High-pressure Raman spectroscopic studies on orthophosphates Ba3(PO4)2 and Sr3(PO4)2

By using diamond anvil cell (DAC), high-pressure Raman spectroscopic studies of orthophosphates Ba₃(PO₄)₂ and Sr₃(PO₄)₂ were carried out up to 30.7 and 30.1 GPa, respectively. No pressure-induced phase transition was found in the studies. A methanol:ethanol:water (16:3:1) mixture was used as pressure medium in DAC, which is expected to exhibit nearly hydrostatic behavior up to about 14.4 GPa at room temperature. The behaviors of the phosphate modes in Ba₃(PO₄)₂ and Sr₃(PO₄)₂ below 14.4 GPa were quantitatively analyzed. The Raman shift of all modes increased linearly and continuously with pressure in Ba₃(PO₄)₂ and Sr₃(PO₄)₂. The pressure coefficients of the phosphate modes in Ba₃(PO₄)₂ range from 2.8179 to 3.4186 cm⁻¹ GPa⁻¹ for ν₃, 2.9609 cm⁻¹ GPa⁻¹ for ν₁, from 0.9855 to 1.8085 cm⁻¹ GPa⁻¹ for ν₄, and 1.4330 cm⁻¹ GPa⁻¹ for ν₂, and the pressure coefficients of the phosphate modes in Sr₃(PO₄)₂ range from 3.4247 to 4.3765 cm⁻¹ GPa⁻¹ for ν₃, 3.7808 cm⁻¹ GPa⁻¹ for ν₁, from 1.1005 to 1.9244 cm⁻¹ GPa⁻¹ for ν₄, and 1.5647 cm⁻¹ GPa⁻¹ for ν₂.     

Vibrational spectroscopy studies of a pressure-induced disproportionation reaction of LaH2

The pressure-induced disproportionation reaction of LaH₂ was investigated by infrared reflection and Raman measurements at ambient temperature. The relative reflection intensity in the 4000-6000 cm⁻¹ region began to decrease significantly at a pressure of about 12 GPa and fell to 10% of the initial value at 20 GPa. Absorption peaks, which appeared around 1200 and 700 cm⁻¹ at pressures above 14 GPa, were assigned to the hydrogen vibrations at the tetrahedral and octahedral sites of the fcc metal lattice, respectively. The peak frequencies measured in the 14-30 GPa range were similar to those observed in LaH₃. These infrared results indicated insulating LaH₃ precipitated from metallic LaH₂. Above 20 GPa, a Raman peak related to the hydrogen vibration in the octahedral sites appeared around 700 cm⁻¹, and was likely due to lattice distortion. The disproportionation reaction of LaH₂ into and solid solution LaH_x (x<1) was confirmed.     

Raman spectroscopy of RuSr2(Eu1.5Ce0.5)Cu2O10 magneto-superconductor

Raman spectroscopy studies are reported for the RuSr₂Eu_1.5Ce_0.5Cu₂O₁₀ (Ru-1222) compound at various temperatures of 300, 250, 200 and 90 K. Three distinct vibrational bands: the first at 110, 140, and 160 cm⁻¹, the second at 295 and 347 cm⁻¹, and third one at 651 cm⁻¹ are seen in Raman spectra of the compound at room temperature. These bands are attached to the Cu atoms’ c-direction, the Ru atoms’ ab-plane stretching and Ru atoms’ c-direction anti-stretching modes. Below 200 K, an extra vibrational mode is also seen at 260 cm⁻¹. Also, with a decrease in temperature, though the Cu vibrational modes remain intact, the Ru atoms’ ab-plane stretching (295 cm⁻¹) and c-direction anti-stretching (651 cm⁻¹) modes shift gradually to higher wave number positions. The frequencies of modes at 260 and 651 cm⁻¹ showed anomalous softening and line-width broadening below 100 K that corroborates well with the spin ordering seen in susceptibility studies. The studied compound is a ferromagnetic superconductor with magnetic ordering of the Ru spins at 200 K and superconductivity below 30 K. A magnetic and electrical transport characterization of the compound is also presented briefly.     

