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Raman Modes (raman + mode)
Selected AbstractsStrong anharmonicity and phonon confinement on the lowest-frequency Raman mode of nanocrystalline anatase TiO2PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 7 2007Kun Gao Abstract We calculated and analyzed the effects of temperature and crystallite size on the Raman properties of nanocrystalline anatase TiO2 by using the related models mentioned previously [Tang and Herman, Phys. Rev. B 43, 2299 (1991); Richter et al., Solid State Commun. 39, 625 (1981)]. The temperature dependence of the Raman spectra of two different crystallite sizes of anatase TiO2 were obtained. Careful measurements of the frequency shifts of Raman modes were carried out at different temperatures. Through fitting the experimental data, pure-volume and the pure-temperature contributions to the frequency shifts of the lowest- and highest-frequency Eg modes, respectively, have been calculated. The results indicated that strongly intrinsic anharmonicity arising from optical-phonon couplings was the origin of hardening upon increasing temperature for the lowest-frequency Raman mode. The phonon-confinement effect on this mode was analyzed and anharmonic shifts of optical phonons with two different kinds of crystallite sizes were also compared. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] XRD studies, vibrational spectra, and molecular structure of 1H-imidazo [4,5-b]pyridine based on DFT quantum chemical calculationsJOURNAL OF RAMAN SPECTROSCOPY, Issue 9 2010L. Dymi Abstract The molecular structures and vibrational properties of 1H -imidazo[4,5-b]pyridine in its monomeric and dimeric forms are analyzed and compared to the experimental results derived from the X-ray diffraction (XRD), infrared (IR), and Raman studies. The theoretical data are discussed on the basis of density functional theory (DFT) quantum chemical calculations using Lee,Yang,Parr correlation functional (B3LYP) and 6-31G(d,p) basis. This compound crystallizes in orthorhombic structure, space group Pna21(C2v9) and Z = 4. The planar conformation of the skeleton and presence of the NH···N hydrogen bond was found to be characteristic for the studied system. The temperature dependence of IR and Raman modes was studied in the range 4,294 K and 8,295 K, respectively. The normal modes, which are unique for the imidazopyridine derivatives are identified. Copyright © 2009 John Wiley & Sons, Ltd. [source] Temperature-dependent Raman scattering studies of the geometrically frustrated pyrochlores Dy2Ti2O7, Gd2Ti2O7 and Er2Ti2O7JOURNAL OF RAMAN SPECTROSCOPY, Issue 4 2008M. Ma, czka Abstract The temperature-dependent Raman studies of A2Ti2O7(A = Dy, Er, Gd) were performed on single crystals and polycrystalline samples in the 4.2,295 K temperature range. The Raman spectra showed softening of the majority of phonon modes upon cooling in the whole temperature range studied and large decrease of linewidths. These changes have been analyzed in terms of strong third-order phonon,phonon anharmonic interactions. Moreover, the 312 and 330 cm,1 modes of Er2Ti2O7(Gd2Ti2O7) showed hardening upon cooling down to about 130 K (100 K) and then anomalous softening below this temperature. The observed anomalous behavior of the Raman modes indicates that some important changes occur in these materials at low temperatures. However, the origin of this behavior is still not clear. Copyright © 2008 John Wiley & Sons, Ltd. [source] Lattice dynamics of tetragonal Nd2BaZnO5JOURNAL OF RAMAN SPECTROSCOPY, Issue 12 2007H. C. Gupta Abstract A short-range force constant model (SRFCM) has been applied to investigate the Raman and the infrared wavenumbers in Nd2BaZnO5 in its tetragonal phase of space group I4/mcm. Calculations of zone-center phonons are made with five stretching and five bending force constants. All the Raman and infrared values are then assigned to their corresponding modes. Two Raman modes are reassigned on the basis of group theoretical calculations. The calculated Raman wavenumbers exhibit good agreement with the observed values. Copyright © 2007 John Wiley & Sons, Ltd. [source] Raman spectroscopic study on the structure, phase transition and restoration of zirconium tungstate blocks synthesized with a CO2 laserJOURNAL OF RAMAN SPECTROSCOPY, Issue 9 2007E. J. Liang Abstract Densely packed zirconium tungstate blocks were synthesized by rapid solidification with a CO2 laser. The structure and phase transition properties of the samples were studied by Raman spectroscopy and X-ray diffraction. Raman spectroscopic study reveals that zirconium tungstate solidifies with an orthorhombic structure. This is attributed to the pressure encountered by the samples during rapid solidification. Several Raman bands change discontinuously at about 390 K, indicating a phase transition from the ,- to the ,-phase occurring at this temperature. In the ,-phase, most of the Raman modes give rise to negative Grüneisen parameters, suggesting contribution of the corresponding optical phonons to the negative thermal expansion coefficient of the material, at least for the ,-phase. A recovery of the ,-phase was observed when the samples were cooled to room temperature. This suggests that the cubic structure is metastable only at temperatures above 390 K, and at room temperature the ,-phase is preferred. Copyright © 2007 John Wiley & Sons, Ltd. [source] Changes in spectral features with varying mole fractions of anisaldehyde in binary mixturesJOURNAL OF RAMAN SPECTROSCOPY, Issue 3 2007A. Anis Fathima Abstract Raman and IR spectra of neat anisaldehyde (4-methoxybenzaldehyde (4MeOBz)) and its binary mixtures (in polar and nonpolar solvents) with varying mole fraction of 4MeOBz were investigated. The concentration dependence of the wavenumber position and line width (full width at half maximum, FWHM) was analyzed to study the interaction of the solute vibrational modes with the microscopic solvent environment. The wavenumbers of Raman modes of 4MeOBz, namely, the carbonyl stretching, aldehydic , (CH) and ring-breathing modes, showed a linear variation in the peak position for varying concentrations of 4MeOBz in the different solvents. The dependence of Raman line width with concentration of 4MeOBz of these modes was also taken into account. The solute,solvent interaction is stronger in 2-propanol and acetonitrile because of the formation of hydrogen bonds between them, whereas in benzene the interaction is too weak to affect the Raman modes. The modes, , (CO) in 2-propanol and aldehydic , (CH) in acetonitrile, gave a Gaussian-type line width variation, which was explained by the concentration fluctuation model, and the linear variation of the line widths was also interpreted by solute,solvent interactions. IR spectra were taken for these binary mixtures, which also give further support to these data. Copyright © 2006 John Wiley & Sons, Ltd. [source] Lattice dynamic investigation of the Raman and infrared wavenumbers of orthorhombic R2BaNiO5 (R = Y, Gd) oxides in Immm structureJOURNAL OF RAMAN SPECTROSCOPY, Issue 2 2007H. C. Gupta Abstract A short-range force constant model has been applied to investigate the Raman and the infrared wavenumbers in R2BaNiO5 (R = Y, Gd) in their orthorhombic phase of space group Immm. Calculations of zone-center phonons are made with seven stretching and four bending force constants. The force constants are evaluated by fitting nine Raman and two infrared modes. Two Raman modes are reassigned on the basis of group theoretical calculations. The calculated Raman and infrared modes show good agreement with the observed values. The infrared values are assigned for the first time in these oxides. Copyright © 2006 John Wiley & Sons, Ltd. [source] Raman phonons and Raman Jahn,Teller bands in perovskite-like manganitesJOURNAL OF RAMAN SPECTROSCOPY, Issue 10 2001Milko N. Iliev The perovskite-like manganites R1,xAxMnO3, where R is a trivalent rare earth or Y and A is a divalent alkaline earth element, are characterized by a strong interplay of magnetism, electric transport and crystallographic distortion. At doping levels 0.15 < x < 0.45 the materials exhibit colossal magnetoresistance near the concomitant ferromagnetic and insulator,metal transitions. At a fractional doping level, such as x = 0.5, the crystallographic and magnetic environment is strongly modified and charge ordering between Mn3+ and Mn4+ or phase separation takes place. In this work, the polarized Raman spectra of the orthorhombic and rhombohedral phases of parent RMnO3 compound were analyzed in close comparison with results of lattice dynamic calculations. We argue that the strong high-wavenumber bands between 400 and 700 cm,1, which dominate the Raman spectra of rhombohedral RMnO3 and magnetoresistive La1,xAxMnO3 are not proper Raman modes for the R3c or Pnma structures. Rather, the bands are of phonon density-of-states origin and correspond to oxygen phonon branches activated by the non-coherent Jahn,Teller distortions of the Mn3+O6 octahedra. The reduction of these bands upon doping of La1,xAxMnO3 and their disappearance in the ferromagnetic metallic phase support the model. The variation with temperature of the Raman spectra of La0.5Ca0.5MnO3 is also discussed. The results give a strong indication for charge and orbital ordering and formation of superstructure at low temperatures. Copyright © 2001 John Wiley & Sons, Ltd. [source] Analysis of Raman modes in Mn-doped ZnO nanocrystalsPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 10 2009Shuxia Guo Abstract Mn-doped ZnO was synthesized using a co-precipitation technique. X-ray diffraction (XRD) measurements and photoluminescence (PL) spectra show that Mn ions are doped into the lattice positions of ZnO. The modes at 202, 330, and 437,cm,1 in the Raman spectrum are assigned as 2E2 (low), E2 (high),E2 (low), and E2 (high) modes of ZnO base, respectively. The mode at 528,cm,1 is ascribed to a local vibrational mode related to Mn. The mode at 580,cm,1 should be an intrinsic mode of ZnO and assigned to E1 longitudinal optical (LO). Its reinforcement should result from a combination of resonance at the excitation wavelength and impurity-induced scattering. [source] Fullerene derivatives encapsulated in carbon nanotubesPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 11 2007W. Plank Abstract We report on the preparation and subsequent Raman analysis of carbon nanotube peapods, using the fullerene C60 and its heterofullerene derivative (C59N)2 as filling materials. The filling with (C59N)2 was done from liquid solution at room temperature and from the gas phase at elevated temperatures. The success of the encapsulation procedure is confirmed through the identification of fingerprint Raman modes and the conversion to double wall nanotubes through heating to 1250 °C. The 2D mode of double wall nanotubes made from (C59N)2 peapods is observed to be downshifted compared to the same mode in double wall nanotubes made from C60 peapods. We interpret this downshift as an evidence for the integration of the nitrogen into the lattice of the inner tube. