Full Phonon Spectrum Research Articles

Quantitative Hole Mobility Simulation and Validation in Substituted Acenes.

Knowledge of the full phonon spectrum is essential to accurately calculate the dynamic disorder (σ) and hole mobility (μh) in organic semiconductors (OSCs). However, most vibrational spectroscopy techniques under-measure the phonons, thus limiting the phonon validation. Here, we measure and model the full phonon spectrum using multiple spectroscopic techniques and predict μh using σ from only the Γ-point and the full Brillouin zone (FBZ). We find that only inelastic neutron scattering (INS) provides validation of all phonon modes, and that σ in a set of small molecule semiconductors can be miscalculated by up to 28% when comparing Γ-point against FBZ calculations. A subsequent mode analysis shows that many modes contribute to σ and that no single mode dominates. Our results demonstrate the importance of a thoroughly validated phonon calculation, and a need to develop design rules considering the full spectrum of phonon modes.

Structural, electronic, elastic, magnetic, phonon and thermodynamic properties of inverse-Heusler-Ti2FeX (X=Si, Ge, and Sn): Insights from DFT-based computer simulation

The structural, mechanical, electronic and lattice dynamic properties of Ti2FeX (X = Si, Ge, and Sn) inverse-Heusler alloys have been explored via first-principles calculations based on density functional theory. The equilibrium lattice constant, bulk modulus, electronic band structure and magnetic moment values of these alloys have been computed to be consistent with prior studies. Several mechanical parameters such as elastic constants Cij, bulk modulus B, Young modulus E, shear modulus G and Poisson’s ratio υ are calculated, and based on these calculations, mechanical stability is examined. The calculated values of the total magnetic moments are in close agreement with the existing theoretical data and comply with the Slater-Pauling rule. From their calculated electronic band structure, Ti2FeSi, Ti2FeGe and Ti2FeSn are found as half-metallic alloys at the equilibrium lattice constant with a minority-spin energy gap of 0.820, 0.850 and 0.780 eV, respectively. The full phonon spectra, and their total and partial density of states of these alloys have been carried out via the direct method. The calculated phonon spectrum points out dynamic stableness of these alloys. Furthermore, the thermodynamic properties such as the heat capacity, thermal expansion, entropy and Grüneisen parameter have been investigated in the Debye model using the Gibbs2 code with a series of temperature from 0 to 1500 K.

An efficient approximation of the supercell approach to the calculation of the full phonon spectrum

The supercell approach to first-principles calculations of an alloy's full phonon spectrum is computationally expensive. It is therefore rarely used for calculating temperature dependent free energies of alloy phases or for calculating alloy phase diagrams. Amid the wider acceptance of uncertainty evaluation and the use of more efficient methods for materials design, the supercell approach is revisited and a simpler and more computationally efficient approximation is proposed. The new method is then used to calculate the Cr-Ni temperature dependent thermodynamic properties and phase diagram, resulting in predictions that are in good agreement with previous thermodynamic assessment. The new approach can facilitate accurate and rapid CALPHAD thermodynamic assessments of multi-component systems.

First-principles calculations to investigate the structural, electronic, elastic, vibrational and thermodynamic properties of the full-Heusler alloys X2ScGa (X = Ir and Rh)

This study has investigated ab initio pseudopotential calculations on the structural, electronic, elastic, vibrational and thermodynamic properties of the full-Heusler X2ScGa (X = Ir and Rh) alloys. The calculations have taken place under consideration of the generalized gradient approximation (GGA) of the density functional theory (DFT) with using the plane-wave ab initio pseudopotential method. According to the calculations, the major contribution to electronic states at the Fermi energy has been achieved by d orbitals, revealing a more active role for transition metals Ir (Rh) and Sc atoms. The reckonings point out that the Ir2ScGa and Rh2ScGa have metallic behavior at the equilibrium lattice constant with the density of states (DOS) at the Fermi level (N (EF)) of 1.412 states/eV and 1.821 states/eV, respectively. The results of the elastic constants showed that these compounds met the criteria for Born mechanical stability. It was also observed that they have a ductile structure and exhibit anisotropic behavior according to Pugh criteria. Besides, the full phonon spectra and their projected partial density of states of the alloys have been analyzed with the first-principle linear-response approach of the density-functional perturbation theory. All the alloys behaved dynamically stable in the L21 phase. Furthermore, internal free energy, entropy, specific heat capacity at constant volume and vibrational free energy changes of Ir2ScGa and Rh2ScGa alloys were analyzed and discussed between the temperature range of 0–800 K using the quasi harmonic approximation. According to the results, these alloys are potential candidate for industrial use.

