Lattice Dynamics Method Research Articles

Computational study of K vacancy with an H interstitial defect in [formula omitted] crystal

The electronic structures, defect formations, defect transition levels and optical properties for VK+Hi defect models in KDP crystals have been studied based on DFT. Lattice dynamics methods give the most reasonable compensation mechanism for VK, namely compensation with the third-nearest Hi neighbor from VK for the paraelectric (PE) phase and compensation with the fourth-nearest neighbor Hi from VK for the ferroelectric (FE) phase. The defect formation energies indicate that the (VK+Hi)x (The superscript represents the charged state, the ‘x’ represents neutral, ‘’’ represents −1 charge state and ‘.’ represents +1 charge state.) is the main defect type in this kind of defect cluster and a self-trapped electron is located at Hi in the (VK + Hi)′ system. For (VK+Hi)x system, one electron is accommodated in VK. There is not a new defect state in the band gap. The Hi bonds with the O ion (0.99 Å) form a hydroxyl. For (VK + Hi)′ system, the hydroxyl is broken, the Hi exists in an atomic form and introduces new defect states in the band gap. As the large relaxation energy leads to a large Huang-Rhys factor, the Stokes red shifts will significantly affect the optical properties. A broad ultraviolet (UV) absorption band and emission band range from UV to visible are obtained originating from the defect cluster. We believe that the far-violet absorption peaks (167 nm and 179 nm) caused by defect cluster VK+Hi can significantly impact the optical damage threshold of the KDP.

Mode-resolved phonon transmittance using lattice dynamics: Robust algorithm and statistical characteristics

Lattice dynamics (LD) enables the calculation of mode-resolved transmittance of phonons passing through an interface, which is essential for understanding and controlling the thermal boundary conductance (TBC). However, the original LD method may yield unphysical transmittance over 100% due to the absence of the constraint of energy conservation. Here, we present a robust LD algorithm that utilizes linear algebra transformations and projection gradient descent iterations to ensure energy conservation. Our approach demonstrates consistency with the original LD method on the atomically smooth Si/Ge interface and exhibits robustness on rough Si/Ge interfaces. The evanescent modes and localized effects at the interface are revealed. In addition, bottom-up analysis of the phonon transmittance shows that the anisotropy in the azimuth angle can be ignored, while the dependency on the frequency and polar angle can be decoupled. The decoupled expression reproduces the TBC precisely. This work provides comprehensive insights into the mode-resolved phonon transmittance across interfaces and paves the way for further research into the mechanism of TBC and its relation to atomic structures.

Studying the Influence of T2O Substitution for H2O on the Dynamic Properties, Density Maximum, and Melting Point of Ice in Terms of the Lattice Dynamics Method

An isotopic effect arising from the substitution of superheavy water molecules for normal water molecules in ice (Ih) has been studied by the lattice dynamics method in a quasi-harmonic approximation using a rigid three-point potential modified to reproduce the superheavy water properties. It has been shown that the considerable variation of the vibrational state density upon substituting 12.5, 50, and 100% of water molecules takes place only in the range of libration. The temperature dependence of the superheavy ice density has been calculated, and the density maximum for this ice near 60 K has been predicted. A relationship between the melting point of (T2O + H2O)-ice Ih and the T2O molecule concentration in its structure has been constructed, and this relationship has been found to be linear.

Studying the Influence of T2O Substitution for H2O on the Dynamic Properties, Density Maximum, and Melting Point of Ice in Terms of the Lattice Dynamics Method

Studying the Influence of T2O Substitution for H2O on the Dynamic Properties, Density Maximum, and Melting Point of Ice in Terms of the Lattice Dynamics Method

Lattice dielectric properties of rutile TiO2 : First-principles anharmonic self-consistent phonon study