Raman spectra of the orthorhombic semiconductor compound Cu2SnTe3

The optical phonon spectrum of the semiconductor Cu₂SnTe₃, that crystallizes in the orthorhombic structure with space group Imm2 (), have been studied by measuring unpolarized Raman scattering between 10 and 300 K. The experimental frequencies of the phonon modes observed were compared to those calculated by using simplified lattice dynamical models reported in the literature. From combined analysis of these results together with the factor group analysis of the zone-center vibrational modes, valuable information about these modes was obtained and their possible symmetry was assigned. A₁ modes at 71, 123, 167, 176 and 190 cm⁻¹; A₂ modes 115 and 131 cm⁻¹; B₁ modes at 76, 142 and 152 cm⁻¹; B₂ modes at 89, 100 and 206 cm⁻¹; a overtone at 246 cm⁻¹, and combinations at 218, 270 and 292 cm⁻¹; have been observed in this compound.     

In situ Raman spectroscopy of cubic boron nitride to 90 GPa and 800 K

The temperature and pressure dependences of the Raman spectrum of the transverse-optical mode of cubic boron nitride were calibrated for applications to a Raman spectroscopy pressure sensor in optical cells to about 800 K and 90 GPa. A significant deviation from linearity of the pressure dependence is confirmed at pressures above 20 GPa. At ambient temperature, dv/dP slopes are 3.41(7) and 2.04(7) cm⁻¹/GPa at 0 and 90 GPa, respectively. A polynomial expression is used to fit the pressure–temperature dependence of the Raman line. The pressure dependence does not significantly change with temperature, as determined from experiments conducted up to 800 K. At 0 GPa, the dv/dP slope is 3.46(7) cm⁻¹/GPa at 800 K. At pressures above 90 GPa, the Raman spectrum of the transverse-optical mode cannot be observed because of an overlap of the signals of cubic boron nitride and diamond used as the anvils in the high-pressure cell.     

Fano line shape and anti-crossing of Raman active E2g peaks in the charge density wave state of NbSe2

Low energy Raman scattering from the ab-plane of the 2H polytype single crystal NbSe₂ has been investigated in the normal (N), incommensurate charge density wave (ICDW) and superconducting (SC) phases. The temperature dependence of the polarization resolved Raman response has been obtained for the excitation wavelength of 647 nm and fitted to phenomenological models for the E_2g and A_1g symmetry channels. The A_1g response can be fitted by a simple damped oscillator peak superimposed on continuous background. The E_2g response displays an anti-resonance interference pattern between the inter-layer phonon and the CDW-induced mode such that a hybridized configuration (Fano line shape [1]) is required for modelling. The polarization specific peak maxima positions and line widths as a function of temperature, deduced in this manner, are presented. Partial suppression of the electronic continuum scattering in the Raman shift range up to 110 cm⁻¹ in the A_1g symmetry channel and beyond 300 cm⁻¹ in the E_2g symmetry channel is indicative of high energy electronic states far away from the Fermi surface participating in the ICDW formation.     

High-pressure Raman and infrared spectroscopic studies of ZrP2O7

High-pressure Raman and mid-infrared spectroscopic studies were carried out on ZrP₂O₇ to 23.2 and 13 GPa respectively. In the pressure range 0.7–4.3 GPa the lattice mode at 248 cm^?1 disappears, new modes appear around 380 and 1111 cm^?1 and the strong symmetric stretching mode at 476 cm^?1 softens, possibly indicating a subtle phase transition. Above 8 GPa all the modes broaden, and all of the Raman modes disappear beyond 18 GPa. On decompression from the highest pressure, 23.2, to 0 GPa all of the modes reappear but with larger full width at half maximum. Lattice dynamics of the high temperature phase of ZrP₂O₇ were studied using first principles method and compared with experimental values.     

