Zone-center Phonon Frequencies Research Articles

Anisotropy in the dielectric function of Bi2Te3 from first principles: From the UV-visible to the infrared range

The dielectric properties of ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$, a layered compound crystallizing in a rhombohedral structure, are investigated by means of first-principles calculations at the random phase approximation level. A special attention is devoted to the anisotropy in the dielectric function and to the local field effects that strongly renormalize the optical properties in the UV-visible range when the electric field is polarized along the stacking axis. Furthermore, both the Born effective charges for each atom and the zone center phonon frequencies and eigenvectors needed to describe the dielectric response in the infrared range are computed. Our theoretical near-normal incidence reflectivity spectra in both the UV-visible and infrared range are in fairly good agreement with the experimental spectra, provided that the free carriers Drude contribution arising from defects is included in the infrared response. The anisotropic plasmon frequencies entering the Drude model are computed within the rigid band approximation, suggesting that a measurement of the reflectivity in the infrared range for both polarizations might allow one to infer not only the type of doping but also the level of doping.

Lattice dynamical and thermodynamic properties study of ceria using density functional theory and Hubbard correction

In this study, the lattice dynamical and thermodynamic properties of CeO2 were investigated with first principles density functional theories LDA, PBE and DFT + U approximations. The phonon dispersion relation, phonon density of states, Brillion Zone-center phonon frequencies, Born effective charge and polarizability of CeO2 were calculated to investigate the lattice dynamical properties. The value of acoustic phonon frequency TA (Γ) , TO1(Γ) and TO2(Γ) calculated with DFT + U approximation is in better agreement with experimental value as compared to LDA and PBE calculations. The polarizability of CeO2 decreases with increasing frequency and approaches zero with further increase in frequency. Moreover, the temperature dependence of Helmholtz free energy, internal energy, entropy, chemical potential and specific heat capacity at constant volume were analyzed to investigate the thermodynamic properties of CeO2. The lattice dynamical and thermodynamic properties of CeO2 calculated with density functional theories are in agreement with both experimental values and theoretical findings.

Exploring the structural, elastic, lattice dynamical stability and thermoelectric properties of semiconducting novel quaternary Heusler alloy LiScPdPb

Quaternary Heusler alloy LiScPdPb with an 18-valence electron count show incredible applications in photovoltaic, spintronics, and thermoelectric materials. The study related to the structural, elastic, mechanical stability, and zone center phonon frequencies of LiScPdPb quaternary Heusler alloy is not investigated so far. Therefore, in the present article, the equiatomic quaternary Heusler alloy ‘LiScPdPb’ is studied in detail for their structural stability, elastic and mechanical stability. The research work presented in this report is undertaken by implementing first-principles Density Functional Theory (DFT). The non-magnetic character of the alloy is determined by the Slater-Pauling rule. Furthermore, density functional perturbation theory is implemented to calculate the phonon dispersion curves, phonon density of states, and zone center phonon frequencies of LiScPdPb. The calculated positive phonon frequencies have established the dynamical stability of LiScPdPb in the cubic phase. Raman and infrared phonon modes are also identified for the first time for LiScPdPb by implementing density functional perturbation theory. The calculated value of the bandgap is 0.22eV using generalized gradient approximations, while the bandgap of 1.62eV is observed by using Gaussian-Perdue-Burke–Ernzerhof (Gau-PBE) hybrid functional, which ensures the semiconducting nature of the alloy. In addition to the structural, elastic, mechanical, electronic, and lattice dynamical properties, the thermoelectric response of this alloy are also evaluated by including semi-classical Boltzmann transport theory. The maximum value of the obtained figure of merit for LiScPdPb is 0.18 ​at 1200 ​K, which opens up new avenues for its applications in electronic industries. Hence, the present work will provide a fertile field for the experimental researchers to synthesis this alloy for thermoelectric applications.