We calculate the lattice dielectric function of strongly anharmonic rutile ${\mathrm{TiO}}_{2}$ from ab initio anharmonic lattice dynamics methods. Since an accurate calculation of the $\mathrm{\ensuremath{\Gamma}}$ point phonons is essential for determining optical properties, we employ the modified self-consistent approach, including third-order anharmonicity as well as fourth-order anharmonicity. The resulting optical phonon frequencies and linewidths at the $\mathrm{\ensuremath{\Gamma}}$ point agree much better with experimental measurements than those from a perturbative approach. We show that the four-phonon scattering process contributes as much as the third-order anharmonic term to phonon linewidths of some phonon modes. Furthermore, incorporating the frequency dependence of phonon linewidth reveals that experimentally known but unidentified peaks of the dielectric function are due to the two-phonon process. This work emphasizes the importance of the self-consistent approach in predicting the optical properties of highly anharmonic materials.

The influence of boundary scattering phase shift on melting behavior of nanoparticles

Based on the Lindemann melting criterion and the lattice dynamical approach, we analytically estimated the melting temperature of nanoparticles with emphasis on the influence of the boundary scattering phase shift parameter α . With the decrease of size, the estimated relation shows that the melting temperature of free standing like nanoparticles will decrease, and the melting temperature of embedded nanoparticles will increase. The increase or decrease of the melting temperature depends on the boundary scattering phase shift parameter. The gradual change of parameter α from − 1 to 1 corresponds to the transition of the surface or environment of nanoparticles from the free boundary condition to the fixed boundary condition. Our analytical estimation qualitatively explains the experimental data, and its predictive capability will help the design of nanoparticles. • Based on the Lindemann criterion, An analytical estimation for size-dependent melting temperature of nanoparticles by lattice dynamical method will be given. • Our theoretical result can fit experimental data of nanoparticles in different environments qualitatively. • A simple boundary scattering phase shift is used to characterize the environment of nanoparticles.

Influence of Quantum Oscillations in the Thermal Scattering Law of Zirconium Carbide on Neutron Thermalization and Criticality

Zirconium carbide (ZrC) is a candidate material for advanced high temperature reactors, including space nuclear thermal propulsion applications. Thermal scattering laws (TSLs) are generated in the incoherent approximation for carbon bound in ZrC [C(ZrC)] and zirconium bound in ZrC [Zr(ZrC)], using ab initio lattice dynamics methods. Disordered alloy theory is introduced to improve treatment of isotopic composition within the elastic scattering cross section. Localized higher-energy vibrational modes and the presence of a phonon band gap in C(ZrC) cause quantized oscillation in the TSL atypical of nonhydrogenous solids. These oscillations yield a significant likelihood of large energy downscattering and upscattering interactions such that the quanta of energy transfer affecting neutron thermalization is substantially greater than classically expected. MC21 critical mass calculations of ZrC mixtures with high-enriched uranium demonstrate an impact of TSLs when compared to a free-gas treatment for thermal neutron–driven 235U loadings. The critical mass of homogenous mixed moderator systems of ZrC and reactor-grade graphite are also sensitive to the ZrC TSL. Moreover, the effect of quantized energy exchange on the neutron spectra is found to influence the temperature feedback coefficient.

Dynamic matrix theory for resonance response of plasmonic metamaterial lattice

In this paper, a lattice dynamics method, named M-K matrix method, is proposed to investigate the near-field resonance response of a plasmonic metamaterial lattice under an oblique incident field with an arbitrary incident angle. By considering the electric, magnetic and field-dipole interactions, we construct a dissipative many-body Lagrange model for a reference lattice. A collective forced vibration equation, with the degree of freedom equals to the number of nanoparticles in a cell, is introduced to describe the lattice resonance under a polarized field. The resonance frequencies can be conveniently obtained from the poles of transfer function matrix. Based on this elegant matrix differential equation, one can calculate the amplitude-frequency and phase-frequency responses of plasmonic lattice, and analysis the normal modes from dispersion relations. The analytical results, which are from three examples: simple square lattice, binary chain and chessboard lattice, are perfectly matched with numerical simulations in a large frequency band, proving it to be an effective tool to calculate the dynamic response of plasmonic lattice.

Phase relations, thermal conductivity and elastic properties of ZrO2 and HfO2 polymorphs at high pressures and temperatures.