First principles study of structural, electronic and vibrational properties of heavily boron-doped diamond

The structural, electronic and vibrational properties of a series of heavily B-doped diamond models have been investigated using the density functional theory within a local density approximation. The doped models C_64 − nB_n (n=1-3) were constructed using supercell techniques. The structural and electronic calculations confirmed that the B dimers are always energetically stable and electrical inactive. The superconducting transition temperature TC is not only decided by the B concentration, but also by the lattice configurations of boron atoms. The vibrational frequencies and eigenmodes were determined using the linear response approach, while Raman intensities were obtained by the second response method. The Raman analysis in terms of atomic vibrations found that the “500 cm⁻¹” and “1230 cm⁻¹” bands are both superposed bands including not only C vibrations but also B-B vibrations and B-C vibrations, respectively. The calculated Raman spectra with isotopic substitutions are in excellent agreement with corresponding experimental results. The reasonable explanation was provided for no obvious Raman shift of main bands from ¹⁰B¹²C to ¹¹B¹²C model.     

The size and strain effects on the Raman spectra of Ce1−xNdxO2−δ (0≤x≤0.25) nanopowders

Ultrafine Ce_1−xNd_xO_2−δ (x=0-0.25) powders were synthesized by self-propagating room temperature synthesis. Raman spectra were measured at room temperature in the 300-700 cm⁻¹ spectral range. The shift and asymmetric broadening of the Raman F_2g mode at about 454 cm⁻¹ in pure and doped ceria samples could be explained with combined size and inhomogenous strain effects. Increased concentration of O²⁻ vacancies with doping is followed by an appearance of new Raman feature at about 545 cm⁻¹.     

Low temperature Raman study on the site symmetry of ZnSe:P

Raman spectra of phosphorous doped ZnSe are recorded from 9 to 300 K. In addition to TO mode at 208 cm⁻¹ and LO mode at 253 cm⁻¹, a new mode around 240 cm⁻¹ is observed between 55 and 270 K. The spectra at lower and higher temperatures do not show the new mode. This new mode confirms that there is a reduction of (Se site) symmetry from T_d to C_3v when P substitutes for Se. This is due to Jahn Teller distortion.     

Photoluminescence properties of Sm-doped BaBr2 under hydrostatic pressure

Photoluminescence spectra of Sm²⁺-doped BaBr₂ have been measured under hydrostatic pressures up to 17 GPa at room temperature. In the low pressure range a red-shift of the broad 5d-4f transition of −145 cm⁻¹/GPa is observed. From 5 to 8 GPa a phase mixture of the initial orthorhombic phase and the high-pressure monoclinic phase gives rise to two 5d-4f bands, which are strongly overlapping. Above 8 GPa the crystal is completely transformed to its high-pressure phase where two different Sm²⁺ sites exist, but only one broad 5d-4f transition is detected. It exhibits a red-shift of −36 cm⁻¹/GPa. In addition, the line shifts of the ⁵D₀→⁷F_J (J=0, 1, 2) transitions are investigated. Linear shifts of −19 cm⁻¹/GPa for J=0, 2 and of −13 cm⁻¹/GPa for J=1 are observed in the pressure range from 0 to 5 GPa.     

Spectroscopic study ofβ-Ni(OH)2 under pressure

Infrared absorption and Raman study ofβ-Ni(OH)₂ has been carried out up to 25 GPa and 33 GPa, respectively. The frequency ofA _2u internal antisymmetric stretching O-H mode decreases linearly with pressure at a rate of −0.7 cm⁻1/GPa. The FWHM of this mode increases continuously with pressure and reaches a value of ∼ 120 cm⁻¹ around 25 GPa. There was no discernible change observed in the frequency and width of the symmetric stretchingA _1g O-H Raman mode up to 33 GPa. The constancy of the Raman mode is taken as a signature of the repulsion produced by H-H contacts in this material under pressure. Lack of any discontinuity in these modes suggests that there is no phase transition in this material in the measured pressure range.     

