Phonon Vibrational Frequencies Research Articles

Electronic structural and lattice thermodynamic properties of MAlO2 and M5AlO4 (M = Li, Na, K) sorbents for CO2 capture applications

The electronic properties and thermal stabilities of MAlO2 and M5AlO4 (M = Li, Na, K) are investigated by density functional theory and lattice phonon dynamics. Based on the calculated electronic and lattice thermodynamic properties, their abilities to capture CO2 as solid sorbents are analyzed. The calculated electronic structural properties of MAlO2 and M5AlO4 indicate that all these alkali aluminates are semiconductors with a bandgap range of 2.4 ~ 6.4 eV. The 1st valence bands of these alkali aluminates are located 0 ~ − 6 eV under Fermi levels and are mainly contributed by p orbitals of O, s and p orbitals of Al and M. The phonon vibrational frequencies of M5AlO4 spread at a lower frequency range compared to their MAlO2 phases. With increasing temperature, the calculated phonon free energies of M5AlO4 decrease faster than their corresponding MAlO2 while their entropies have opposite trends. The reaction 2MAlO2 + CO2 = M2CO3 + Al2O3 has higher reaction heat and Gibbs free energy change than those of corresponding reaction 2/5M5AlO4 + CO2 = M2CO3 + 1/5Al2O3, which shows the former reaction possesses lower turnover temperature. Among the alkali aluminates studied, the β-NaAlO2, lt-KAlO2, and γ-LiAlO2 are better candidates that could be applied for CO2 capture technologies.Graphical

First principles calculation of mechanical, dynamical and thermodynamic properties of MnO2 with four crystal phases

MnO2 has four crystal phases, whose stability and vibrational properties can affect significantly its application. In this study, a spin-polarized DFT + U method was performed to investigate the structural, mechanical, lattice dynamical, and thermodynamic properties of α-, β-, γ- and δ-MnO2. The analysis of the mechanical properties and phonon dispersion curves confirmed that all four kinds of MnO2 were mechanically and dynamically stable. The calculated mechanical properties indicated that α- and β-MnO2 were ductile, while γ-MnO2 was brittle. The phonon dispersion curves and the phonon density of states were calculated using DFPT. The phonon vibrational frequencies were analyzed in detail by the LO-TO splitting considering the non-analytical term correction. Raman and infrared active normal modes were assigned and discussed briefly to resolve the uncertainty in experiment of the assignment for the four kinds of MnO2. The symmetry of Au, Au, B3u were IR characteristic band for α-, β-, γ-MnO2, and the symmetry of Ag, A1g, Ag, A1g were Raman characteristic band for α-, β-, γ- and δ-MnO2. Finally, the thermodynamic functions of the enthalpy change and the entropy were evaluated and compared with the experimental values.

Time-Domain Terahertz Spectroscopy and Solid-State Density Functional Theory Analysis of p-Nitrophenol Polymorphs

Accurate interpretations of THz vibrational spectra using density functional theory (DFT) must account for the often significant thermal expansion exhibited by molecular crystal systems. In this study, the THz spectra of two p-nitrophenol polymorphs were obtained in the spectral bandwidth of 20–95 cm−1, and solid-state DFT was used to assign the observed absorptions to specific vibrational mode characters. Computational treatments of the crystal systems involved comparison of phonon frequencies produced from both fixed- and full-unit cell geometry optimizations using a variety of functional and basis set combinations. Arbitrary frequency scaling was applied to overcome the temperature dependence of unit cell volumes and associated phonon vibrational frequencies. Fully relaxed cell geometries producing contracted cell volumes resulted in a large overestimation of THz vibrational frequencies. Employing appropriate frequency scaling factors to the computed mode frequencies allowed for proper vibrational mode assignments and enhanced the quality of spectral simulations versus fixed-cell calculations. Optimal frequency scalars in the range of 0.675 to 0.827 were determined to best reproduce the THz spectra of the p-nitrophenol polymorphs. This treatment of calculated phonon modes offers the key advantage of eliminating the temperature correlation between the crystal structure data and the THz data acquisition.

First-principles investigation of elastic, mechanical, electronic and thermodynamic properties of Al3Li compound under pressure

First-principles calculations are employed to study the elastic, mechanical, electronic and thermodynamic properties of Al3Li compound under pressure up to 25.9 GPa. Based on the elastic constant and phonon calculations, we found that Al3Li has mechanical and dynamical stability in the considered pressure range. The elastic constants and mechanical properties of Al3Li under pressure are calculated. The values of hardness, thermal conductivity and melting temperature under ambient pressure are compared with that of the pure Al and Al3Sc. The wave velocities, Debye temperatures, phonon vibrational frequencies positively increase with the increasing pressure. The results from the analysis of electronic density of states exhibit a metallic bonding behavior of Al3Li. Finally, the temperature dependent behaviors of thermodynamic properties of Al3Li as a function of temperature are determined within the quasi-harmonic approximation theory and compared with the available experimental and theoretical data.

