Zone-center Modes Research Articles

Specific features of Raman spectra of III–V nanowhiskers

Raman spectra of GaAs nanowhiskers that are grown on different substrates and differ from one another by the content of the sphalerite and wurtzite phases have been investigated. Special attention has been focused on the manifestation of structural features in the scattering spectra of nanowhiskers. It has been established that the nanowhiskers are characterized both by random inclusions of wurtzite layers in the sphalerite structure and by the continuous growth in the wurtzite phase. The interpretation of the scattering spectrum agrees with the concept of summation of the dispersion curves of the sphalerite structure upon transition to the wurtzite structure, which leads to a transformation of zone-boundary modes at the L point of the Brillouin zone into zone-center modes of the wurtzite structure and, as a consequence, to the appearance of a number of new fundamental modes of different symmetries. An analysis of the Raman spectra has revealed the formation of the hexagonal 4H polytype in narrow layers of nanowhiskers due to a random packing of hexagonal layers. The coexistence of the sphalerite and wurtzite phases in GaAs nanowhiskers completely correlates with the photoluminescence spectra measured for the same samples.

Theory of prospective perovskite ferroelectrics with double rocksalt order

Using first-principles methods, we predict the energy landscape and ferroelectric states of double perovskites of the form AA$'$BB$'$O$_6$ in which the atoms on both the A and B sites are arranged in rock-salt order. While we are not aware of compounds that occur naturally in this structure, we argue that they might be realizable by directed synthesis. The high-symmetry structure formed by this arrangement belongs to the tetrahedral $F\bar{4}3m$ space group. If a ferroelectric instability occurs, the energy landscape will tend to have minima with the polarization along tetrahedral directions, leading to a rhombohedral phase, or along Cartesian directions, leading to an orthorhombic phase. We find that the latter scenario applies to CaBaTiZrO$_6$ and KCaZrNbO$_6$, which are weakly ferroelectric, and the former one applies to PbSnTiZrO$_6$, which is strongly ferroelectric. The results are modeled with a fourth- or fifth-order Landau-Devonshire expansion, providing good agreement with the first-principles calculations. Computations of zone-center soft modes are also carried out in order to characterize the polar and octahedral-rotation instabilities in more detail. Prospects for synthesis of ferroelectric materials belonging to this class are discussed.

The WURM project--a freely available web-based repository of computed physical data for minerals

The WURM project is a database of computed Raman and infrared spectra and other physical properties for minerals. The calculations are performed within the framework of the density-functional theory and the density-functional perturbation theory. The database is freely available for teaching and research purposes and is presented in a web-based format, hosted on the web site. It provides the crystal structure, the parameters of the calculations, the dielectric properties, the Raman spectra with both peak positions and intensities and the infrared spectra with peak positions for minerals. It shows the atomic displacement patterns for all the zone-center vibrational modes and the associated Raman tensors. The web presentation is user friendly and highly oriented toward the end user, with a strong educational component in mind. A set of visualization tools ensures the observation of the crystal structure, the vibrational pattern, and the different spectra. Further developments include elastic and optical properties of minerals.

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 (C2v20), 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.

Angular dispersion of oblique phonon modes inBiFeO3from micro-Raman scattering

The angular dispersion of oblique phonon modes in a multiferroic ${\mathrm{BiFeO}}_{3}$ has been obtained from a micro-Raman spectroscopic investigation of a coarse grain ceramic sample. Continuity of the measured angular dispersion curves allows conclusive identification of all pure zone-center polar modes. The method employed here to reconstruct the anisotropic crystal property from a large set of independent local measurements on a macroscopically isotropic ceramic sample profits from the considerable dispersion of the oblique modes in ferroelectric perovskites and it can be in principle conveniently applied to any other optically uniaxial ferroelectric material.