Crystal structure and phase transition of TlReO4: a combined experimental and theoretical study

The present work describes a density-functional theory (DFT) study of TlReO4 in combination with powder x-ray diffraction experiments as a function of temperature and Raman measurements at ambient temperature. X-ray diffraction measurements reveal three different structures as a function of temperature. A monoclinic structure (space group P21/c) is observed at room temperature while two isostructural tetragonal structures (space group I41/a) are found at low- and high-temperature. In order to complement the experimental results first-principles DFT calculations were performed to compute the structural energy differences. From the total energies it is evident that the monoclinic structure has the lowest total energy when compared to the orthorhombic structure, which was originally proposed to be the structure at room temperature, which agrees with our experiments. The structural and vibrational properties of the low- and room-temperature phase of TlReO4 have been calculated using DFT. Inclusion of van der Waals correction to the standard DFT exchange correlation functional is found to improve the agreement with the observed structural and vibrational properties. The Born effective charge of these phases has also been studied which shows a combination of ionic and covalent nature, resembling metavalent bonding. Calculations of zone-center phonon frequencies lead to the symmetry assignment of previously reported low-temperature Raman modes. We have determined the frequencies of the eight infrared-active, 13 Raman-active and three silent modes of low-temperature TlReO4 along with 105 infrared-active and 108 Raman-active modes for room-temperature TlReO4. Phonons of these two phases of TlReO4 are mainly divided into three regions which are below 150 cm−1 due to vibration of whole crystal, 250 to 400 cm−1 due to wagging, scissoring, rocking and twisting and above 900 cm−1 due to stretching in ReO4 tetrahedron. The strongest infrared peak is associated to the internal asymmetric stretching of ReO4 whereas the strongest Raman peak is associated to the internal symmetric stretching of ReO4. We have also measured the room-temperature Raman spectra of monoclinic TlReO4 identifying up to 28 modes. This Raman spectrum has been interpreted by comparison with the previously reported Raman frequencies of the low-temperature phase and our calculated Raman frequencies of low- and room-temperature phases of TlReO4.

Lattice dynamics of novel Heusler alloys MnY2Z (Z=Al and Si)

Mn-based Heusler alloys ‘MnY2Z (Z= (Al and Si)) have ‘L21’ crystal structure at the ambient condition of temperature and pressure. MnY2Z have a high melting temperature, shows half-metallic character and shape memory effects where numerous of its applications takes place. In this report, theoretical investigations of the phonon properties of MnY2Z in the cubic phase has been investigated by using density functional theory followed by density functional perturbation theory to comprehend the role of phonons in these alloys. In these calculations, zone center phonon frequency, phonon modes in symmetric directions and phonon density of states are investigated for these alloys. The phonon modes are found to be positive in all symmetric directions indicate the stability of the present alloys in the L21 crystal structure. Additionally, Raman and infrared phonon modes have also been identified for each alloy. The investigated results are interpreted with other existing similar type of Heusler alloys.

First Brillouin zone-centre phonon frequencies and elastic stiffness of the Ln2Hf2O7 (Ln = La, Nd, Sm and Eu) pyrochlore

First Brillouin zone-centre phonon frequencies, elastic stiffness and mechanical properties of the Ln2Hf2O7 [Ln: La, Nd, Sm and Eu] pyrochlore structure were predicted by using an eight parameter bond-bending force constant model. One of the preliminary results of our study is that all the examined compounds are mechanically stable, and the elastic stiffness constants, and bulk and shear moduli decrease in the following sequence: La2Hf2O7→Nd2Hf2O7→Sm2Hf2O7→Eu2Hf2O7. The Poisson’s ratio reveals that the interatomic bonding of the studied compounds has an ionic character and their ionicity decreases when one moves from La to Eu. The elastic properties are anisotropic and the anisotropy increases from La to Eu. The Raman and infrared active and inactive modes of the studied materials were calculated. Our findings are in good accordance with the related available data.

Zone-center phonons and elastic properties of ternary chalcopyrite ABSe2 (A = Cu and Ag; B = al, Ga and In)

Abstract The zone-center phonon frequencies and the elastic properties of six ternary chalcopyrite compounds represented by ABSe2 (A = Cu and Ag; B = Al, Ga and In) are systematically calculated by using the rigid ion model. In the present paper, we calculate four bond-stretching and three bond-bending interactions, along with three effective dynamical charges. The calculated results are compared and found to be in fairly good agreement with the available experimental and theoretical results. The elastic anisotropy of the studied compounds is analyzed and visualized by plotting three-dimensional surfaces representing the directional dependences of Young's modulus.

