Raman Spectra Of Phases Research Articles

Mechanochemical induced structural changes in sucrose using the rotational diamond anvil cell

The response of sucrose to high-pressure and shear conditions has been studied in a rotational diamond anvil cell. Previous experiments conducted by Bridgman and Teller showed divergent behavior in regard to the existence of a rheological explosion under mechanochemical stimuli. Raman spectroscopy confirmed the existence of the isostructural Phase I to Phase II transition near 5 GPa. When subjected to high-pressure and shear, Raman spectra of Phase I showed evidence that while the sucrose molecule underwent significant molecular deformation, there was no evidence of a complete chemical reaction. In contrast, Phase II showed a near-total loss of the in-situ Raman signal in response to shear, suggesting the onset of amorphization or decomposition. The divergent behaviors of Phase I and Phase II are examined in light of the differences in the hydrogen bonding and plasticity of the material.

Raman Scattering Study Temperature Phase Transitions of Rb2KInF6 Crystal

Raman scattering spectra of Rb2KInF6 crystal have been studied in a wide temperature range, including two phase transitions: from cubic to tetragonal, and then – to the monoclinic phase. Hardening of soft phonon modes was observed that attributes them to the transitions of displacive type. Parameters of Raman lines have been quantitatively analyzed and found in good agreement with known thermodynamic characteristics of these transitions. No evidences of order – disorder processes were observed in the Raman spectra of the high-symmetry phase.

ChemInform Abstract: Raman Study of the High-Temperature Phase Transition in CsH2PO4.

Abstract The Raman spectra of the high temperature superionic phase of CsH 2 PO 4 were investigated in the 4000-20 cm −1 range. There was no evidence for an irreversible phase transition at 422 K and the only reversible high temperature transition was observed near 504 K. The high temperature phase corresponds well to CsH 2 PO 4 and not to CsH 2 P 2 O 7 and its Raman spectra are characteristic of a plastic phase, implying high dynamical disorder of H 2 PO − 4 ions but with a relatively ordered Cs + sublattice.

Raman study of carbon dioxide up to 80 GPa

In order to investigate the phase transition of solid CO2 under high pressure, Raman measurements were performed up to 80 GPa at room temperature using a diamond anvil cell. To reduce the strain in the sample, an indium metal was used as the pressure medium. The Raman spectrum of the molecular phase III was found to be changed around 65 GPa; the Raman intensities for the lattice modes were suddenly weakened without gradual broadening, strongly suggesting a structural phase transition. In the decompression process, the high-pressure phase did not return to phase III but to phase I, which is the lowest pressure phase of CO2.

Raman spectra of ilmenite-type phases on the Mg4Si4O12-Mg3Al2Si3O12 join

Raman spectra of ilmenite-type phases on the Mg 4 Si 4 O 12 -Mg 3 Al 2 Si 3 O 12 join have been collected. With increasing Al content, bandwidths increase while band positions remain nearly constant, consistent with the occurrence of disorder in the solid solution. However, at least eight active Raman bands are observed up to the highest Al content, which is inconsistent with the seven bands expected for an R 3 c corundum-type structure. It implies that Al, Mg and Si are ordered at short range distance and that the occurrence of a fully disordered corundum-type phase is unlikely within the narrow P-T range of stability of the ilmenite-structure phases in this system, unless ordering occurs during temperature quenching as is the case in Mg-rich majorites. The present Raman spectra also provides a basis for identification of silicate ilmenites in shocked meteorites.

Raman spectra of phase C (superhydrous phase B) at various pressures and temperatures

Raman spectra of phase C (superhydrous phase B) at various pressures and temperatures

Non-destructive NIR-FT-raman analyses in practice. Part II. Analyses of 'jumping' crystals, photosensitive crystals and gems.

Using an improved sampling arrangement we observed the FT Raman spectra of the different phases of a 'jumping crystal', an inositol derivative. The phase transition produced--as consequences of large changes of the unit cell constants--changes in frequency and intensity mainly of CH deformation vibrations. Photochemical reactions, usually produced with light quanta in the visible range, are not activated with the quanta from the Nd:YAG laser at 1064 nm. The Raman spectra of the 'dark' form of a dinitrobenzyl pyridine and afterwards the 'light' form, the product of its illumination in the visible range, were recorded. We could not observe changes of most bands, especially not of the NO2-vibrations; however, a new strong band appeared at 1253 cm(-1), which may be due to the expected NH-photo-isomer. Genuine gemstones and fakes can be unambiguously identified by FT Raman spectroscopy. This is especially useful for the stones whose physical properties are quite similar to those of diamonds--moissanite and zirconia. The quality of diamonds can be estimated from relative band intensities; however, this is not in complete agreement with the internationally accepted visual qualification. Synthetic diamonds produced by CVD (chemical vapor deposition) show remarkable differences from natural ones in their FT-Raman spectra.