In the present study, P-T phase diagrams of ZrO2 and HfO2 for a wide pressure range of 0-150 GPa at 0-2500 K were calculated for the first time using density functional theory with the method of lattice dynamics within the quasi-harmonic approximation. We calculated P-T conditions for a full sequence of high-pressure transformations, P21/c → Pbca → Pnma → P6̄2m, for both compounds. At low temperatures, these transformations for ZrO2 are obtained at 7.6 GPa (P21/c → Pbca), 13.4 GPa (Pbca → Pnma), and 143 GPa (Pnma → P6̄2m), while for HfO2 similar polymorphic transitions are obtained at 9 GPa (P21/c → Pbca), 16 GPa (Pbca → Pnma), and 126 GPa (Pnma → P6̄2m), correspondingly. At high temperatures, for both ZrO2 and HfO2 the P21/c and Pbca structures transform into the P42/nmc modification. In addition, the thermal conductivity and elastic properties of the ZrO2 and HfO2 polymorphs were calculated and compared with the available experimental and theoretical data.

Phonon interference effects in graphene nanomesh

Graphene nanomesh (GNM) is a single-layer graphene material that has a periodic distribution of nanoscale pores. GNM shows great potential applications in various fields such as thermoelectric energy conversion, energy storage, and field-effect transistors. In this study we utilize non-equilibrium molecular dynamics and lattice dynamics method to investigate the thermal transport mechanism of GNM. The thermal conductivity of GNM is mainly affected by the number of nanoscale pores and their horizontal and vertical spacing. Our study finds that as the number of nanoscale pores increases, the thermal conductivity of GNM decreases significantly. Additionally, the increase of the number of nanoscale pores causes phonon branch to be folded and confined, which results in a flatter dispersion curve, wider bandgap, and slower phonon group velocity. Moreover, the horizontal and vertical spacing of the nanoscale pores jointly affect the thermal transport process of GNM. When the horizontal spacing is small, the thermal conductivity of GNM decreases monotonically with the increase of vertical spacing, and increases monotonically with an increase of horizontal spacing. However, as the horizontal spacing increases, the interference effect caused jointly by phonon reflection and superposition leads to significant fluctuations in thermal conductivity. The analysis of the spectral heat flow, density of states, participation rate, and group velocity of GNM indicate that the variation in vertical spacing leads to different phonon contributions to heat flow, resulting in fluctuations in the thermal conductivity of GNM. These findings could serve as a reference for controlling the thermal transport of graphene nanomesh, and are of great significance in regulating the thermal conductivity and designing nanoscale pores in GNM.

Correlation of rattlers with thermal transport and thermoelectric performance.

The presence of rattlers in the host-guest structure has sparked great interest in the field of thermoelectrics, as it allows for the suppression of thermal transport in materials through vigorous anharmonic vibrations. This work predicts a ternary half-Heusler compound, LiAgTe, with good thermoelectric properties and high-temperature stability, which possesses a host-guest structure. Furthermore, it provides a detailed analysis of the role of rattlers in the transport process. By microscopically exploring rattlers, we have revealed that rattlers (Ag atoms), while suppressing the thermal transport properties of the host framework, provide a significant enhancement of the electronic transport capability through the provision of nearly free weakly bound electrons. Using self-consistent phonon theory combined with compressive sensing lattice dynamics method, we captured the exact lattice thermal conductivity considering quartic anharmonicity and four-phonon scattering, and obtained the electronic transport parameters through the calculation of τe, which includes full anisotropic acoustic deformation potential scattering, polar optical phonon scattering, and ionized impurity scattering. We systematically dissected the role of rattlers in the host-guest structure by combining methods such as electron local function, Bader charge density, and Vibration visualization. The anharmonic vibrations of rattlers enhance the temperature response of scattering, resulting in rapid deterioration of thermal transport at high temperatures. Moreover, the extended d-orbital electrons of the rattlers, together with the p-orbital electrons of the Te atom in the host framework, result in the coexistence of maximum degeneracy and high dispersion bands in the valence band, which greatly enhances the electronic transport properties.

