Results Of Phonon Dispersion Research Articles

Pressure Dependence of Mechanical and Thermodynamic Properties of MAX‐Phase M2GaC (M = Nb and Ta) from First‐Principles Calculations

Herein, the high‐pressure impacts on the electronic, mechanical, and thermodynamic properties of MAX‐phase M2GaC (M = Nb and Ta) are calculated through the first principles. The phonon dispersion results indicate that M2GaC (M = Nb and Ta) is dynamically stable. The elastic constants and elastic modulus of Nb2GaC and Ta2GaC increase with the pressure increase, while the elastic anisotropic 3D surface structure and projection diagram show that bulk modulus, shear modulus, and Young's modulus all show anisotropy. M2GaC (M = Nb and Ta) has metallic, covalent, and ionic bonds. In addition, based on the quasiharmonic Debye model, the effects of high pressure (0–50 GPa) and temperature (0–2000 K) on the thermodynamic properties of Nb2GaC and Ta2GaC are systematically studied. The constant pressure heat capacity (CP) and thermal expansion coefficient (α) of Nb2GaC and Ta2GaC decrease with the increase of pressure, while the internal energy (U) and Gibbs free energy (G) of Nb2GaC and Ta2GaC increase with the increase of pressure. The sound velocity and kmin increase with the pressure increase, and Nb2GaC has a higher thermal conductivity than Ta2GaC.

A novel monoclinic phase and electrically tunable magnetism of van derWaals layered magnet CrTe2

Exploring the novel structural phase of van der Waals (vdW) magnets would promote the development of spintronics. Here, through first-principles calculations, we report a novel monoclinic structure of vdW layered 1T-CrTe2, which is one of the popular vdW magnets normally exhibiting a trigonal structure. The new monoclinic phase emerges from a switchable magnetic state between ferromagnetism and antiferromagnetism through changing hole doping concentration, which suggests a practical approach to obtain such a structure. The results of phonon dispersion and energy analysis convince us that the monoclinic structure is a metastable phase even without hole doping. When the hole doping concentration increases, the stability analysis indicates the preference for a novel monoclinic phase rather than a conventional trigonal phase, and meanwhile, the magnetic properties are accordingly tuned. This work provides new insights into the phase engineering of the chalcogenide family and the electrical control of magnetism of vdW layered magnets.

Theoretical insights on structural, mechanical and thermodynamic properties of MCoB (M=Nb, Mo, and W) ternary borides under high pressure

In this work, the effects of the high pressure on the structural, mechanical and thermodynamic properties of MCoB (M = Nb, Mo, and W) ternary borides were calculated through the first-principles. The formation enthalpy and phonon dispersion results indicate that these ternary borides are thermodynamically and dynamically stable. In addition, the high pressure can enhance the thermodynamic stability of these ternary borides. The elastic constants and elastic modulus of these ternary borides increased with the pressure increase, and the elastic anisotropy sequence is NbCoB > MoCoB > WCoB. The states density proved that no phase transition occurs at pressures between of 0 and 50 GPa, and that the stability of these ternary borides also increased with the pressure increase. Furthermore, the influence of high pressure (0–50 GPa) and temperature (0–1500 K) on the thermodynamic properties of these ternary borides are systematically studied based on the Quasi-harmonic Debye model. The findings show that the isothermal bulk modulus and the Debye temperature of these ternary borides increase by increasing the pressure and decreasing the temperature, while the volume showed a reverse change trend. The constant volume heat capacity and the constant pressure heat capacity of these ternary borides slowly decrease with the pressure increase, while the thermal expansion coefficient of these ternary borides is more sensitive to the pressure than to the temperature.

Modulation of optical absorption in m-Fe1\u2212xRuxS2 and exploring stability in new m-RuS2

A first-principle computational method has been used to investigate the effects of Ru dopants on the electronic and optical absorption properties of marcasite FeS2. In addition, we have also revealed a new marcasite phase in RuS2, unlike most studied pyrite structures. The new phase has fulfilled all the necessary criteria of structural stability and its practical existence. The transition pressure of 8 GPa drives the structural change from pyrite to orthorhombic phase in RuS2. From the thermodynamical calculation, we have reported the stability of new-phase under various ranges of applied pressure and temperature. Further, from the results of phonon dispersion calculated at Zero Point Energy, pyrite structure exhibits ground state stability and the marcasite phase has all modes of frequencies positive. The newly proposed phase is a semiconductor with a band gap comparable to its pyrite counterpart but vary in optical absorption by around 106 cm−1. The various Ru doped structures have also shown similar optical absorption spectra in the same order of magnitude. We have used crystal field theory to explain high optical absorption which is due to the involvement of different electronic states in formation of electronic and optical band gaps. Lӧwdin charge analysis is used over the customarily Mulliken charges to predict 89% of covalence in the compound. Our results indicate the importance of new phase to enhance the efficiency of photovoltaic materials for practical applications.