Raman spectroscopic studies of phase E to 19 GPa

Raman spectra of phase E have been measured between 100 cm -1 and 4000 cm -1 in a diamondanvil cell to 19 GPa at 300 K. We observe several broad bands in the region below 1200 cm -1 and three bands in the OH stretching region. All modes shift continuously with increasing pressures to 19 GPa and there is no indication for a major change in the crystal structure of phase E. First constraints on hydrogen sites and hydrogen bond strengths are derived from the OH frequencies together with their pressure dependencies and the correlation of OH stretching frequencies with O···O bond distances. The 3429 cm -1 band decreases linearly with pressure at -7.8 cm -1 /GPa and is clearly involved in hydrogen bonding. The corresponding O···O distance at ambient conditions is estimated to be around 2.82 Å. The 3617 cm -1 band shows a positive pressure trend allowing only very weak or absent hydrogen bonds. Corresponding O···O distances would be greater than 3 Å.

Polarized Raman spectroscopy study of phase transitions in 0.915Pb(Zn1/3Nb2/3)O3-0.085PbTiO3 relaxor ferroelectric single crystals

A complex perovskite 0.915Pb(Zn1/3Nb2/3)O3-0.085PbTiO3 solid solution with remarkable ferroelectric and related properties was studied by microprobe Raman scattering in a wide temperature range from 70 K to 580 K and in two different polarizations. The specimen chosen was in single crystal form and the spectra were recorded in both its parallel and perpendicular backscattered configurations. The spectra were complex due to short-range ordering on the B-site. It was established that the Raman spectra of the cubic phase is of the first-order in nature and is associated with the local structural distortions. The integrated intensities of the low frequency (50 cm−1) and high frequency (780 cm−1) bands were shown to exhibit the anomalies at the points of structural phase transitions. However, two phase transitions were determined corresponding to the cubic (C) to tetragonal (T) and from T to rhombohedral (R) structure with the former quite obvious and the later less apparent. The results were explained well by a description of change in ordered nanoregions existing at B- site almost at all temperatures.

Characterization of Conductive RuO2 Thin Film as Bottom electrodes for Ferroelectric Thin Films

AbstractRuthenium Oxide (RuO2) thin films were prepared on silicon substrates by solution chemistry technique. X-ray Diffraction (XRD), Atomic Force Microscopy (AFM), micro-Raman, X-ray photoelectron spectroscopy (XPS), and four probe Van-der-paw technique were used for the film characterization. X-ray analysis shows a rutile structure in these films. The films annealed at 700°C showed lowest resistivity of 29 × 10−5 ohm-cm. The presence of Eg, A1g, and B2g modes is consistent with the Raman spectrum of rutile phase. These modes as well as additional unidentified band at about 477 cm−1 were investigated by temperature dependent Raman studies. Based on the result, band at 477 cm−1 that disappears above 370 K is attributed to hydrated RuO2 present in the films. XPS analysis show stoichiometric rutile RuO2 present in the films. Small concentrations of RuCl3, RuO3 and hydrated RuO2 were also detected. Pb0.9La0.15TiO3 (PLT15) thin films were deposited on RuO2/Si substrates and characterized for its ferroelectric properties to demonstrate that solution deposition technique offers an alternative approach for preparing high quality RuO2 bottom electrodes.

The Structural Phase Transitions in LiK0.50Rb0.50SO4Mixed Crystal

The structural phase transitions of the LiK 0.50 Rb 0.50 SO 4 mixed crystal were investigated by Raman spectroscopy and neutron powder diffraction up to 800 K. The results show two first order structural transitions, quite analogous to the transitions in LiKSO 4 . Raman spectra and diffraction profiles were compatible with average symmetries C 3v 4 , C 6 6 and D 2h 16 for the three observed phases, with increasing temperature. The transitions, both reconstructive, are smeared over large temperature intervals, leading to the coexistence of Raman bands and neutron peaks of the different phases and to an anomalous behavior of the lattice parameters into these intervals. In the hexagonal phase, the cell volume presents a hysteretic behavior depending on the sample thermal history. Raman spectra of the orthorhombic phase shows the high rotational disorder of the SO 4 ions, associated with the high Li positional disorder in this high conducting phase.