Advanced McMillan’s equation and its application for the analysis of highly-compressed superconductors

Any theory of electron-phonon mediated superconductivity requires knowledge of the full phonon spectrum in order to calculate superconducting transition temperature, Tc. However, there is currently no experimental technique for measuring in highly-compressed near-room-temperature (NRT) superconductors. In this paper, we propose to advance McMillan’s approach (1968 Phys Rev 167 331), which utilises the Debye temperature (an integrated parameter for the full phonon spectrum), deduced via the fit of experimentally measured temperature-dependent resistance data R(T) to the Bloch-Grüneisen equation for highly-compressed black phosphorous, boron, GeAs, SiH4, HxS, DxS, LaHx, and LaDy. By utilizing the relations between Tc, Tθ, and the electron-phonon coupling strength constant λe-ph (which can be computed from first-principles calculations), it is possible to affirm/disprove the electron-phonon coupling mechanism in given superconductors. We show that computed λe-ph for highly-compressed black phosphorous, boron, GeAs, SiH4, and for one sample of LaH10 are in a good agreement with λe-ph values deduced from experimental data. A remarkable constancy of for H3S at different ageing stages is also found. We also show that if the phonon spectra of two isotopic counterparts share an identical shape (or, in the case of highly-compressed superconductors, the same material at different pressures), then within electron-phonon phenomenology, these materials should obey the relation of Tθ,1/Tθ,2 = Tc,1/Tc,2 = ωln,1/ωln,2 (where subscripts 1 and 2 designate two isotopic counterparts). We further report that H3S-D3S pair ratios of Tc,H3S/Tc,D3S = ωln,H3S/ωln,D3S = 1.27 are largely different from deduced Tθ,H3S/Tθ,D3S = 1.65. This implies that NRT superconductivity in H3S-D3S systems originates from more than one mechanism, where the electron-phonon coupling lifts Tc in H3S vs D3S, but the primary origin for the NRT background of Tc ∼ 150 K in both H3S and D3S remains to be discovered.

Structural electronic and dynamic properties of Li3Bi and Li2NaBi

We report a study of structural, electronic and dynamic properties of Li3Bi and Li2NaBi via density functional theory. It is found that Li3Bi and Li2NaBi show semiconducting property with an indirect electronic band gap. The calculated structural and electronic parameters (lattice parameters, bulk modulus, bulk modulus derivation) are in a good agreement with the available experimental data. Full phonon spectra of Li3Bi and Li2NaBi materials in the Rock Salt structure were collected using the linear response method. At 0 GPa pressure, Li3Bi is dynamically stable while Li2NaBi, which can be synthesized from Li3Bi by replacing one Bi atom with the Na atom, is unstable. In this study we searched to find the pressure value that makes Li2NaBi dynamically stable. Calculations showed that the Li2NaBi structure becomes stable when 8.62 GPa pressure is applied to the Li2NaBi structure. This study is thought to give direction to the future studies.