Influence of cetyltrimethylammonium bromide on the morphology of AWO4 (A = Ca, Sr) prepared by cyclic microwave irradiation

AWO₄ (A = Ca, Sr) was prepared from metal salts [Ca(NO₃)₂·4H₂O or Sr(NO₃)₂], Na₂WO₄·2H₂O and different moles of cetyltrimethylammonium bromide (CTAB) in water by cyclic microwave irradiation. The structure of AWO₄ was characterized by X-ray diffraction (XRD) and selected area electron diffraction (SAED). Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) revealed the presence of nanoparticles in clusters with different morphologies; spheres, peaches with notches, dumb-bells and bundles, influenced by CTAB. Six Raman vibrational peaks of scheelite structure were detected at 908, 835, 793, 399, 332 and 210 cm⁻¹ for CaWO₄ and 917, 833, 795, 372, 336 and 192 cm⁻¹ for SrWO₄, which are assigned as ν₁(A_g), ν₃(B_g), ν₃(E_g), ν₄(B_g), ν₂(A_g) and ν_f.r.(A_g), respectively. Fourier transform infrared (FTIR) spectra provided the evidence of W-O stretching vibration in [WO₄]²⁻ tetrahedrons at 793 cm⁻¹ for CaWO₄ and 807 cm⁻¹ for SrWO₄. The peaks of photoluminescence (PL) spectra were at 428-434 nm for CaWO₄, and 447-451 nm for SrWO₄.     

Softening of charge density wave excitations at the superstructure transition in 2H-TaSe2

Raman scattering measurements performed between 5 K and 300 K on 2H-TaSe₂ reveal new modes which are assigned to the modes of the charge density wave, observed in light scattering due to the Fermi surface induced distortion. The mode at 49 cm^?1 of E_2g symmetry softens (with concurrent line-width broadening) towards 122 K, the transition temperature from the incommensurate distorted to the undistorted phase. The mode at 82 cm^?1 of A_1g symmetry appears to be connected with the transition at 90 K from the commensurate to the incommensurate superstructure. The mode at 24.5 cm^?1 of E_2g shows no temperature dependence and is clearly due to the rigid-layer vibration.     

Raman spectra of Ba6−3xSm8+2xTi18O54 solid solution

Raman spectra of Ba_6−3xSm_8+2xTi₁₈O₅₄ solid solution were investigated as the function of x and sintering time. Reasonable explanations were provided about the Raman shifts and their intensities at 1013, 590, 751, 280, 232 cm⁻¹. 1013 cm⁻¹ demonstrates the existence of BaCO₃ phase in solid solution, 590 cm⁻¹ is the symmetric stretching mode of the basal oxygens of the octahedral; 280 and 232 cm⁻¹ are the symmetric stretching modes resulted from the tilt of octahedral when large cation sites are Sm³⁺ and Ba²⁺. The shoulder peak appearing around 302 cm⁻¹ is related to the vacancy produced by the unequal valence of Sm³⁺ and Ba²⁺.     

Raman spectroscopic study of carbon substitution in MgB2

We study the effect of carbon doping on the Raman spectrum of the MgB₂ superconductor. Out data show that significant changes in the Raman spectra of the MgB_2−xC_x compounds occur for carbon concentrations x>0.04. The E_2g mode at ∼580 cm⁻¹ hardens only moderately upon increased doping despite direct carbon substitution in the boron layers, while the dependence of its full width at half maximum (FWHM) on x reveals the competing effects of reduced electron-phonon coupling and increased disorder. The results are discussed in the framework of anisotropic lattice contraction and the position of the σ sub-bands with respect to the Fermi energy level. The relative intensity of the phonon peak at ∼770 cm⁻¹, associated with a peak in the phonon density of states, increases considerably and dominates for x=0.08. Additionally, it hardens and broadens with increasing x in the range 0.04-0.08. These changes can be associated with carbon substitution-induced disorder in the investigated samples.     