Phase stabilities and vibrational analysis of hydrogenated diamondized bilayer graphenes: A first principles investigation

The phase stabilities as well as some intrinsic properties of hydrogenated diamondized bilayer graphenes, 2-dimensional materials adopting the crystal structure of diamond and of lonsdaleite, are investigated using a first-principles approach. Our simulations demonstrate that hydrogenated diamondized bilayer graphenes are thermodynamically stable with respect to bilayer graphene and hydrogen molecule even at 0 GPa, and additionally they are found to withstand the physical change in structure up to at least 1000 K, ensuring their dynamical and thermal stabilities. The studied hydrogenated diamondized bilayer graphenes are predicted not only to behave as direct and wide band gap semiconductors, but also to have a remarkably high resistance to in-plane plastic deformation induced by indentation as implied by their high in-plane elastic constants comparable to those of diamond and of lonsdaleite. The mechanical stability of the materials is confirmed though the fulfilment of the Born stability criteria. Detailed analysis of phonon vibrational frequencies of hydrogenated diamondized bilayer graphenes reveals possible Raman active and IR active modes, which are found to be distinctly different from those of hydrogenated diamond-like amorphous carbon and defective graphene and thus could be used as a fingerprint for future experimental characterization of the materials.

Ab Initio Study of the ZnSnSb2 Semiconductor

For the chalcopyrite-like ZnSnSb2 crystal, the equilibrium crystal-lattice parameters a = 6.2893 A, c = 12.5975 A, and u = 0.2314 and the band structure involving the band gap Eg = 0.43 eV are determined by ab initio calculations based on density functional theory. The phonon vibrational frequencies, the elastic constants (C11 = 89.3, C12 = 41.9, C13 = 41.8, C33 = 90.4, C44 = 43.9, and C66 = 44.1), the phase velocities of elastic waves, the elasticity moduli, the microhardness (2.29 GPa), and the Gruneisen elastic parameter (1.5) are calculated. The temperature dependences of the heat capacity and thermodynamic potential are considered (in the range from 20 to 633 K).

Arsenene and Antimonene: Two-Dimensional Materials with High Thermoelectric Figures of Merit

Graphene has a big head start, but other two-dimensional (2D) materials continue to pique interest---and not just for electronics. These systems also might be good, for example, in thermoelectric devices to scavenge wasted energy, and many basic questions need to be addressed. The authors use density functional theory plus semiclassical Boltzmann transport to study As and Sb monolayers, and find that both show excellent thermoelectric response. With low phonon group velocities and vibrational frequencies reducing lattice thermal conductivity, they outperform all known 2D materials, and are serious competition even for well-established thermoelectrics.

Building a Fast Lane for Mg Diffusion in α-MoO3 by Fluorine Doping

Following previous experimental work examining a layered oxyfluoride as cathode material for Mg-ion batteries [Incorvati et al., Chem. Mater. 2016, 28, 17], we study the role of fluorination on the structural and electronic properties of molybdenum trioxide and its impact on Mg intercalation and diffusion using first-principles methods. Although bulk α-MoO3 is a 2D layered compound, we find that it provides 3D channels for Mg diffusion. When F atoms are incorporated into the α-MoO3 lattice, they replace the O atoms sitting at a specific crystallographic site that is linked by two nearest Mo atoms within a single Mo–O layer. As a consequence of F substitution, the local Mo-anion bonds are distorted, which leads to closure of the electronic band gap. From the analysis of zone center phonon vibrational frequencies, it is found that the local Mo-anion bonding strength is weakened by replacing O2– with F–, which ultimately facilitates Mg diffusion through the F-substituted lattice. For example, it is shown tha...

Evidence for photo-induced monoclinic metallic VO2 under high pressure

We combine ultrafast pump-probe spectroscopy with a diamond-anvil cell to decouple the insulator-metal electronic transition from the lattice symmetry changing structural transition in the archetypal strongly correlated material vanadium dioxide. Coherent phonon spectroscopy enables tracking of the photo-excited phonon vibrational frequencies of the low temperature, monoclinic (M1)-insulating phase that transforms into the metallic, tetragonal rutile structured phase at high temperature or via non-thermal photo-excitations. We find that in contrast with ambient pressure experiments where strong photo-excitation promptly induces the electronic transition along with changes in the lattice symmetry, at high pressure, the coherent phonons of the monoclinic (M1) phase are still clearly observed upon the photo-driven phase transition to a metallic state. These results demonstrate the possibility of synthesizing and studying transient phases under extreme conditions.