Raman spectrum of monoclinic semiconductor [formula omitted

The Raman-active phonons in semiconductor Cu 2SnSe 3 that crystallizes in the monoclinic structure with space group Cc were studied by measuring unpolarized Raman spectrum. The experimental Raman line wave numbers for the phonon modes were compared to those reported and to the results of lattice dynamical calculations based on simplified current models reported in the literature. From the factor group analysis of the zone-center vibrational modes, the symmetry of the observed Raman lines were tentatively assigned. The most intense A ′ mode at 178 cm −1; the lowest- and highest-frequency A ′ modes at 83 and 244 cm −1, respectively; A ″ modes at 204, 231 and 291 cm −1; and a band at about 363 cm −1 which probably corresponds to an overtone on the strong peak at 178 cm −1, have been observed in this compound.

Equation of state and anharmonicity of carbon dioxide phase I up to 12 GPa and 800 K

We present an extended investigation of phase I of carbon dioxide by x-ray diffraction and spectroscopic techniques at simultaneous high pressure and high temperature, up to 12 GPa and 800 K. Based on the present and literature data, we show that a Mie-Grüneisen-Debye model reproduces within experimental uncertainties the equation of state of CO(2) over the entire range of stability of phase I. Using infrared and Raman spectroscopy, we have determined the frequencies of the zone-center lattice modes as a function of pressure and temperature. We have then extracted the volume and temperature dependencies of the optical lattice mode frequencies and their respective Grüneisen parameters. We find a large difference between the thermodynamic Grüneisen parameter obtained from the P-V-T data and those associated with the optical lattice modes. This suggests, within the quasiharmonic approximation, that acoustic modes have a dominant contribution to the anharmonicity of the system.

ChemInform Abstract: First‐Principles Study of the Electronic, Optical Properties and Lattice Dynamics of Tantalum Oxynitride.

First-principles calculations of the electronic, optical properties and lattice dynamics of tantalum oxynitride are performed with the density functional theory plane-wave pseudopotential method. The analysis of the electronic structure shows a covalent nature in Ta−N bonds and Ta−O bonds. The hybridization of anion 2p and Ta 5d states results in enhanced dispersion of the valence band, raising the top of the valence band and leading to the visible-light response in TaON. It has a high dielectric constant, and the anisotropy is displayed obviously in the lower energy region. Our calculation indicated that TaON has excellent dielectric properties along [010] direction. Various optical properties, including the reflectivity, absorption coefficient, refractive index, and the energy-loss spectrum are derived from the complex dielectric function. We also present phonon dispersion relation, zone-center optical mode frequency, density of phonon states, and some thermodynamic properties. The experimental IR modes...

Raman spectroscopy of selected carbonaceous samples

Abstract This paper presents the results of Raman spectra measured on carbonaceous materials ranging from greenschist facies to granulite–facies graphite (Anchimetamorphism and Epimetamorphism zones). Raman spectroscopy has come to be regarded as a more appropriate tool than X-ray diffraction for study of highly ordered carbon materials, including chondritic matter, soot, polycyclic aromatic hydrocarbons and evolved coal samples. This work demonstrates the usefulness of the Raman spectroscopy analysis in determining internal crystallographic structure (disordered lattice, heterogeneity). Moreover, this methodology permits the detection of differences within the meta-anthracite rank, semi-graphite and graphite stages for the samples included in this study. In the first order Raman spectra, the bands located near to c.a. 1350 cm − 1 (defects and disorder mode A 1g ) and 1580 cm − 1 (in plane E 2g zone—centre mode) contribute to the characterization and determination of the degree of structural evolution and graphitization of the carbonaceous samples. The data from Raman spectroscopy were compared with parameters obtained by means of structural, chemical and optical microscopic analysis carried out on the same carbonaceous samples. The results revealed some positive and significant relationships, although the use of reflectance as a parameter for following the increase in structural order in natural graphitized samples was subject to limitations.