Strong phonon localization in PbTe with dislocations and large deviation to Matthiessen\u2019s rule

Dislocations can greatly enhance the figure of merit of thermoelectric materials by prominently reducing thermal conductivity. However, the evolution of phonon modes with different energies when they propagate through a single dislocation is unknown. Here we perform non-equilibrium molecular dynamics simulation to study phonon transport in PbTe crystal with dislocations by excluding boundary scattering and strain coupling effect. The frequency-dependent heat flux, phonon mode analysis, and frequency-dependent phonon mean free paths (MFPs) are presented. The thermal conductivity of PbTe with dislocation density on the order of 1015 m−2 is decreased by 62%. We provide solid evidence of strong localization of phonon modes in dislocation sample. Moreover, by comparing the frequency-dependent phonon MFPs between atomistic modeling and traditional theory, it is found that the conventional theories are inadequate to describe the phonon behavior throughout the full phonon spectrum, and large deviation to the well-known semi-classical Matthiessen’s rule is observed. These results provide insightful guidance for the development of PbTe based thermoelectrics and shed light on new routes for enhancing the performance of existing thermoelectrics by incorporating dislocations.

Modeling of Hypothetical BeXAs2 (X = Si, Ge, Sn) Crystal Structure and Phonon Spectra

The paper presents ab initio density functional calculations of the geometry optimization of BeXAs2 (X = Si, Ge, Sn) crystal structure, primitive cells and phonon spectra in the centre of the Brillouin zone. Interpolation of phonon spectra was performed over the Brillouin zone using the Keating model. Ab initio frequency calculations are used to ascertain the Keating parameters via program optimization. The values of phenomenological parameters are in good agreement with the parameters of the chemical bond obtained by ab initio calculations. The partial phonon spectra are investigated. The structure of acoustic and optical branches of the full phonon spectrum is determined mostly by the ion mass ratio of the crystal compounds.

Investigation of structural, electronic, elastic and phonon properties of cubic spinel ZnM2O4(M = Co, Rh and Ir) compounds

The results obtained from ab initio calculations on ZnM2O4 (M = Co, Rh and Ir) compounds have been reported. The elastic constants, Bulk, Shear and Young modulus, and Poisson's ratios of the compounds are presented. In addition, full phonon dispersion curves and projected density of states of the compounds have been computed using the direct method. The lattice parameters (a) and internal parameters (u) are found to be in a good agreement with experimental results. According to both the B/G values and the Poisson's ratio, these compounds have covalent bondings. The analysis of the band structure of these compounds have indicated indirect band gaps of 1.25 eV for ZnCo2O4 and 1.14 eV for ZnRh2O4 and 0.86 eV for ZnIr2O4. The full phonon spectra of these compounds show that they are dynamically stable in the cubic spinel structure.

Computational investigations of mechanic, electronic and lattice dynamic properties of yttrium based compounds

Electronic, mechanic and lattice dynamic properties of yttrium-based compounds, X3Y, where X = Pd, Pt and Rh were investigated using the density functional theory. The electronic band calculations demonstrated that X3Y compounds are metallic at the cubic crystal structures. The calculated elastic constants using the energy-strain method indicate that the three materials are mechanically stable. The calculated bulk modulus and Young’s modulus values suggest that the Pt3Y is stiffer than that of the other two. The type of bonding and ductility in the X3Y compounds were also evaluated based on their B/G ratios, Cauchy pressures (C[Formula: see text]–C[Formula: see text]) and band structure calculations. These compounds were found to be ductile in nature. The density functional perturbation theory was used to derive full phonon frequencies and total and projected phonon density of states. The computed full phonon spectra for X3Y compounds show that these compounds in the L12 phase are dynamically stable. Debye temperature and specific heat of these compounds were also calculated and evaluated using quasi harmonic approximation.

Reply to “Comment on ‘Comparative study of ab initio nonradiative recombination rate calculations under different formalisms’ ”

As pointed out by the comments of Wickramaratne et al., there was indeed an algebra error in our original publication [Phys. Rev. B 91, 205315 (2015)] in calculating the 1D model. We have corrected this error, and did a renewed calculation and comparison of the 1D model and other approximated models. The revised comparison provides some insights for the origin of the difference between the full phonon spectrum electron-phonon coupling static formula results and other approximated results, especially for how to evaluate the amplitude of electron-phonon coupling.