Bandgaps and topological interfaces of metabeams with periodic acoustic black holes

Acoustic black hole (ABH) with extraordinary properties of wave manipulation and energy focalization has been drawing growing attention in mechanical metamaterial, and the development of topological phenomena provides a novel way for investigation of ABH characteristics. This paper explores the physical mechanism of dispersion curves of three metabeams with embedded periodic ABHs and studies the topological phenomena of the compound metabeam for energy localization. First, the mass-spring models are employed according to lattice dynamics method to clarify the formation mechanism of bandgaps and the effect of the parameters on dispersion relation. Meanwhile, the diatomic mass-spring model provides an analytical framework to investigate the band inversion and the displacement distribution of topological states. Furthermore, numerical analyses are carried out to investigate the band structures, eigenmodes, transmission, and the achievement of topological interface state by proposed compound metabeam. The adjustment of structural parameters, changing the mass and stiffness of the mass-spring model, can broaden the bandgap in low frequency for better vibration attenuation performance. Besides, the modal formation in low frequency is explained by masses vibration distribution. The correctness of the method is verified by comparing the transmission numerically with previous experiments. This work enriches the existing knowledge of ABH on wave manipulating and energy harvesting.

Thermal conduction in a densified oxide glass: Insights from lattice dynamics

Thermal conductivity is an important property of oxide glasses, but its structural origins remain largely unknown. Here, we provide detailed modal information on thermal conductivity in a calcium aluminosilicate glass by relying on recent advances in lattice dynamics methods. We probe various structural features using molecular dynamics simulations by densifying the glass at pressures up to 100 GPa and studying the vibrational, mechanical, and thermal properties. We demonstrate good agreement between these simulations and complementary experiments, both of which indicate significant pressure-induced alteration of mechanical moduli, vibrational density of states, boson peak behavior, and thermal conductivity. We also find an intriguing correlation between the boson peak frequency and the total thermal conductivity in both the current glass series and a lithium borate glass series reported in literature. This correlation scales with the Debye frequency, suggesting that both parameters are associated with the transformation of the elastic medium under pressure.

High-Temperature Thermodynamics Modeling of Graphite

We present high-temperature thermodynamic properties for graphite from first-principles anharmonic theory. The ab initio electronic structure is obtained from density-functional theory coupled to a lattice dynamics method that is used to model anharmonic lattice vibrations. This combined approach produces free energies and specific heats for graphite that compare well with available experiments and results from models that empirically represent experimental data, such as CALPHAD. We show that anharmonic theory for the phonons is essential for accurate thermodynamic quantities above about 1000 K.

Thermodynamics Modeling for Actinide Monocarbides and Mononitrides from First Principles

The high-temperature thermodynamical properties for the actinide monocarbides and mononitrides ThC, ThN, UC, UN, PuC, and PuN are calculated from first-principles electronic-structure theory. The electronic structure is modeled with density-functional theory (DFT) and is fully relativistic, including the spin-orbit interaction. Furthermore, the DFT is extended to account for orbital–orbital interactions, by means of a parameter-free orbital-polarization (OP) technique, that has proven to be essential for the 5f electrons in plutonium. Strong anharmonicity and the temperature dependence of the lattice vibrations are captured with the self-consistent ab initio lattice dynamics (SCAILD) method. The calculated free energies and heat capacities are compared to published results from quasi-harmonic (QH) theory, and experiments, where available. For the uranium and plutonium compounds, we make use of CALPHAD assessments to help evaluate the theory. Generally, our anharmonic relativistic approach compares well with both CALPHAD and experiments. For the thorium compounds, our theory is in good accord with QH modeling of the free energy at lower temperatures but for the heat capacity the comparison is less favorable.