YS2 monolayer as a high-efficient anode material for rechargeable Li-ion and Na-ion batteries

Here, we propose a new two-dimensional (2D) monolayer YS2 and investigate its properties as electrode material for Li-ion and Na-ion batteries (LIBs and NIBs), by combining the density functional theory (DFT) and the crystal structure prediction techniques. The results of phonon dispersion and ab-initio molecular dynamics (AIMD) simulations confirm that it is dynamically and thermally stable. The adsorption energies of Li and Na on YS2 are −2.038 and −1.939 eV, respectively. Besides, the superior electrical conductivity is maintained during the whole lithiation/sodiumation process. The calculated diffusion energy barriers for Li and Na on this YS2 monolayer are 0.33 and 0.24 eV, respectively, demonstrating a fast ion transfer rate. The theoretical specific capacities for Li and Na adatoms on YS2 are both 350 mA h g−1, comparable to the graphite electrode. These features indicate that 2D YS2 monolayer could be considered as a promising electrode material for both LIBs and NIBs.

A computational study of structural, electronic and carrier mobility of boron and phosphorus/nitrogen co-doped graphene

Opening a bandgap in graphene by doping with lighter elements plays a vital role in the next generation nanoelectronic devices. Here, we present the structural, electronic, and mobility of graphene co-doped with boron/nitrogen (BCN) and boron/phosphorus (BCP) using density functional theory with the inclusion of van der Waals interactions. By analyzing the band structure, it is found that BCP shows a direct bandgap whereas BCN exhibits an indirect bandgap. The bandgap values calculated using PBE (HSE) functional are (1.97 eV) (3.19 eV) for BCN and 0.55 eV (1.18 eV) for BCP. From phonon dispersion results, it is apparent that both BCN and BCP shows positive frequencies within the Brillouin zone demonstrating the lattice dynamical stability of these materials. Deformation potential theory is applied to calculate the electron/hole mobility by applying the uniaxial strain along x- and y-directions. It is seen that BCP possess significantly larger mobility compared to BCN. For BCP, the mobility of electron is 1588 (2999) and that of the hole is and 1607 (838) along x-direction (y-direction) in units of cm2V−1s−1, respectively, which are larger than MoS2. Also, Boltzmann theory within the constant relaxation time approximation is used to determine the temperature dependent conductivity, and the results are consistent with the deformation theory.

Wave Propagations in Metamaterial Based 2D Phononic Crystal: Finite Element Analysis

In the present work, the acoustic band structure of a two-dimensional (2D) phononic crystal (PnC) containing composite material were investigated by the finite element method. Two-dimensional PC with triangular and honeycomb lattices composed of composite cylindrical rods are in the air and liquid matrix. The existence of stop bands are investigated for the waves of certain frequency ranges. This phononic band gap - forbidden frequency range - allows sound to be controlled in many useful ways in structures. These structures can be used as sonic filters, waveguides or resonant cavities. Phononic band diagrams ω=ω(k) for a 2D PnC were plotted versus the wavevector k along the Γ-X-M- Γ path in the first Brillouin zone. The calculated phonon dispersion results indicate the existence of full acoustic modes in the proposed structure along the high symmetry points.

First-principles calculations of dynamical and thermodynamic properties of cuprite doped with silver (Cu2(1−x)Ag2xO)

Cuprite (Cu2O) is a solid mineral and a compound whose simplicity of preparation, non toxic nature, low band gap and its abundance has made it a prospective candidate for the realisation of low cost photovoltaic applications. The present work successfully dopes Cuprite with Ag at different concentrations x = 0, 0.25, 0.5, 0.75 and 1, their first-principle calculations of their electronic, dynamical and thermodynamic properties have been investigated extensively within the generalised gradient approximation. Direct band gap energies at are predicted for all the studied systems. A small bowing parameter for lattice constants ba and bulk modulus bB of and 0.8747 GPa were obtained when compared to Vegard’s law. The results of phonon dispersion when x = 0 and 1 indicate stability, these agree with available theoretical and experimental results while negative frequencies observed along the Brillouin zone for the doped systems when x = 0.25, 0.5 and 0.75 imply that they are dynamically unstable. The thermodynamic properties between 0 to 800 K were determined using the calculated phonon density of states within the harmonic approximation and the values of the specific heat capacity at constant volume at ambient temperature and the temperature at which lattice vibrations and thermal motion of electrons contribute to the constant volume specific heat capacity are presented for all the systems.