Lattice dynamical and elastic properties of mixed quaternary semiconducting alloys CuGa1−xAlxS2 and AgxCu1−xGaS2

Ten parameter rigid-ion model is used to study the vibrational and elastic properties of mixed quaternary semiconducting CuGa1−xAlxS2 and AgxCu1−xGaS2 chalcopyrites alloys. The zone-centre phonon frequencies and the elastic properties of above mentioned alloys are calculated for the composition range 0 ≤ x ≤ 1. The main outcome of the present study is that the studied compounds are mechanically stable at all compositions of x. While the elastic constants C33 and C44 change sharply in comparison with other elastic constants for AgxCu1−xGaS2, all elastic constants of the CuGa1−xAlxS2 do not change too much with the increase in the concentration of Al-and Ag-atoms in CuGa1−xAlxS2 and AgxCu1−xGaS2 compounds.

Structural, electronic, elastic and lattice dynamical properties of CdIn2Te4 under pressure from first principle

The first-principle calculations have been performed to study the pressure-induced effects on the structural, electronic, elastic and lattice dynamical properties of defect chalcopyrite CdIn2Te4. The optimized structural parameters are well consistent with the available experimental and theoretical data. The evolutions of energy gaps, elastic constants, elastic moduli, phonon dispersion spectra, and Raman scattering spectra, as well as zone-center phonon frequencies with pressure have been investigated in detail. Our results show that CdIn2Te4 has a direct bandgap and energy gaps decrease nonlinearly with pressure. A pressure-induced transition from brittle fracture to plastic flow exists, and DC-CdIn2Te4 is elastic anisotropy. Most Raman-active phonon frequencies have positive pressure dependence, the exception is for the E2 and B3 modes.

Vibrational and elastic properties of Ln2Sn2O7 (Ln = La, Sm, Gd, Dy, Ho, Er, Yb, or Lu)

In this study, an eight-parameter bond-bending force constant model was used to calculate the zone center phonon frequencies, elastic constants, and related properties of the stannate compounds Ln2Sn2O7 (Ln = La, Sm, Gd, Dy, Ho, Er, Yb, or Lu) with a pyrochlore structure. We found that the SnO bond strengths dominate the Ln-O and OO bonds. We also found that all of the materials are ductile and anisotropic in nature. The anisotropic nature of the compounds increases in the order of: La2Sn2O7 < Sm2Sn2O7 < Gd2Sn2O7 < Dy2Sn2O7 < Ho2Sn2O7 < Er2Sn2O7 < Yb2Sn2O7 < Lu2Sn2O7.

Structural, electronic, elastic, optical and vibrational properties of MAl2O4 (M = Co and Mn) aluminate spinels

First principles density functional theory calculations were performed to study structural, electronic, elastic, optical and vibrational properties of CoAl2O4 and MnAl2O4 aluminate spinels. Computed ground state properties such as unit-cell parameter and oxygen positional parameter differ by less than 1% from previously available theoretical and experimental results. However, the bulk modulus differs by less than 4% difference from available theoretical and experimental values for CoAl2O4 and less than 10% for MnAl2O4. Zone-center phonon frequencies and the phonon spectrum along high symmetry direction together with the phonon density of states were calculated using supercell method. Bandgaps of CoAl2O4 and MnAl2O4 were obtained as 1.78 and 2.21eV respectively. CoAl2O4 was found to be more ionic than the MnAl2O4 spinel. And quasi harmonic method was used to calculate the Debye temperature for the studied compounds.

