First-order Raman Spectra Research Articles

An experimental and theoretical investigation of phonons and lattice instabilities in metastable decompressed perovskite

We report detailed Raman and IR spectroscopic measurements for the decompressed high-pressure perovskite phase of . The appearance of a first-order Raman spectrum and slight splittings in the infrared bands suggest that the symmetry of the recovered metastable perovskite phase is lowered from Pm3m. This interpretation is fully supported by first-principles LDA calculations using the LAPW method, which indicate a small tetragonal distortion. The static lattice energy is lowered by 3.3 meV (per formula unit) by allowing rotational relaxation of the octahedra. The calculations permit a reliable assignment of the zone centre phonon modes of perovskite. The calculated pressure dependence of the ferroic IR-active modes is in excellent agreement with our measured data and reveals an incipient soft-mode behaviour in the tension regime. Further calculations of the unit as a function of octahedral volume reveal instabilities to local off-centre displacements as the octahedra are expanded. This provides a detailed understanding of the large dielectric constant for this non-transition-metal-containing decompressed perovskite phase.

Raman spectra of isotopic GaN

First-order Raman spectra of wurtzite GaN made from natural Ga and isotopically pure ${}^{15}\mathrm{N}$ and natural N (99.63% ${}^{14}\mathrm{N}$) were measured at low temperature. The Raman frequencies of the polar optical phonons [${A}_{1}(\mathrm{TO},\mathrm{LO})$, ${E}_{1}(\mathrm{TO},\mathrm{LO})$] shift according to the inverse square root of the reduced masses, as expected. The isotope shifts of the two nonpolar ${E}_{2}$ modes deviate significantly from the reduced-mass behavior as well as from the dependence expected for pure N or Ga vibrations which might be expected in view of their large frequency difference. This indicates that the ${E}_{2}$ modes involve mixed Ga and N vibrations. From fits to the experimental data with a coupled two-mode model we determine the mode eigenvectors. They confirm the results of an ab initio calculation which we have done for the zone-center vibrations of wurtzite GaN. Our calculations also predict a vanishing coupling between the two ${B}_{1}$ silent modes. We also measured a ${}^{\mathrm{nat}}{\mathrm{Ga}}^{14}{\mathrm{N}}_{0.5}^{15}{\mathrm{N}}_{0.5}$ alloy sample. Additional negative frequency shifts due to isotope disorder are observed for the ${A}_{1}(\mathrm{TO},\mathrm{LO})$, ${E}_{1}(\mathrm{TO},\mathrm{LO})$, and the high-frequency ${E}_{2}$ modes as compared to the reduced-mass or coupled-mode behavior. An estimate from second-order perturbation theory provides a quantitative explanation of this result.

Raman-scattering criteria for characterization of anneal-restored zinc blende single crystals: Application to Si+-implanted InP

We have studied the lattice recovery by rapid thermal annealing of Si+-implanted InP using Raman spectroscopy. The crystallinity recovery for different annealing temperatures of samples totally amorphized by the implantation can be monitored by means of their Raman spectra. However, free-charge coupling with the LO mode and possible misorientation of the recrystallized material may alter substantially the first-order Raman spectrum, making it unreliable for a good characterization of the lattice recovery. The study of second-order Raman spectrum overcomes the problems present in the analysis of first-order Raman spectrum and provides suitable criteria to assess the recrystallization of the implanted and annealed samples. After rapid thermal annealing at 875 °C for 10 s, the intensity of the second-order peaks approaches 70% of its value in virgin InP, and third-order Raman peaks are also clearly detected, evidencing the good lattice recovery achieved.

Raman microspectrometry of accumulated non-graphitized solid bitumens

Solid bitumens (SB) occur accumulated in sedimentary, metasedimentary and volcanic rocks from the central and western part of the Bohemian Massif (Czech Republic). Structural features of asphaltic-like (ALSB) and hard (HSB) solid bitumens from sedimentary and metasedimentary rocks of Paleozoic and Precambrian age have been obtained using Raman microspectrometry. Structural and microtextural imaging were carried out using the lattice fringes mode of transmission electron microscopy (TEM). Weakly carbonized ALSB of sedimentary origin are characterized by a relatively high H/C atomic ratio (about 1) and differ from highly carbonized HSB from basaltic rocks (H/C atomic ratio about 0.1) which cross-cut a black shale formation of Upper Proterozoic age. Both these groups of accumulated forms of SB differ from dispersed kerogens, isolated from regionally metamorphosed black shales of the same primary sedimentary basin. First-order Raman spectra of both ALSB and HSB differ considerably from those obtained on dispersed carbonaceous matter forms from black shales and schists (from non-metamorphic to medium range biotite metamorphic zone). SB display a higher surface ratio of the 1350 to the 1582 cm-1 Raman bands (0.8–2.2) and higher half-width of the 1582 cm-1 (E2g) Raman band (65–73 cm-1) in comparison with dispersed forms (0.1–1.2 and 21–55 cm-1, respectively). Only HSB display second-order Raman spectra: the first band at 2680 cm-1 is broad and the second band at 2930 cm-1 is sharp. These features of the Raman spectra confirm the low structural order of SB, which is in agreement with X-ray and TEM results. © 1997 John Wiley & Sons, Ltd.