Phase stability, mechanical and optoelectronic properties of two novel phases of AlN

Two novel aluminum nitride (which is bct-AlN at ambient pressure, and h-AlN at higher pressure) were predicted using first-principles calculations. The mechanical and phonon dispersion results indicate that bct-AlN is mechanically and dynamically stable at zero pressure, h-AlN phase can be stabilized by increasing pressure and it is mechanically and dynamically stable at 10 GPa. bct-AlN is more favorable than rs-AlN in thermodynamics at ambient pressure. Our calculated band gap of bct-AlN is 5.85 eV. It can be used as semiconductor device and optoelectronic device due to its inherent wide direct band gap. For bct-AlN, the shortest Al–N bond length is 1.8476 Å and its bond order index is 1.28, which shows that strong covalent bonds are formed between Al atoms and N atoms. Moreover, the anisotropy of Young’s modulus and optical properties can be noticed obviously for bct-AlN.

Effects of Na, Mg, and Al substitution on the electronic, optical, lattice dynamical, and thermodynamic properties of semiboride Be2B

The structural, electronic, and optical properties of Be2B and its ternary compounds XBeB (X = Al, Na, and Mg) have been calculated using the full-potential linearized augmented plane wave (FP_LAPW) method, which is based on the first-principles method with the generalized gradient approximation (GGA) and the local density approximation (LDA). While, the lattice dynamical and thermodynamic properties were performed using the density functional perturbation theory (DFPT). The calculated lattice constants are in good agreement with the available theoretical and experimental values. The partial density of states (PDOS), and phonon dispersion curves have been obtained and compared with previous works. An accurate calculation of the dielectric function, refractive index, reflectivity, optical conductivity, absorption spectra, extinction coefficient, and energy-loss spectrum were performed. The first time calculated optical properties suggest that AlBeB can be used as possible shields for UV radiations. The phonon dispersion results of the investigated compounds confirm the dynamical stability of both structures Be2B and AlBeB. However the soft modes are observed for both compounds MgBeB and NaBeB, indicating their dynamical instability. Finally, the first time calculated thermodynamic properties showed clearly the thermodynamic instability of NaBeB compound.

Electronic, vibrational, superconducting and thermodynamic properties of cubic antiperovskite ZnNNi3

We present results of ab initio theoretical investigations of the structural and electronic properties of the cubic superconductor ZnNNi by employing the plane wave pseudopotential method within the generalized gradient approximation. The density of states at the Fermi level is found to be governed by the Ni 3d electrons. A linear-response approach to the density functional theory is used to derive the phonon dispersion curves, vibrational density of states and the electron–phonon coupling parameter. The calculated electron–phonon coupling constant and the logarithmically averaged phonon frequency are calculated to be 0.654 and 169.89 K, respectively, giving the superconducting transition temperature T=2.925 K according to the Allen–Dynes formula. Our calculated value of T is in excellent accord with the corresponding experimental value of 3 K. Using the phonon dispersion results, we further present an assessment of important thermodynamical properties such as internal energy (E), Helmholtz free energy (F), constant-volume specific heat (C), entropy (S) and Debye temperature () in the framework of quasi harmonic approximation theory.