Vibrational, Elastic Properties and Sound Velocities of MgTi2O4 Spinel

Zone-centre phonon frequencies, elastic properties and sound velocities along high-symmetry directions and Debye temperature of MgTi2O4 spinel is calculated using rigid ion model. The interatomic interactions up to third nearest neighbours are also calculated. The main results of the present paper reveals that the tetrahedral bonding is stronger than the octahedral bonding. Calculated results are compared with previously available experimental and theoretical data. It is observed that the obtained results are in good agreement with the available results in literature.

Lattice Dynamical Properties and Elastic Constants of the Ternary Chalcopyrite Compounds CuAlS2, CuGaS2, CuInS2, and AgGaS2

Lattice dynamics calculations have been performed for ternary chalcopyrite compounds CuAlS2, CuGaS2, CuInS2, and AgGaS2 using the proposed theoretical model. This model is applied to study the zone-centre (GAMMA = 0) phonon frequencies of CuAlS2, CuGaS2, CuInS2, and AgGaS2. The interatomic interactions up to third nearest neighbours were calculated. The calculated zone-centre phonon frequencies are found to be in very good agreement with observed and previous calculated data available in the literature. Single crystal elastic constants and related properties for these materials were also calculated and compared with the available data in the scientific literature.

Investigation of structural, elastic, electronic, optical and vibrational properties of silver chromate spinels: Normal (CrAg2O4) and inverse (Ag2CrO4)

Abstract Structural and elastic properties of silver chromate in normal (CrAg 2 O 4 ) and inverse (Ag 2 CrO 4 ) spinel structure have been studied using density functional theory (DFT) together with Perdew–Burke–Ernzerhof (PBE) exchange–correlation functional implemented in VASP package. It is observed that studied compound satisfy the well-known requirements of mechanical stability for cubic crystals. So we can infer that studied compound is stable both in normal (CrAg 2 O 4 ) and inverse (Ag 2 CrO 4 ) spinel structure. Electronic, optical and vibrational properties of Ag 2 CrO 4 at 0 and 11 GPa have also been studied. Zone-centre phonon frequencies and the phonon dispersion relations along high symmetry directions of the silver chromate have been calculated using phonon software with the forces calculated with VASP. Our calculated lattice constant values, Raman spectrum modes and fundamental bandgap values are in good agreement with recent studies which can motivate further experimental and theoretical attention.

Structural, vibrational and bonding properties of hydro-nitrogen solids under high pressure: An ab-initio study

Hydro-nitrogen solids are potential high energy density materials (HEDMs) due to high mass ratio of nitrogen which find wide range of applications as propellants and explosives. In the present work, we report the structural and vibrational properties of Tetramethyl Ammonium Azide (TMAA) and HydroZonium Azide (HZA) using density functional theory calculations by treating weak intermolecular interactions. The obtained ground state parameters using vdW-TS method are in good agreement with the experimental data. The pressure dependent lattice constants, compressibility and equation of state are discussed. The obtained equilibrium bulk moduli show the soft nature of these materials. The compressibility curves reveal that these compounds are highly compressible along crystallographic a-axis. We have also calculated the zone-center phonon frequencies and made a complete analysis of vibrational spectra at ambient as well as at high pressure. Contraction and elongation of C-H and N-H (NH3 stretching) bonds under pressure lead to blue- and red-shift of the frequencies in the mid-IR region for TMAA and HZA compounds, respectively. Structural, equation of state, and vibrational properties of Tetramethyl Ammonium Azide and HydroZonium Azide were calculated using van der Waals density functional theory calculations and their implications are discussed to explore the stability and hydrogen bonding.

Lattice Dynamics of the Rhenium and Technetium Dichalcogenides.

The rhenium and technetium dichalcogenides are layered van der Waals semiconductors which show a large number of Raman-active zone-centre phonon modes as a result of their unusually large unit cells and deviation from hexagonal symmetry. They thus offer the possibility of introducing in-plane anisotropy into composite heterostructures based on van der Waals materials, and Raman spectroscopy is generally used to determine their in-plane orientation. We show that first-principles calculations give a good description of the lattice dynamics of this family of materials and thus predict the zone-centre phonon frequencies and Raman activities of TcS2. We consider the distribution of the phonon modes in frequency and their atomic displacements and give a unified understanding of the phonon frequencies and Raman spectra of ReS2, TcS2 and ReSe2 in terms of the scaling of Raman frequency with the chalcogen mass.