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Strong anharmonicity and phonon confinement on the lowest-frequency Raman mode of nanocrystalline anatase TiO2PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 7 2007Kun Gao Abstract We calculated and analyzed the effects of temperature and crystallite size on the Raman properties of nanocrystalline anatase TiO2 by using the related models mentioned previously [Tang and Herman, Phys. Rev. B 43, 2299 (1991); Richter et al., Solid State Commun. 39, 625 (1981)]. The temperature dependence of the Raman spectra of two different crystallite sizes of anatase TiO2 were obtained. Careful measurements of the frequency shifts of Raman modes were carried out at different temperatures. Through fitting the experimental data, pure-volume and the pure-temperature contributions to the frequency shifts of the lowest- and highest-frequency Eg modes, respectively, have been calculated. The results indicated that strongly intrinsic anharmonicity arising from optical-phonon couplings was the origin of hardening upon increasing temperature for the lowest-frequency Raman mode. The phonon-confinement effect on this mode was analyzed and anharmonic shifts of optical phonons with two different kinds of crystallite sizes were also compared. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Annealing crystallization of a-Ge/Al/Si and a-Ge/Si thin filmsPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 9 2005F. Fajardo Abstract This work describes the temperature-induced crystallization of amorphous Ge (a-Ge) as a function of the thickness of the a-Ge films (in the 12,2600 nm range), which were deposited both onto c-Si substrates and c-Si substrates covered with aluminium. After deposition, the samples were submitted to cumulative thermal annealing treatments. It is shown that the temperature of crystallization depends on the thickness of the a-Ge films and to the presence (or not) of the Al layer. For an annealing temperature (Ta) of ,700 °C, for example, the Raman spectra of films thinner than ,1000 nm and deposited onto c-Si substrates are completely dominated by the sharp phonon mode of crystalline Si. Films with thicknesses equal to 300, 1000 and 2600 nm, deposited onto Al/c-Si, and treated at Ta = 600 °C, on the other hand, clearly display two additional peaks at 405 and 490 cm,1. They correspond to the Raman modes of Si,Ge and Si,Si modes, suggesting the formation of a SiGe alloy during the thermal anneal of the films. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Pressure evolution of the phonon modes and force constants of Tb3Al5O12 and Lu3Al5O12PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 2 2003K. Papagelis Abstract The pressure evolution of the Raman modes of Tb3Al5O12 and Lu3Al5O12 has been measured at room temperature and analyzed theoretically with the rigid ion model. Satisfactory agreement between experiment and theory has been found. The extracted pressure variation of the bond bending and bond stretching force constants shows that the compressibility of the various polyhedra types increases with increasing cation coordination, indicating that the dodecahedra play an important role in the compressibility of the garnet structure. Finally, an estimation of the pressure coefficients for the transverse optical infrared active mode frequencies is given. [source] Boson peak in low-frequency Raman spectra of AsxS100-x glasses: nanocluster contributionPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 3-4 2010Roman Holomb Abstract The origin of the Boson peak was investigated using Raman spectroscopy and ab initio calculations. Low frequency Raman active vibrational modes of different branchy-, ring- and cage-like AsmSn nanoclusters were calculated and compared to experimentally determined Raman spectra of AsxS100-x glasses with different compositon. A good correlation was found between the spectral features and the calculated Raman modes. The possible structural nature of the Boson peak in arsenic chalcogenides is proposed and discussed. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Persistence of the stereochemical activity of the Bi3+ lone electron pair in Bi2Ga4O9 up to 50,GPa and crystal structure of the high-pressure phaseACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2010Alexandra Friedrich The crystal structure of the high-pressure phase of bismuth gallium oxide, Bi2Ga4O9, was determined up to 30.5,(5),GPa from in situ single-crystal in-house and synchrotron X-ray diffraction. Structures were refined at ambient conditions and at pressures of 3.3,(2), 6.2,(3), 8.9,(1) and 14.9,(3),GPa for the low-pressure phase, and at 21.4,(5) and 30.5,(5),GPa for the high-pressure phase. The mode-Grüneisen parameters for the Raman modes of the low-pressure structure and the changes of the modes induced by the phase transition were obtained from Raman spectroscopic measurements. Complementary quantum-mechanical calculations based on density-functional theory were performed between 0 and 50,GPa. The phase transition is driven by a large spontaneous displacement of one O atom from a fully constrained position. The density-functional theory (DFT) model confirmed the persistence of the stereochemical activity of the lone electron pair up to at least 50,GPa in accordance with the crystal structure of the high-pressure phase. While the stereochemcial activity of the lone electron pair of Bi is reduced at increasing pressure, a symmetrization of the bismuth coordination was not observed in this pressure range. This shows an unexpected stability of the localization of the lone electron pair and of its stereochemical activity at high pressure. [source] | |||||||||||||