Lattice dynamic properties of Rh 2 XAl (X=Fe and Y) alloys

Abstract The electronic band structure, elastic and vibrational spectra of Rh2FeAl and Rh2YAl alloys were computed in detail by employing an ab-initio pseudopotential method and a linear-response technique based on the density-functional theory (DFT) scheme within a generalized gradient approximation (GGA). Computed lattice constants, bulk modulus and elastic constants were compared. Rh2YAl exhibited higher ability to resist volume change than Rh2FeAl. The elastic constants, shear modulus, Young modulus, Poisson's ratio, B/G ratio electronic band structure, total and partial density of states, and total magnetic moment of alloys were also presented. Rh2FeAl showed spin up and spin down states whereas Rh2YAl showed none due to being non-magnetic. The calculated total densities of states for both materials suggest that both alloys are metallic in nature. Full phonon spectra of Rh2FeAl and Rh2YA1 alloys in the L21 phase were collected using the ab-initio linear response method. The obtained phonon frequencies were in the positive region indicating that both alloys are dynamically stable.

Abnormal negative thermal expansion of sodium: A first-principles discovery at high pressures

Negative thermal expansion coefficients ($\ensuremath{\alpha}$) are found in the fcc and cI16 (a distorted bcc structure) phases of sodium at pressures above 90 GPa from first principles. At room temperature, the most significant negative $\ensuremath{\alpha}$ of about $\ensuremath{-}4.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\phantom{\rule{4pt}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ is predicted in fcc sodium at 120 GPa, which represents the first prediction of a robust negative $\ensuremath{\alpha}$ in a simple metal. Anharmonic lattice dynamics calculations were carried out to rationalize this intriguing behavior. We find that the negative thermal expansion of fcc sodium originates from the transverse acoustic modes with large negative Gr\uneisen parameters. Furthermore, full phonon spectra of fcc sodium at elevated temperatures are calculated from perturbation theory. The phonon-phonon couplings in fcc sodium are found to increase abnormally under high pressures.

ICONE23-1349 FIRST-PRINCIPLES STUDY ON THERMAL NEUTRON CROSS SECTIONS OF ELECTRON-DOPED LITHIUM HYDRIDE

LiH is designated as a promising moderator and shielding material because of its low density, high melting point and large fraction of H atoms. Previous work was done to produce full and partial phonon spectra of LiH in a primitive cell and then be used to generate thermal neutron scattering cross sections of H in LiH and Li in LiH using the NJOY code system. A re-evaluate work with super cells was done in this paper to modify DFT (Density Function Theory) calculations' error in First Principles. The overall agreement of the lattice constants for pure LiH with the experiments is excellent if the zero-point motion is taken into account. From the theoretical calculation for electron (n)-doped LiH, it is indicated that metallic n-doped LiH was found to be a good superconductor. The phonon spectrum was predicted to be significantly softened, with both Li and H vibrations softened with the introduction of the dopant. Analysis was done in this paper to study thermal neutron scattering cross sections of this electron (n)-doped LiH. A comparison of the generated cross sections shows that accounting for the dopant introduced in the calculations affects the cross sections mainly in some energy range, and the phonon effect mainly occurs at low temperature, which is helpful to study the neutronic properties of LiH, especially for the reactors used LiH moderator.

Phonon and thermal transport properties of the misfit-layered oxide thermoelectric Ca3Co4O9 from first principles

The full phonon spectra of two rational approximants to the incommensurate thermoelectric oxide material Ca3Co4O9 (CCO) are calculated from first principles within density functional theory (DFT) and its DFT + U extension. The computed phonon dispersion curves are then used to evaluate the constant volume heat capacity, as well as the full and partial (subsystem-projected) lattice thermal conductivity of CCO. The results are discussed in terms of the effects of the Hubbard U on the computed properties, the size of the rational approximants used to model CCO, the relative contributions of the two subsystems that comprise the misfit-layered material, and the anisotropic nature of the computed thermal conductivity. We obtain good agreement with available experimental data.