Gupta potential for rare earth elements of the fcc phase: lanthanum and cerium

The potential parameters for a Gupta-type many-body potential are fitted for the first two rare earth elements, La and Ce. The experimental cohesive energies, lattice parameters and elastic constants of β-La and γ-Ce solids of the face-centered cubic (fcc) phase are well reproduced. The theoretical P–V curves, sound velocities and Debye temperatures of β-La and γ-Ce solids are in reasonable agreement with experimental data. The vacancy formation energies and surface energies are also predicted. In particular, the phonon dispersion relationship and vibrational frequencies at high symmetric points within the first Brillouin zone from our potential are consistent with experimental ones. Molecular dynamics simulation are performed to determine the melting temperature of La and Ce solids as well as the radial distribution function of liquid La, which are also in line with experimental data. All these agreements indicate the validity of the current set of potential parameters. Thus, the Gupta potential developed here would be useful in future simulation of La, Ce solids and their alloys.

Observation of Γ-point phonon frequency in ultrathin metallic films confirmed by ab initio calculation and lattice dynamics

Vibrations in ultrathin metallic films excited by ultrafast light pulses have been studied based on continuum mechanics. However, this paper shows that they are Γ-point phonon vibrations of plate-phonon modes. Ab initio and lattice dynamics calculations are made to compare Γ-point phonon vibrational frequencies with measurements obtained by the picosecond ultrasound spectroscopy. The standing-wave frequencies of specific Γ-point phonon modes of the slab model show good agreement with measurements without any fitting parameters. This study informs us of a limitation of the continuum-mechanics theory for explaining the mechanics of ultrathin metallic films.

Phonon vibrational frequencies of all single-wall carbon nanotubes at the Lambda point: Reduced matrix calculations

With a simple method—the reduced matrix method, we simplified the calculation of the phonon vibrational frequencies according to SWNTs structure and their phonon symmetric property and got the dispersion properties of all SWNTs at Γ point in Brillouin zone, whose diameters lie between 0.6 and 2.5 nm. The calculating time is shrunk about 2–4 orders. A series of the dependent relationships between the diameters of SWNTs and the frequencies of Raman and IR active modes are given. Several fine structures including “glazed tile” structures in ω ∼ d figures are found, which might predict a certain macro-quantum phenomenon of the phonons in SWNTs.

Low-frequency Raman scattering of Ge and Si nanocrystals in silica matrix

Ge and Si nanocrystals (nc-Ge and nc-Si) with average sizes in the range of 2–7nm, embedded in silica matrix, were fabricated for investigating their acoustic-phonon vibrational properties. The freely elastic sphere theory was found to be unsuitable for explaining the low-frequency phonon vibration character of both nc-Ge and nc-Si in our current experiments. We have assigned the observed low-frequency Raman peaks to “LA-like mode” and “TA-like mode” in terms of their polarization and depolarization behaviors. In addition, it is revealed that the lattice contraction phenomenon exists in nc-Ge and nc-Si with sizes smaller than 4nm, which leads to a contrary effect against matrix traction on the phonon vibrational frequencies.

Phonon vibrational frequencies and elastic properties of solid SrFCl. An ab initio study

The phonon vibrational frequencies, electronic and elastic properties of SrFCl, one of the members of the alkaline-earth fluorohalide family crystallizing with the PbFCl-type structure, have been investigated, for the first time, at the ab initio level, by using the periodic CRYSTAL program. Both Hartree-Fock (HF) and density functional theory (DFT) Hamiltonians have been used, with the latter in its local density, gradient-corrected (PW91), and hybrid (B3LYP) versions. The structural and elastic properties are in good agreement with experiment, with the exception of those calculated within the local density approximation, which were found to be systematically under-estimated (distances) or over-estimated (elastic properties). As regards the phonon frequencies, B3LYP and PW91 provide excellent results, the mean absolute difference with respect to the experimental Raman data being 4.1% and 3.6%, respectively.

STRONGLY CORRELATED QUASI-ONE-DIMENSIONAL BANDS: GROUND STATES, OPTICAL ABSORPTION, AND PHONONS

Using the Lanczos method for exact diagonalization on systems up to 14 sites, combined with a novel phase randomization'' technique for extracting more information from these small systems, we investigate several aspects of the one-dimensional Peierls-Hubbard Hamiltonian, in the context of trans-polyacetylene: the dependence of the ground state dimerization on the strength of the electron-electron interactions, including the effects of off-diagonal'' Coulomb terms generally ignored in the Hubbard model; the phonon vibrational frequencies and dispersion relations, and the optical absorption properties, including the spectrum of absorptions as a function of photon energy. These three different observables provide considerable insight into the effects of electron-electron interactions on the properties of real materials and thus into the nature of strongly correlated electron systems. 29 refs., 11 figs.

Vibrational Spectrum and Specific Heat of Thallous Chloride

The phonon vibrational frequencies of thallous chloride have been calculated in various symmetry and off-symmetry wavevector directions using shell model. The calculated temperature dependence of specific heat from the vibrational spectrum is in good agreement with the experimental results. However, out results predict higher value for Debye-temperature \(\varTheta_{D}\) at 0°K than the one reported earlier based on Houston's series expansion method using elastic constant data.