Vibrational properties of vacancy in bcc transition metals using embedded atom method potentials

The embedded atom method (EAM) potentials, with the universal form of the embedding function along with the Morse form of pair potential, have been employed to determine the potential parameters for three bcc transition metals: Fe, Mo, and W, by fitting to Cauchy pressure (C 12 − C 44)/2, shear constants \(G_\textrm{v} =({C_{11} -C_{12} +3C_{44}})/5\) and C 44, cohesive energy and the vacancy formation energy. The obtained potential parameters are used to calculate the phonon dispersion spectra of these metals. Large discrepancies are found between the calculated results of phonon dispersion using the EAM and the experimental phonon dispersion results. Therefore, to overcome this inadequacy of the EAM model, we employ the modified embedded atom method (MEAM) in which a modified term along with the pair potential and embedding function is added in the total energy. The phonon dispersions calculated using potential parameters obtained from the MEAM show good agreement with experimental results compared to those obtained from the EAM. Using the calculated phonons, we evaluate the local density of states of the neighbours of vacancy using the Green’s function method. The local frequency spectrum of first neighbours of vacancy in Mo shows an increase at higher frequencies and a shift towards the lower frequencies whereas in Fe and W, the frequency spectrum shows a small decrease towards higher frequency and small shift towards lower frequency. For the second neighbours of vacancy in all the three metals, the local frequency spectrum is not much different from that of the host atom. The local density of states of the neighbours of the vacancy has been used to calculate the mean square displacements and the formation entropy of vacancy. The calculated mean square displacements of the first neighbours of vacancy are found to be higher than that of the host atom, whereas it is lower for the second neighbours. The calculated results of the formation entropy of the vacancy compared well with other available results.

Magnetic spectral response and lattice properties in mixed-valenceSm1−xYxSsolid solutions studied with x-ray diffraction, x-ray absorption spectroscopy, and inelastic neutron scattering

Mixed-valence phenomena occurring in the "black" (B) and "gold" (G) phases of Sm1-x Yx S have been studied by x-ray diffraction, x-ray absorption spectroscopy, and inelastic neutron scattering. Lattice-constant and phonon-dispersion results confirm that the valence instability occurs already inside the B phase. On the other hand, pronounced temperature anomalies in the thermal expansion \alpha(T), as well as in the Sm mean-square displacements denote the onset of the B-G transition for the compositions x = 0.33 and 0.45. It is argued that these anomalies primarily denote an effect of electron-phonon coupling. The magnetic spectral response, measured on both powder and single crystals, is dominated by the Sm2+ spin-orbit component close to 36 meV. A strongly overdamped Sm3+ contribution appears only for x >= 0.33 near room-temperature. The quasielastic signal is strongly suppressed below 70 K, reflecting the formation of the singlet mixed-valence ground state. Quite remarkably, the signal around 36 meV is found, from the single-crystal spectra, to arise from two distinct, dispersive, interacting branches. The lower peak, confirmed to exist from x = 0.17 to x = 0.33 at least, is tentatively ascribed to an excitation specific to the mixed-valence regime, reminiscent of the "exciton" peak reported previously for SmB6 .

Interaction potential for ZnTe: A molecular dynamics study

An effective interaction potential is proposed in order to describe the structural and dynamics properties of ZnTe as a function of temperature and pressure. The interaction potential is used in molecular dynamics simulation and consists of two- and three-body terms. At ambient pressure conditions melting temperature, linear thermal expansion coefficient, and vibrational density of states is compatible with experimental phonon dispersion results. The structural phase transformation induced by hydrostatic pressure at $12\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ is in good agreement with experimental reported values for this material. The reverse transformation, from orthorhombic sixfold to a zinc-blende fourfold coordinate structure, shows a large hysteresis.

Dynamical Correlations in Liquid Sodium

AbstractThe theory developed by Glass and Rice is used to study the atomic motions in liquid sodium in terms of the velocity autocorrelation function, power spectrum, and mean square displacement at 373 K. The interatomic pair potential for liquid sodium at the melting temperature is obtained by using Ashcroft's pseudopotential theory with the dielectric function of Vashishta and Singwi. Results of these calculations are compared with the results obtained by Miranda and Tora from molecular dynamics at 373 K. These results of dynamical variables obtained by the theory of Glass and Rice show a good agreement with the molecular dynamics results for liquid sodium. Besides, calculations are also done for the coherent behaviour of liquid sodium in terms of the dynamical structure factor S(k, ω). Quasi dispersion curves derived from the peak positions of the longitudinal current‐current correlation function ω2S(k, ω)/ω−2 against k, are in good agreement with experimental phonon dispersion results of Cocking for liquid sodium.

An improved Fielek model for transition metals

The lattice dynamical model for FCC transition metals due to Fielek has been further improved by introducing the contribution of next-nearest neighbour d shell-d shell interaction. The computed phonon dispersion results of platinum and palladium along the three principal symmetry directions on the basis of the Fielek model (1975) and the improved Fielek model are compared with the existing experimental data.