First-Principles Study of the Electronic, Optical Properties and Lattice Dynamics of Tantalum Oxynitride

First-principles calculations of the electronic, optical properties and lattice dynamics of tantalum oxynitride are performed with the density functional theory plane-wave pseudopotential method. The analysis of the electronic structure shows a covalent nature in Ta-N bonds and Ta-O bonds. The hybridization of anion 2p and Ta 5d states results in enhanced dispersion of the valence band, raising the top of the valence band and leading to the visible-light response in TaON. It has a high dielectric constant, and the anisotropy is displayed obviously in the lower energy region. Our calculation indicated that TaON has excellent dielectric properties along [010] direction. Various optical properties, including the reflectivity, absorption coefficient, refractive index, and the energy-loss spectrum are derived from the complex dielectric function. We also present phonon dispersion relation, zone-center optical mode frequency, density of phonon states, and some thermodynamic properties. The experimental IR modes (B(u) at 808 cm(-1) and A(u) at 863 cm(-1)) are reproduced well and assigned to a combination of stretching and bending vibrations for the Ta-N bond and Ta-O bond. The thermodynamic properties of TaON, such as heat capacity and Debye temperature, which were important parameters for the measurement of crystal physical properties, were first given for reference. Our investigations provide useful information for the potential application of this material.

First-principles study of the elasticity, piezoelectricity, and vibrational modes inLiGaO2compared with ZnO and GaN

The lattice-dynamical properties of ${\text{LiGaO}}_{2}$ have been studied by means of first-principles density-functional calculations using the pseudopotential linear-response approach in the local-density approximation. For comparison, similar calculations are performed for the related materials ZnO and GaN. The quantities calculated are the elastic constants, the piezoelectric constants, the static and high-frequency dielectric constants, and the zone-center vibrational modes. The latter are used to calculate infrared optical spectra and compared with experiment. The results are compared to available experimental data, indicating overall satisfactory agreement typical of the methodology used. The nature of the vibrational modes is examined in terms of the mode eigenvectors and indicates a large mixing of wurtzite derived modes due to the lower symmetry.

Zinc incorporation into CdTe quantum dots in glass

We report zinc incorporation into CdTe nanoparticles grown by two-step solid phase precipitation in commercial borosilicate glass quenched from the melt, based on a co-evaluation of the results of resonant Raman and optical absorption measurements. Resonant Raman spectra display a two-peak structure at wave-number positions corresponding to ternary compound Zn x Cd 1− x Te. We attribute the higher intensity peak between 190 and 195 cm −1 to the first harmonic of the zone-center longitudinal optical mode (LO 1) and, the lower intensity peak between 157 and 160 cm −1 to the second harmonic (LO 2) of Zn x Cd 1− x Te crystal. The wave-number of vibrational Raman modes indicates that zinc content varies between 39 and 50% during the growth of quantum dots. The asymptotic absorption edge against heat-treatment time plot extrapolates to a bulk band gap of 1.714 eV which sets a lower limit of 31% for zinc incorporated into quantum dots which is consistent with the results of resonant Raman measurements. The energetic position of asymptotic absorption edge of 1.592 eV and an additional unresolved weak structure in Raman spectrum between 166 and 182 cm −1 observed for as-received glass might serve as a evidence for the occurrence of a different nanocrystalline phase with 13% zinc content.

First-principles study of the electronic, optical, and lattice dynamics properties of LiInSe2 polymorph

First-principles calculations of the electronic, optical, and lattice dynamics properties of LiInSe2 polymorph were performed with the density functional theory plane-wave pseudopotential method. The results of electronic structure reveal that the different coordinated structure for α-NaFeO2-type LiInSe2 leads to the broadening of density of states and the smaller band gap. Various optical properties, including the dielectric function, reflectivity, absorption coefficient, refractive index, and the energy-loss spectrum as functions of the photon energy were calculated and are found to be in good agreement with experiments. We also presented phonon dispersion relation, zone-center optical mode frequency, density of phonon states, and some thermodynamic properties, such as constant volume heat capacity and Debye temperature. The results show that the α-NaFeO2-type LiInSe2 does not only have special optical properties but also demonstrates special vibrational properties and thermodynamic properties, which may lead to significance research meaning and application value.