Variable-temperature Raman scattering study on anatase titanium dioxide nanocrystals

The temperature dependence of the Raman modes in anatase TiO₂ nanocrystals has been investigated over the temperature range 77-873 K. With increasing temperature, the frequency of the E_g mode at 639 cm⁻¹ shifts sublinearly to the lower frequencies, however, the frequency of the lowest-frequency E_g mode shifts sublinearly to the higher frequencies from 138 cm⁻¹ at 77 K to 152 cm⁻¹ at 873 K and the frequency of the B_1g mode at 397 cm⁻¹ increases firstly and attains a maximum near 350 K. The linewidth of all of the three modes increases linearly with increasing temperature. The anharmonic effects contribute a lot to the temperature dependence behavior of the frequency and linewidth of Raman modes in anatase TiO₂ nanocrystals.     

Pressure‐induced anomalous phase transformation in nano‐crystalline dysprosium sesquioxide

The phase transformation in nano‐crystalline dysprosium sesquioxide (Dy₂O₃) under high pressures is investigated using in situ Raman spectroscopy. The material at ambient was found to be cubic in structure using X‐ray diffraction (XRD) and Raman spectroscopy, while atomic force microscope (AFM) showed the nano‐crystalline nature of the material which was further confirmed using XRD. Under ambient conditions the Raman spectrum showed a predominant cubic phase peak at 374 cm⁻¹, identified as F_g mode. With increase in the applied pressure this band steadily shifts to higher wavenumbers. However, around a pressure of about 14.6 GPa, another broad band is seen to be developing around 530 cm⁻¹ which splits into two distinct peaks as the pressure is further increased. In addition, the cubic phase peak also starts losing intensity significantly, and above a pressure of 17.81 GPa this peak almost completely disappears and is replaced by two strong peaks at about 517 and 553 cm⁻¹. These peaks have been identified as occurring due to the development of hexagonal phase at the expense of cubic phase. Further increase in pressure up to about 25.5 GPa does not lead to any new peaks apart from slight shifting of the hexagonal phase peaks to higher wavenumbers. With release of the applied pressure, these peaks shift to lower wavenumbers and lose their doublet nature. However, the starting cubic phase is not recovered at total release but rather ends up in monoclinic structure. The factors contributing to this anomalous phase evolution would be discussed in detail. Copyright © 2010 John Wiley & Sons, Ltd.     

Pressure dependence study on the electron-phonon coupling of thulium hexachloroelpasolite

Raman spectra of Cs₂NaTmCl₆ have been recorded using a diamond anvil cell at ambient temperature. The vibrational energy of each of the Raman-active TmCl₆⁻³ moiety modes increases linearly with pressure. The integrated band areas of the ν₁(a_1g) and ν₂(e_g) modes are independent of applied pressure. However, the band area of the ν₅(t_2g) mode shows an anomalous behaviour, which has been qualitatively interpreted as due to electron-phonon coupling of the aΓ₅ electronic state with the Γ₁+ν₅(t_2g) vibronic state. This interaction between the coupled states is strongest between ca. 10 and 13 GPa at ambient temperature. The results serve to emphasize the specificity of the occurrence of strong electron-phonon coupling for particular transitions of a given rare earth ion.     

Analysis of the high-resolution infrared spectrum of cyclopropane

The high-resolution infrared spectrum of cyclopropane (C₃H₆) has been measured from 100 cm⁻¹ to 2200 cm⁻¹. In that region we have identified 24 absorption bands attributed to six fundamental bands, five combination bands, three hot bands and 10 difference bands. Long pathlength spectra, up to 32 m, facilitated the identification and analysis of many previously unstudied infrared inactive, and Raman and infrared inactive vibrational states, including direct access to two forbidden fundamental states, ν₄ and ν₁₄. An improved set of constants for the ground vibrational state as well as for the fundamental vibrations ν₇, ν₉, ν₁₀, ν₁₁ are also reported. The spectral resolution of the measurements varied from 0.002 cm⁻¹ to 0.004 cm⁻¹.