Raman spectroscopy and electron microscopy were helpful techniques to unveil the microstructural properties of the pristine nanostructured electrodes. Thus, Raman spectroscopy revealed the presence of amorphous NiO, α‐Fe₂O₃ (hematite) and γ‐Fe₂O₃ (maghemite). Also, this technique allowed the calculation of an average particle size of 23.4 Å in the amorphous carbon phase in situ generated on the positive electrode. The full sodium‐ion cell performed with a reversible capacity of 100 mA h g^?1 at C/2 with an output voltage of about 1.8 V, corresponding to a specific energy density of about 180 W h kg^?1. These promising electrochemical performances allow these transition metal thin films obtained by electrochemical deposition to be envisaged as serious competitors for future negative electrodes in sodium‐ion batteries.     

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.     

The study of phase transition of KHF2 doped with sodium ions with Raman intensities of the lattice modes as the order parameter

The phase transition of doped K_1-xNa_xHF₂ is studied with the K⁺ translatory mode and the HF⁻₂ librational mode, which are Raman active in the low temperature phase while inactive in the high temperature phase, as the order parameter. Particular attention is paid to the elucidation of the critical exponent β which is defined as the parameter relating the lattice Raman intensity to the temperature near T_c through (T_c - T)^β. β values for various dopings are interpreted to understand the dynamics of the phase transition in the critical region. The analysis shows that a very small amount of doping results in a drastically large effect in the critical region. Further doping does not induce further a critical effect in a proportional way. This enables one to define the undoped and doped systems with a very small amount of dopant as two distinct states.     

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.     

Vanadium(V) environments in bismuth vanadates: A structural investigation using Raman spectroscopy and solid state V NMR

The Bi₂O₃---V₂O₅ system was examined using Raman spectroscopy and solid state ⁵¹V wideline, magic-angle spinning (MAS), and nutation NMR spectroscopy. The methods are shown to be complementary in the identification of the various phases and in the characterization of their vanadium site symmetries. Most of the compositions examined (1:1 ≤ Bi:V ≤ 60:1) are multiphasic. Depending on the Bi:V ratio, the following phases have been identified: BiVO₄, Bi₄V₂O₁₁, a triclinic type-II phase, a cubic type-I phase, γ-Bi₂O₃ doped with V(V) (sillenite), and β-Bi₂O₃. Detailed spectroscopic characterization reveals that vanadium is tetrahedrally coordinated in all these compounds, and that the degree of symmetry increases with increasing Bi:V ratio. At the highest Bi:V ratios, the combined interpretation of the Raman and NMR data provides strong evidence for the presence of Bi⁵⁺O₄ tetrahedra.     

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.     

Strain correlated effect on structural,magnetic, and dielectric properties in Ti4+ substituted Bi0.8Ba0.2Fe1−xTixO3

Ti⁴⁺ substituted Bi_0.8Ba_0.2Fe_1−xTi_xO₃ for x = 0.0, 0.1 and 0.2 are prepared by modified solid state reaction method. The prepared samples sintered at 850 °C for 1 h show a single phase nature. A structural change was observed on Ti⁴⁺ substitutions are confirmed through X-ray Diffraction, Fourier Transform Infrared spectroscopy and Raman spectra. An anomalous phase transition is observed in Bi_0.8Ba_0.2FeO₃ at 1173 K. The absence of ferroelectric transition and enhancement of decomposition temperature is observed in the substituted samples from the thermal analysis. A dielectric spectroscopic measurement shows that on Ti⁴⁺ substitutions, the magnitude of dielectric constant and loss tangent (tan δ) value is decreased. Vibrating Sample Magnetometer (VSM) study shows both antiferromagnetic and ferromagnetic phases coexist in the M−H curve. On Ti⁴⁺ substitutions in Bi_0.8Ba_0.2FeO₃, the antiferromagnetism dominates over the ferromagnetic phase. In corroboration to magnetisation process, ZFC–FC measurement confirms it that on Ti⁴⁺ substitution, the antiferromagnetic behaviour gets dominated. The report suggests that the interplay of strain upon Ti⁴⁺ substitution causes the structural and magnetic phase transition.     

Laser Raman study of single crystals of deuterated cadmium fluorosilicate

A laser Raman study of oriented single crystals of CdSiF₆:6D₂O at room temperature as well as at 10 K in all the six polarization geometries has been made. Also a detailed temperature dependent Raman study from room temperature to 10 K in (yy) polarization in the region of internal vibrations and lattice modes has been made. Abrupt changes have been observed in frequency shift, line-width and intensity of some of the bands at about 235 K. Some doubly degenerate modes show splitting at the same temperature. From these observations a phase change at about 235 K is inferred for this salt. It has been suggested that when the system is cooled, the lattice contracts and the water molecules lose their reorientational freedom to some extent which might result in the distortion of the [Cd(OD₂)₆]²⁺ octahedra triggering a phase transition.     