Calculation of the vibrational properties of LiMgAs

We have studied the vibrational properties of the filled tetrahedral semiconductor LiMgAsand its binary analog AlAs by using the plane-wave pseudopotential method within densityfunctional theory. The calculated lattice constants for the studied compounds are in goodagreement with previous theoretical and experimental results. The phonon dispersioncurves and phonon density of states are calculated by using density functional perturbationtheory. The sound speeds in different directions are quantitatively similar in LiMgAs andAlAs. The assignment of the zone center modes to the relative motion of the atoms showsthat the lower optic modes are due to the Mg–As pair vibrations, while for theupper ones the Li–Mg pair dominates, which is attributed to the smaller Mg atommass. The longitudinal interatomic force constant of Mg–As is about 66% higherthan that of Li–As, showing the relatively high covalency of the former bond.

Low-pressure clino- to high-pressure clinoenstatite phase transition: A phonon-related mechanism

We have investigated by first principles the compressional behavior of low-pressure (LP) and high-pressure (HP) MgSiO 3 clinoenstatite. We have carefully examined unit-cell shapes, chain angles, and polyhedral volume responses, such as angle variances and quasi-elongations, under pressure at room temperature. We have observed opposite behavior of the tetrahedra in the S-rotated and O-rotated chains with pressure in the LP phase, with a slight increase (decrease) in angle variance and quasi-elongation in the former (latter). Inspection of zone center modes of both phases under pressure reveals a transition path that converts the S-rotated chain in the LP phase into the O-rotated chain in the HP phase. This conversion is related to a slight softening of an A g “metastable” Raman mode under pressure.

Electronic structure and lattice dynamical properties of different tetragonal phases ofBiFeO3

From first-principles calculations based on the plane-wave pseudopotential method within the density-functional $+U$ scheme, we have investigated the atomic geometry, electronic band-structure, and lattice dynamical properties of four tetragonal phases of the multiferroic ${\text{BiFeO}}_{3}$. In contrast to the indirect Kohn-Sham band gap of the rhombohedral phase, the most stable of the single-phase stable tetragonal structure is semiconducting with a smaller and direct band gap. It is found that the highest optical phonon branch is split from the rest of the phonon continuum by a small gap. The presently calculated zone-center optical modes have been compared and contrasted with the available polarized Raman-scattering studies and previous calculations based on a simple short-range force-constant method. The highest phonon mode with frequency $\ensuremath{\sim}662\text{ }{\text{cm}}^{\ensuremath{-}1}$ of ${A}_{1}$ representation in the tetragonal phase $(P4mm)$ can be readily distinguished from the frequency $\ensuremath{\sim}590\text{ }{\text{cm}}^{\ensuremath{-}1}$ of ${A}_{2}$ representation in the rhombohedral phase $(R3c)$.

Lattice vibration spectrum of GaN from first-principle calculations

We report first-principle calculations in the framework of the density functional perturbation theory of the lattice vibration properties of GaN in the wurtzite and rocksalt phases at zero and under pressures ranging from 0 to 40 GPa. The calculations predict the full phonon spectrum throughout the Brillouin zone and phonon density of states. Generally, the calculated zone-center modes for the wurtzite structure are found to be in good agreement with previous theoretical and experimental results. For the phonons of rocksalt GaN in the presence of hydrostatic pressure, these are the first results to our knowledge. The pressure dependence of the zone-center optical frequencies is examined and discussed which allowed the calculation of the pressure coefficients and mode Grüneisen parameters.

Polarized hyper-Raman scattering study of the silentF2umode inPbMg1/3Nb2/3O3

A single crystal of the relaxor ${\text{PbMg}}_{1/3}{\text{Nb}}_{2/3}{\text{O}}_{3}$ is studied by hyper-Raman scattering. The relative scattering intensities obtained for the band near $250\text{ }{\text{cm}}^{\ensuremath{-}1}$ in various polarization geometries are fully compatible with hyper-Raman spectroscopy tensor of the ${F}_{2u}$ zone-center vibrational mode of prototype $Pm\overline{3}m$ $({O}_{h}^{1})$ cubic perovskite structure. The mode was investigated between room temperature and 775 K, thus covering the range around the Burns temperature ${T}_{d}$ and the crossover temperature ${T}^{\ensuremath{\ast}}$. At variance with recent anticipations, no softening was observed.