High-quality cubic and hexagonal InN crystals studied by micro-Raman scattering and electron backscatter diffraction

Large InN microcrystals grown by molecular beam epitaxy are investigated by micro-Raman spectroscopy and electron backscatter diffraction (EBSD). High-quality (phonon linewidths between 1.5 and 2 cm−1) cubic and hexagonal crystals are identified with Raman mapping by the observation of the respective characteristic phonon modes. The unexpected occurrence of metastable cubic InN crystals is confirmed by EBSD measurements. The cubic microcrystals are revealed by EBSD to be single-crystalline and to exhibit <1 1 1> orientation. The transverse (TO) and longitudinal-optical (LO) zone-center phonon frequencies of cubic InN are found to be 463 and 584 cm−1, respectively. The bulk carrier density in the microcrystals lies in the range of 2–3 × 1017 cm−3 as determined by the analysis of LO phonon-plasmon-coupled modes in the Raman spectra.

Vibrational, mechanical and thermodynamical properties of indium thiospinelsMIn2S4(M= Cd, Zn and Mg)

In this paper, interatomic interactions, zone-center phonon frequencies, mechanical properties, sound velocities and Debye temperature of indium thiospinels MIn2S4(M = Cd, Zn and Mg) have been calculated using rigid-ion model. We found that the first neighbor interaction is stronger than the second neighbor interaction. We have compared our calculated results with the available experimental and theoretical data and find good agreement with the experimental results.

Investigation of structural, mechanical, electronic, optical, and dynamical properties of cubic BaLiF3, BaLiH3, and SrLiH3

The structural, mechanical, electronic, optical, and dynamical properties of BaLiF3, BaLiH3, and SrLiH3 cubic perovskite materials are theoretically investigated by using first principles calculations. Obtained results are in reasonable agreement with other available theoretical and experimental studies. The considered materials are found to be mechanically stable in the cubic structure. We found that all materials are brittle. The modified Becke–Johnson (mBJ) exchange potential has been used here to obtain an accurate band order. The calculated band-gap energy value of BaLiF3 (8.26 eV) within the mBJ potential agrees very well with the experimentally reported value of 8.41 eV. In order to have a deeper understanding of the bonding mechanism and the effect of atomic relaxation on the electronic band structure, the total and partial density of states have also been calculated. We have investigated the fundamental optical properties, such as the real ε1(ω) and imaginary ε2(ω) parts of the dielectric function, absorption coefficient α(ω), reflectivity R(ω), and refractive index n(ω) in the energy range from 0 to 40 eV within the mBJ potential. The band-gap energy obtained from the absorption spectrum is around 8.76, 3.99, and 3.31 eV for BaLiF3, BaLiH3, and SrLiH3 crystals, respectively. It should be noted that BaLiF3 could be a strong potential candidate as a laser material for the development of a vacuum-ultraviolet light emitting diode once direct transition is confirmed by experimental studies. Finally, we have calculated the lattice dynamical properties of BaLiF3, BaLiH3, SrLiH3, and SrLiF3 crystals. The full phonon dispersion curves of these materials are reported for the first time. Our results clearly indicate that the materials are dynamically stable, except for SrLiF3, in the cubic structure. The obtained zone-center phonon frequencies of BaLiF3, BaLiH3, and SrLiH3 accord very well with previous experimental measurements.

Experimental evidence of zone-center optical phonon softening by accumulating holes in thin Ge

We discuss the impact of free carriers on the zone-center optical phonon frequency in germanium (Ge). By taking advantage of the Ge-on-insulator structure, we measured the Raman spectroscopy by applying back-gate bias. Phonon softening by accumulating holes in Ge film was clearly observed. This fact strongly suggests that the phonon softening in heavily-doped Ge is mainly attributed to the free carrier effect rather than the dopant atom counterpart. Furthermore, we propose that the free carrier effect on phonon softening is simply understandable from the viewpoint of covalent bonding modification by free carriers.