Probing anisotropic magnetotransport in manganese perovskites using Raman spectroscopy

We report an electronic Raman scattering study of the colossal magnetoresistance (CMR) manganese perovskites as a function of temperature, magnetic field, symmetry, and doping. The low-frequency electronic Raman spectrum in the paramagnetic-insulating phase of these materials is characterized by a diffusive Raman-scattering response, while a nearly flat continuum response is observed in the ferromagnetic-metallic state. We found that the ${B}_{1g}$-symmetry electronic scattering intensity is significantly reduced with applied magnetic field near ${T}_{C},$ in a manner reminiscent of the dc magnetoresistivity. The strongly field-dependent scattering rate in the ${B}_{1g}$ channel appears to reflect the highly field-dependent mobility along the Mn-O bond direction expected in the double exchange mechanism. In addition, we observe a persistent field dependence in the ${B}_{1g}$ electronic scattering response for $T\ensuremath{\ll}{T}_{C},$ suggesting that the ferromagnetic phase is inhomogeneous, perhaps consisting of both metallic and insulating components.

Raman spectra of phase B at various pressures and temperatures

Variations in Raman spectra of phase B were investigated up to 320 kbar at room temperature and over the temperature range 83–775 K at atmospheric pressure. Two well separated OH Raman bands of phase B were demonstrated. During heating, OH Raman bands were observable up to about 673 K, and bands corresponding to silicate vibrations were detected up to 775 K, above which no spectrum was observed. Unlike other dense hydrous magnesium silicates studied so far, both temperature and pressure variations of the OH Raman bands of phase B behave uniquely. Raman frequencies corresponding to silicate vibrations of phase B increase with increasing pressure but decrease with increasing temperature, as found for other silicates. For silicate vibrations temperature dependency was observed to be linear and the pressure dependency for the most reliably determined bands was found to be nonlinear within experimental uncertainties and over the range investigated.

Raman spectra of phase E at various pressures and temperatures with geophysical implications

Variations in Raman spectra of phase E were investigated up to 340 kbar at room temperature and in the range 83–870 K at atmospheric pressure. Phase E appears to remain intact up to at least 870 K. However, the OH Raman band of phase E was not observable at temperatures above 473 K during heating, and it was not detected in the recovered sample. The temperature variation in Raman frequencies of phase E also showed abnormal behavior above 473 K. Thus, it is suggested that phase E dehydrated and converted to an imperfect forsterite above 473 K. Only the two most intense Raman bands of phase E were reliably measured in the high- pressure experiments. With the exception of the OH Raman band, the Raman frequencies of phase E increase with increasing pressure but decreased with increasing temperature. The frequency of the OH Raman band increases with increasing temperature, which parallels that observed in both phase A and the hydrous β-phase. The temperature dependency was observed to be linear below 500 K and the pressure dependency to be non-linear within the experimental uncertainties and the range investigated. It appears that phase E may be regarded as the hydrous form of forsterite and it is further suggested that phase E and the hydrous β-phase may be competitive hydrous phases in the upper mantle and transition zone.

Raman microscopy examination of phase evolution in Bi(Pb)–Sr–Ca–Cu–O superconducting ceramics

Raman microspectroscopy (RMS) and imaging Raman microscopy (IRM) were used to probe the composition and spatial distribution of chemical phases in Bi(Pb)–Sr–Ca–Cu–O (BSCCO) ceramic superconductor powders and silver-BSCCO composites. The Raman techniques were used to identify various phases, including alkaline earth cuprates, CuO, Bi-2212, Bi-2223, and Pb-containing phases. Changes in the Ca/Sr ratios in (Ca, Sr)2CuO3 phases were distinguished by differences in orientation with respect to polarization of the exciting radiation. Variations were observed in the content and distribution of lead in various phases formed during intermediate stages of the thermal processing of composite conductors. The spatial distribution of the various phases detected in powder and composite conductors was established to a resolution of a few microns by collecting images of the Raman scattering at wavelengths corresponding to the signature peaks of the observed phases. Reference Raman spectra of the major phases observed in the BSCCO system are also reported. The Raman techniques, when combined with complementary techniques, such as x-ray diffraction and electron microscopy, can provide valuable information about the reaction paths and mechanisms of the high temperature BSCCO superconducting ceramics.

Condensation of a soft mode in the Raman spectrum of the second tetragonal phase of CsScF4