Effect of fictive temperature on medium range order in v-SiO 2

The first-order Raman spectra of vitreous SiO 2 between 10 and 650 cm −1 have been measured as a function of sample fictive temperature, T F, achieved by laser quenching. The changes observed in the high frequency range of the spectra, > 100 cm −1, which is related to the short range order, are correlated with the shift of the ‘Boson peak’ (BP) which is present in the low frequency range, < 100 cm −1, and characteristic of the medium-range order. Based on the non-continuous glassy structure model, these observations are attributed to a decrease in the size of domains, associated with a modification of the SiOSi angle distribution, with the increase of T F.

Temperature Dependence of the Linewidth of the First-Order Raman Spectra for CaWO4Crystal

The first-order Raman spectra of A g (1) , A g (2) + B g (2) and B g (1) modes in CaWO 4 crystal were measured in the temperature range of 77–771 K and the linewidths at these temperatures were obtained. The temperature dependence of the linewidths was analyzed using the phonon dispersion curves calculated on the basis of a rigid-ion model, and the results showed that it was caused approximately by the cubic anharmonic term in the crystal potential energy.

Model investigation of the Raman spectra of amorphous silicon

A model for calculating the first-order Raman spectra of amorphous silicon (a-Si) without adjustable parameters is proposed. Calculations on the original 216-atom model of a-Si, generated by the algorithm of Wooten, Winer, and Weaire (WWW) are in very good agreement with experimental spectra and give further indication that the WWW cluster is a realistic model of moderately disordered a-Si. The TA-TO assignment of the low and high frequency bands is supported by direct numerical calculations of the phase quotient and the stretching character of the vibrational modes. The calculated participation ratios and correlation lengths of the vibrational modes indicate that the high-frequency TO-like modes are strongly localized on defects. The relative intensities of the TA-, LA-, and LO-like bands depend on the intermediate-range order, while that of the TO-like band mainly on the short-range order.

Raman spectra of virgin and damaged graphite edge planes

First-order Raman spectra of virgin and ion irradiated highly oriented pyrolytic graphite were performed along the basal and edge planes. Ion irradiation was performed in the low fluence regime (≈ 1012–1013 ions cm−2) using a 400 keV Ar+ beam in order to a introduce controlled amount of defects in the structure. Virgin edge Raman spectra reveal the presence of a 1351 cm−1 structure (D∗) shifted 10 cm−1 away from the disorder induced D-line found upon irradiating or grinding the graphite surface.The 1351 cm−1 D∗ structure is then considered as the feature coming from the rupture of the D6h4 space group symmetry. A comparison between D∗ and the classical 1360 cm−1 D-line is discussed in terms of induced disorder.

Raman Spectra and lattice dynamics of PbCO3 crystal.

The first-order Raman spectra for A1g and B1g mode in PbCO3 crystal were measured in the room temperature.Using these experimental Raman frequency values, the phonon dispersion curves of PbCO3 were analyzed by use of a modified rigid-ion model. The calculated phonon dispersion curves allowed us to calculate the one phonon density of states and the temperature dependence of the phonon specific heat of PbCO3.

Lattice dynamics of CuAlS 2 and CuAlSe 2

We have measured the first-order Raman spectra of CuAlSe 2 and the second-order Raman spectra of CuAlS 2 and CuAlSe 2, and reexamined two-phonon absorption spectra of CuAlS 2. Lattice dynamical calculations have been performed using a rigid ion model. Force constants and charges as fitting parameters have been determined so as to reproduce zone-center phonon frequencies and the two-phonon spectra observed. Calculated two-phonon density of states shows reasonable agreements with those spectra.

Temperature Dependence of the Linewidth of the First-Order Raman Spectra for Aragonite Crystal

The first-order Raman spectra of A 1 g and B 1 g mode in aragonite crystal were measured in the temperature range of 4.4∼630 K and the linewidths at these temperature were obtained. The temperature dependence of the linewidths was analyzed by the phonon dispersion curves based on the Born-von Karman model, and the results showed that it was caused approximately by the cubic anharmonic term in crystal potential energy.

Crystal structure and Raman scattering in Zn1−xMgxSe alloys

Zn1−xMgxSe alloys have been grown on (100) GaAs substrates by molecular beam epitaxy. Their crystal structures are determined form x-ray diffraction spectra and are found to be zinc blende, rocksalt, and their mixture for x&amp;lt;0.5, x=1, and x∼0.6, respectively. The Raman spectra are measured and show two-mode behavior for those alloys with zinc blende structure. The long-wavelength frequencies of ZnSe- and MgSe-like optical phonons are determined and the results can be expressed as linear functions of x. The first-order Raman spectra are observed for MgSe and Mg-rich Zn1−xMgxSe alloys with rocksalt structure. Those Raman spectra are attributed to the scattering from optical phonons activated by disorders and impurities, and their line shapes represent the phonon density of states.

Defectlike nature of the interface in AB/CD-type superlattices.