Strong enhancement of phonon scattering through nanoscale grains in lead sulfide thermoelectrics

We present nanocrystalline PbS, which was prepared using a solvothermal method followed by spark plasma sintering, as a promising thermoelectric material. The effects of grains with different length scales on phonon scattering of PbS samples, and therefore on the thermal conductivity of these samples, were studied using transmission electron microscopy and theoretical calculations. We found that a high density of nanoscale grain boundaries dramatically lowered the thermal conductivity by effectively scattering long-wavelength phonons. The thermal conductivity at room temperature was reduced from 2.5 W m−1 K−1 for ingot-PbS (grain size >200 μm) to 2.3 W m−1 K−1 for micro-PbS (grain size >0.4 μm); remarkably, thermal conductivity was reduced to 0.85 W m−1 K−1 for nano-PbS (grain size ∼30 nm). Considering the full phonon spectrum of the material, a theoretical model based on a combination of first-principles calculations and semiempirical phonon scattering rates was proposed to explain this effective enhancement. The results show that the high density of nanoscale grains could cause effective phonon scattering of almost 61%. These findings shed light on developing high-performance thermoelectrics via nanograins at the intermediate temperature range.

Lattice dynamics and thermal properties of phononic semiconductors

An enhanced adiabatic bond charge model has been employed to study the lattice dynamics of phononic metamaterials based on group-IV and group--III-V semiconductors. Using the full phonon spectrum and a realistic Brillouin zone summation method, we have developed theories of phonon scattering rates from interface formation and anharmonicity. Numerical results for specific-heat capacity and phonon conductivity of thin Si/Ge and GaAs/AlAs superlattices are then presented and compared with available experimental measurements. The roles of various phonon scattering mechanisms in controlling the thermal conductivity in different temperature ranges have been quantified.

Effects of nonframework metal cations and phonon scattering mechanisms on the thermal transport properties of polycrystalline zeolite LTA films

We present a systematic study to investigate the effects of nonframework cations and the role of phonon scattering mechanisms on the thermal transport properties of zeolite LTA, via experiment and semiempirical lattice dynamics calculations. Our study is motivated by the increasing interest in accurate measurements and mechanistic understanding of the thermal transport properties of zeolite materials. The presence of a nanostructured pore network, extra-framework cations, and tunable framework structure and composition confer interesting thermophysical properties to these materials, making them a good model system to investigate thermal transport in complex materials. Continuous films of zeolite LTA with different nonframework cations (Na+, K+, and Ca+2) were synthesized and characterized. The thermal conductivity was measured using the three-omega method over a wide range of temperature (150–450 K). These are the first thermal conductivity measurements performed on bulk LTA, so they are more accurate than previous measurements, which involved the use of compacted zeolite powders. Our data showed significant dependence of the thermal conductivity on the extra-framework cations as well the temperature. The thermal conductivities of the zeolite LTA samples were modeled with the relaxation time approximation to the Boltzmann transport equation. The full phonon spectra for each type of LTA zeolite were calculated and used in conjunction with semiempirical relaxation time expressions to calculate the thermal conductivity. The results both validated, and suggested the limitations of, this modeling approach. Optical phonons dominated the thermal conductivity and boundarylike scattering was found to be the strongest phonon scattering mechanism, as also observed in MFI zeolite.

Optical phonon spectrum and the Fröhlich Hamiltonian in würtzite-type nanotubes

Dispersion laws of the full optical phonon spectrum of a nanotube made of würtzite-typematerials as well as the corresponding Fröhlich electron–phonon interaction terms arederived and studied within the framework of the dielectric continuum model for a uniaxialcrystal. The coupling coefficients describing electron–phonon interaction are obtained in ananalytical closed form, depending on the dispersion law of the involved phononbranch. We present and discuss results of numerical calculations of optical phononspectrum and Fröhlich coupling coefficients for some chosen würtzite AlN nanotubes.Observed features, induced by the anisotropy of würtzite-type materials, are discussed.

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.