Thermo-physical characterization of some paraffins used as phase change materials for thermal energy storage

Three phase change paraffinic materials (PCMs) were thermophysically (phase-transition temperatures, latent heat, heat capacity at constant pressure, density, and thermal conductivity) investigated in order to be used as latent heat storage media in a pilot plant developed in Plovdiv Bulgaria. Raman structural investigation probes aliphatic character of the E53 sample, while the E46 and ECP samples contain also unsaturated components due to their Raman features within 1,500–1,700 cm^?1 range. Orthorhombic structure of the three PCMs was evidenced by the Raman modes at the 1,417 cm^?1. The highest latent heat value, ΔH, of phase transitions among the three materials was represented by summation of a solid order–disorder, and melting latent heat was encountered by the E53 paraffin, i.e., 194.32 J g^?1 during a μ-DSC scan of 1 °C min^?1. Conversely, the ECP composite containing ceresin component shows the lowest latent heat value of 143.89 J g^?1 and the highest thermal conductivity of 0.46 W m^?1 K^?1 among the three phase change materials (PCMs). More facile melt-disordered solid transition with the activation energy of 525.45 kJ mol^?1 than the lower temperature transition of disorder–order (E _a of 631.73 kJ mol^?1) during the two-step process of solidification for the E53 melt are discussed in terms of structural and molecular motion changes.     

Vibrational spectroscopy at very high pressures: Part 21. Raman and infrared study of mercury(II) iodide [1]

The solid phase behaviour of HgI₂ is reviewed, together with IR and Raman data for the red and the high-temperature yellow forms. New IR transmission, single-crystal reflectance, and Raman data were obtained at temperatures down to 80 K for red HgI₂ but no evidence of a temperature-induced transition to HgI₂ IV was found.The high pressure results showed a possible break in the slope of the pressure versus frequency plot for the 29 cm^?1 Raman mode of red Hgl, at 6 ± 0.5 kbar, close to the reported III/IV phase boundary. Both Raman and far-IR spectra of the high pressure form of yellow HgI₂ differ significantly from those of high temperature yellow HgI₂ : the two forms are regarded as having different structures. These, and other data, reveal a much closer similarity between the phase diagrams of HgBr₂ and HgI₂ than has been suspected hitherto.     

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.     

Structural and low temperature Raman scattering studies in SrTi<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Co<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>3</Subscript> nanoparticles synthesized by sol–gel method

We investigate effects of Co dopant concentration on the structure and low temperature Raman scattering properties in SrTi_1−x Co _x O₃ (x = 0.00, 0.10, 0.20, 0.30) nanoparticles prepared by sol–gel method. The dopant induced changes are studied by XRD, and Raman scattering measurements. The results show an average particle size of about 20 nm depending on the Co content and the lattice parameters decrease as increasing the Co content. In the Raman spectra, a broad structure in the region 100–500 cm⁻¹ is almost absent and the peaks in the region 600–800 cm⁻¹ show different weights with respect to SrTiO₃, relating to structural changes. The anomalous change in the area ratio of Raman peaks as function of temperature suggests a phase transition in our samples in the range of 110–130 K. These results indicate that the Co ion has replaced the site of Ti in unit cell. This novel route also demonstrates the advantage of synthesizing the compound with low annealing temperature.     

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.     

Mesophase formation in lead(II) decanoate

Abstract

Structures of the thermotropic mesophases of lead(II) decanoate are reassigned following optical and X-ray diffraction studies. These results, and those of D.S.C., Raman and ²⁰⁷Pb N.M.R. spectroscopy, indicate formation of a lower temperature mesophase involving mainly increased lateral disorder, and a higher temperature L_α (smectic A) phase resulting from chain disordering and decreased lead-carboxylate interaction. Comparison of experimental thermodynamic data for the phase transitions with theoretical data in the literature indicates that the entropy change for the lower to higher mesophase transition is dominated by the increase in chain disorder.     

Vibrational spectra and dynamics of crystalline malonic acid at room temperature

The i.r. and Raman spectra of malonic acid and its isotopic derivatives CD₂(COOD)₂ and CH₂(C¹⁸O¹⁸OH)₂ were studied in the β phase. They are assigned and discussed with the help of a normal coordinate analysis and of a lattice-dynamical calculation which give information about the couplings and the intermolecular forces. These calculations confirm a mechanism proposed for the β ↔ α phase transition at 360 K.     

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.