40 cording to the data obtained in that study, the structure of its high-temperature G0 phase ~space group D4h 1 , Z51) consists of a series of perovskite-like square layers of ScF6 octahedrons separated by Cs ions ~Fig. 1!. Lowering the temperature causes a transition to the G1 phase ~space group D4h 5 , Z52) at T15490 K and then to the G2 phase (D2h 13 , Z54) at T25317 K ~the temperatures cited are based on the data obtained in the present work and differ somewhat from the data in Ref. 1; the existence of such a spread of values was noted in Ref. 1!. Such a sequence is not typical of cesium-containing perovskites, but is similar to that previously observed in RbAlF4, 2,3 in which reorganization of the structure occurs as a result of displacive phase transitions associated with turning of the AlF6 octahedra. This similarity, as well as the closeness of several macroscopic characteristics of the phase transitions in these crystals, provided Aleksandrov et al.. with some basis to postulate that the phase transitions in CsScF6 are of the same type. In that case the phase transitions should be accompanied by the condensation of soft phonon modes, which can be observed in the Raman spectra. The present work was devoted to testing this hypothesis. Investigations were carried out in the vicinity of the lowtemperature D4h 1 2D4h 5 phase transition at T15490, below which condensation of the Raman-active totally symmetric soft mode should be observed according to the selection rules. Samples measuring 23334 mm, which were taken from the same crystallization batch as in Ref. 1 were cut so that they would be oriented along the crystallographic axes in the G1 phase. The crystals were optically transparent and did not contain microscopically visible defects or inclusions. The spectra were obtained on a DFS-24-based automated spectrometer and a Jobin and Yvon U-1000 Raman spectrometer. The spectral slit width was 2 cm, and the scan step was 1 cm. The precision of the stabilization of the sample temperature during the recording of a spectrum was better than 60.2 K. In accordance with the selection rules and the structural data, the spectra of the high-temperature G0 phase displayed two intense lines, at 160 cm (Eg) and 495 cm 21 (A1g), which correspond to vibrations of the axial fluorine atoms in the xy plane and along the z axis, respectively. The frequency of the latter is close to the frequency of the longitu-

Raman study of critical fluctuations near the phase transition in ferroelectric Li2Ge7O15

Abstract The anomalous behavior of the soft mode in the Raman spectra of the Li2Ge7O15 polar phase has been found to be in good agreement with renormalization group (RG) theoretical results in the short-range scaling region 7 < Tc -T < 43 K. The values and the temperature dependence of the dimensionless critical susceptibility, the order parameter correlation lengths along the orthorhombic axes and the bare soft phonon damping constant have been obtained along with the scaling indexes r = 0.631 ± 0.002, γ = 1.23 ± 0.03 and the Ginzburg parameter Gi ≈ 0.41 from a least-squares fit of the RG formula to the Raman data.

Fluctuation anomalies in the Raman spectra of the ferroelectric tris-sarcosine calcium chloride (TSCC).

The anomalous behavior of the soft mode in the Raman spectra of the tris-sarcosine calcium chloride polar phase has been found to be in good agreement with the renormalization-group (RG) theoretical results in the short-range scaling region 10 K<T c -T<37 K. The values and the temperature dependencies of the bare soft-phonon damping constant and the order-parameter correlation length in the a-c plane have been obtained, along with the scaling indices v=0.630±0.004 and γ=1.21±0.06, from the RG formulas least-squares fitted to the Raman intensity data

Raman scattering and phase changes of CulnSe2and LiInSe2at high pressure

Abstract We have investigated the effect of pressure on the Raman spectra of the ternary chalcogenides CulnSe2 (chalcopyrite structure) and LiInSe2 (β-NaFeO2 structure) for pressures extending well above their first pressure-induced phase transitions. Sign and magnitude of Griineisen parameters are discussed by comparing to related tetrahedrally coordinated chalcogenides. Discontinuous changes of the Raman frequencies indicate pressure-induced phase transitions at 8.0±0.3 and 4.2±0.2 GPa in CuInSe2 and in LiInSe2, respectively. The Raman spectra of the low-pressure phases are not recovered after pressure release but a highly disordered structure is induced. In the case of LiInSe2, the effect of laser heating on the Raman spectra of the high-pressure phase is investigated and discussed in light of recent high-pressure x-ray investigations.

The polymorphism of alkali metal formates. Part 4. A Raman study of the phase transition in KHCOO

KHCOO II is orthorhombic at ambient conditions and it is shown that traces of moisture affect the polymorphism of these very hygroscopic crystals. Dry KHCOO II transforms into phase I at 417 K (144 °C), and this phase can be supercooled to room temperature, remaining metastable for several days before transforming back to the orthorhombic phase II. The Raman spectra of phases I and II, as well as of supercooled phase I, are reported in the present study. The absence of some prominent translational modes in the Raman spectra of KHCOO II, compared to NaHCOO II, can be explained on the basis of a group-theoretical treatment. From the temperature dependence of the linewidths of various Raman-active librational and internal modes, activation energies are obtained for intramolecular motions of the formate ions. Fermi resonance occurs between the overtone of the bending mode 2ν5 and the C—H stretching mode ν1 in KHCOO II and the coupling constant W increases with temperature. The Raman and X-ray data show that KHCOO I is structurally different from NaCHOO I, but it is not possible to assign a definite structure to this phase on the basis of a Raman spectrum alone. Key words: Potassium formate, phase transition, Raman spectra.