Thermal emission of midinfrared GaAs photonic crystals

We have investigated the thermal emission of midinfrared dielectric photonic crystals. We show that the optical properties of the photonic crystals can be probed by the photomodulation of the room-temperature thermal emission. The technique is illustrated in the case of membrane-type two-dimensional photonic crystals. The GaAs-based photonic crystal membranes were fabricated by wafer bonding followed by standard e-beam lithography and etching processes. Thermal radiation collected from the surface of the sample allows one to investigate experimentally a broad spectral range from 500 to $1500\text{ }{\text{cm}}^{\ensuremath{-}1}$. Zone-center modes at the $\ensuremath{\Gamma}$ point coupled to the radiative continuum are evidenced around $900\text{ }{\text{cm}}^{\ensuremath{-}1}$ ($11\text{ }\ensuremath{\mu}\text{m}$ wavelength). The lattice periodicity of the photonic crystals allows one to spectrally tailor the emissivity. Three-dimensional finite-difference time-domain modeling is used to explain the thermal radiation spectra of the photonic crystal characterized by optical sources embedded in the whole structure.

Ab initiostudy of the physical properties ofγ-Al2O3: Lattice dynamics, bulk properties, electronic structure, bonding, optical properties, and ELNES/XANES spectra

Based on the most recently determined noncubic structure for $\ensuremath{\gamma}{\text{-Al}}_{2}{\text{O}}_{3}$ by Menendez-Proupin and Gutierrez, a comprehensive list of physical properties is investigated theoretically. These include lattice dynamics and phonon spectra, elastic constants and bulk structural parameters, electronic structure and interatomic bonding, optical properties, and x-ray absorption near-edge structure (XANES) spectra. Compared to similar calculations of $\ensuremath{\alpha}{\text{-Al}}_{2}{\text{O}}_{3}$, we find a smaller lowest zone-center vibrational mode at $97.6\text{ }{\text{cm}}^{\ensuremath{-}1}$, a lower heat capacity, a smaller bulk modulus, and a much larger thermal-expansion coefficient. The threefold bonded O ions introduce highly localized vibrational modes near $751\text{ }{\text{cm}}^{\ensuremath{-}1}$. The calculated thermal Gr\"uneisen parameter indicates a strong anharmonicity in $\ensuremath{\gamma}{\text{-Al}}_{2}{\text{O}}_{3}$. The elastic tensor and the elastic wave velocities are also evaluated showing the longitudinal wave to be nearly isotropic. For the electronic structure, we find that $\ensuremath{\gamma}{\text{-Al}}_{2}{\text{O}}_{3}$ has a smaller band gap but a refractive index similar to $\ensuremath{\alpha}{\text{-Al}}_{2}{\text{O}}_{3}$. Highly localized states at the top of the valence band originating from threefold bonded O in the more covalently bonded ${\text{AlO}}_{4}$ tetrahedra are identified. The calculated Mulliken effective charges and bond order values indicate that the structural model for $\ensuremath{\gamma}{\text{-Al}}_{2}{\text{O}}_{3}$ has a high degree of disorder. The octahedral unit $({\text{AlO}}_{6})$ is a stronger polyhedron than the tetrahedral unit $({\text{AlO}}_{4})$ although the latter has stronger Al--O bonds. The calculated $\text{Al-}K$, $\text{Al-}{L}_{3}$, and $\text{O-}K$ edges for Al and O in $\ensuremath{\gamma}{\text{-Al}}_{2}{\text{O}}_{3}$ show strong dependence on their local coordination and environments. These results are in good agreement with available experimental data but the effect of the $\ensuremath{\gamma}{\text{-Al}}_{2}{\text{O}}_{3}$ samples' porosity should be properly assessed. It is argued that the traditional view that stoichiometric $\ensuremath{\gamma}{\text{-Al}}_{2}{\text{O}}_{3}$ is a defective spinel with cation vacancies (or its variations) should be modified. $\ensuremath{\gamma}{\text{-Al}}_{2}{\text{O}}_{3}$ is better described as an amorphous networklike structure such that the ratio of tetrahedrally coordinated Al to octahedrally coordinated Al is close to 0.6; and the O ions are bonded to Al in either a threefold or fourfold configurations in about equal proportion.