Based on the calculation of lattice dynamic properties and observation of first-order Raman spectra for the Zn-Se localized microscopic interface mode of the (CdSe${)}_{4}$/(ZnTe${)}_{4}$ superlattice, multiphonon (MP) Raman scattering of up to five orders has been observed. The linewidth variation with MP order and temperature as well as the frequency and intensity behavior with MP order can be interpreted by the anharmonic decay of the localized mode into extended band modes. These results show the defectlike nature of interfaces in AB/CD-type superlattices.

Temperature Dependence of the Linewidth of the First-Order Raman Spectrum for SnO2 Crystal

Temperature Dependence of the Linewidth of the First-Order Raman Spectrum for SnO2 Crystal

Temperature Dependence of the Linewidth of the First-Order Raman Spectrum for SnO2Crystal

The first-order Raman spectrum of the A 1g mode in SnO 2 crystal was measured in the temperature range of 300–973 K and the linewidths at these temperatures were obtained. The temperature dependence of the linewidths was analyzed using the phonon dispersion curves based on the shell model, and the results showed that it may have been caused provably by the cubic anharmonic term in crystal potential energy.

Raman spectra of coal-based graphite

Graphite formed in response to thermal contact metamorphism of coal bodies with magmatic intrusion is referred to as coal-based graphite. The first-order Raman spectrum of all the coal-based graphite taken from the Lutang area, Hunan Province exhibits a single Raman band near 1585cm-1, which comes directly from in-plane vibration of aromatic layer assigned to the E2g mode. Their Raman band of the structural defect in-plane can be divided into 2 types: one is the defect band (D peak) caused by the primary structural delect of the graphite in graphitization process, which is called D2-peak located at 1 360 cm-1; the other is the defect bank caused by the secondary structural defect in the graphite subject to tectonic shearing stress, which is called D1 peak located at 1370cm-1. The second Raman spectrum of the coal-based graphite shows three-dimensional lattice degree in the coal-based graphite. If the three-dimensional lattice of graphite is not well developed, it exhibits only a band of 2700cm-1; if the three-dimensional lattice of the coal-based graphite has been formed, the band of 2700cm-1 is split into two Raman bands, which are respectively located near 2681 cm-1 and near 2726cm-1, Raman band intensity ratio increases synchronously as the ordering degree of three-dimensional lattice increases regularly.

Temperature Dependence of the Linewidth of the First-Order Raman Spectrum for TeO2Crystal

The first-order Raman spectrum of A 1 (648 cm -1 ) mode in TeO 2 was measured at temperatures ranging from 77 to 473 K and the linewidths at these temperatures were obtained. The temperature dependence of the linewidths was analyzed by the phonon dispersion curves based on the shell model, and the result showed that it was caused approximately by the cubic anharmonic term in crystal potential energy.

Raman study of gallium arsenide thin films

Abstract The influence of deposition parameters on the structure of the gallium arsenide thin films was investigated by Raman scattering. The study was based on the analysis of the first-order Raman spectra which allows for a differentiation between the amorphous component and crystallites of various sizes. The amorphous and crystalline volume fractions were calculated from the integrated intensities of the deconvoluted peaks. It was demonstrated that a transition occurs from μ-GaAs to a-GaAs for particular plasma conditions and substrate temperature. As a function of the deposition parameters the entire range from mostly microcrystalline to completely amorphous films can be obtained. These properties were consistent with the results obtained on the same samples by transmission high-energy electron diffraction and conventional transmission electron microscopy.

Resonance Raman and infrared spectroscopy of carbon nanotubes

We present a comparative analysis of the vibrational and structural properties of carbon nanotubes. The first-order Raman spectrum exhibits two lines at 1582 cm −1 and at 1350 cm −1. The observed ratio of the integrated intensity of these lines was found to be different as compared to polycrystalline graphite. The position and intensity of the line around 1350 cm −1 strongly depend on the energy of the exciting laser line. This dispersion effect is again different from the dispersion in nanocrystalline graphite. It is discussed in terms of a photoselective resonance process. Transmission infrared spectra of the nanotubes show one broad and asymmetric line at 1575 cm −1 and a weaker line at 868 cm −1.

Synthesis of diamond nanocrystallites using the low-energy cluster beam deposition; An indirect proof of small fullerene existence?

Using a laser vaporization source, we have deposited low-energy neutral carbon clusters in the range C 10–C 32 in order to grow thin films ( e ≈ 100 nm). Compared to common DLC films, the first-order Raman spectrum does not give at all the same results, since it does not exhibit the usual G and D bands, but is comparable to the vibrational density of states of diamond. However, unlike amorphous covalent materials, a fine structure due to a perfect short range order is observed. Furthermore, we measure a large red shift (about 110 cm −1), which can be related to the decrease of the material force constants compared to the diamond ones. Finally, taking into account the specific nucleation and growth mechanisms involved in the low-energy cluster beam deposition as well as the crystallographic and electronic structures of our films, the hypothesis of small fullerene formation (C 10–C 32) in the nonequilibrium conditions of the laser vaporization source seems conceivable.