Phase diagram prediction and high pressure melting characteristics of GaN

The III-V compound semiconductor, GaN, has become an excellent semiconductor material for developing the high-frequency and high-power electronic devices because of its excellent characteristics, including large band width, high thermal conductivity and fast electron saturation rate, and has received extensive attention in recent years. However, the decomposition temperature of GaN is lower than the melting temperature, some of its fundamental properties, such as melting temperature and high temperature phase transition pressure, are still unclear, and so, now the investigation of fundamental properties dominates the whole process of this material from development to mature applications. In the present work, the classical molecular dynamics simulations combined with the first-principles calculations and lattice dynamics methods are adopted to predict the phase diagrams of GaN with wurtzite and rocksalt structures in a pressure range of 0–80 GPa. The phase transition pressures, 44.3 GPa and 45.9 GPa, obtained from the first-principles calculations and molecular dynamics simulations from wurtzite to rocksalt structure in GaN at zero temperature, are in agreement with the available experimental results (Sadovyi B, et al. <ext-link ext-link-type="uri" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://doi.org/10.1103/PhysRevB.102.235109">2020 <i>Phys. Rev. B</i> <b>102</b> 235109</ext-link>). The melting temperature at 0 GPa is 2295 K obtained by extrapolating the GaN melting curve of the wurtzite structure. With the pressure increasing to 33.3 GPa, the melting curve of wurtzite structure in GaN intersects with the melting curve of rocksalt structure, and the melting temperatures of both structures increase with pressure increasing. It is found that GaN may have a superionic phase and the superionic phase transition occurs in the wurtzite structure at pressures greater than 2.0 GPa and temperatures above 2550 K, whereas the rocksalt structure undergoes a superionic phase transition at pressures and temperatures higher than 33.1 GPa and 4182 K, respectively, and both of the phase transition temperatures increase with pressure increasing. The slope of the phase boundary line of GaN is positive at high temperatures and gradually changes into a curve with a negative slope as the temperature decreases.

Effect of ion irradiation on thermal conductivity of phosphorene and underlying mechanism

Defects produced by ion irradiation can effectively modulate many physical properties of phosphorene. In this paper, the molecular dynamics method is used to simulate the ion irradiation process of phosphorene. The relations between the formation probability of defects and the energy of incident ions, ion species and incident angle of ions are revealed. The non-equilibrium molecular dynamics simulation is used to calculate the thermal conductivity of irradiated phosphorene. The effects of the energy of ions, the irradiation dose, the type of ions and the incident angle of ions on the thermal conductivity of phosphorene are systematically investigated. The influence of the vacancies on the phonon participation rate of phosphorene is studied by lattice dynamics method, and the spatial distribution of localized modes is demonstrated. According to the quantum-mechanical perturbation theory and bond relaxation theory, we point out that the dominant physical mechanism of vacancy defects which significantly reduce the thermal conductivity of phosphorene is the strong scattering of phonons by the low-coordinated atoms near the vacancies. This study provides a theoretical basis for tuning the heat transport properties of phosphorene by defect engineering.

Thermodynamic properties of propane and methane hydrates doped with sodium hydroxide

Pure and sodium hydroxide doped with single-component hydrates of methane and propane are studied using molecular and lattice dynamics methods. Vibration density of states and the dependence of Helmholtz free energy on temperature and cell volume are calculated. Dynamic stability of empty and filled by gas sI and sII structures doped with 1 or 2 NaOH molecule is shown in the wide range of temperature values. Comparison of free energy values allows calculating the thermal expansion coefficient and demonstrating the possibility of self-preservation effect in NaOH-doped methane and propane hydrates with ~1.8 and ~1.4 mol% of sodium hydroxide, respectively.

Vacancy and phonon dispersion properties of Be, Co, Hf, Mg, and Re by modified embedded atom method potentials.

The modified embedded atom method (MEAM) potentials improved by Jin et al. (Appl. Phys. A120 (2015), p. 189) were applied to calculate the mono- and bi-vacancy properties as well as the phonon dispersions for hexagonal close-packed (HCP) metals Be, Co, Hf, Mg, and Re. We expressed the formulas for calculating the mono- and bi-vacancy properties by the molecular static (MS) method based on the MEAM potentials for HCP metals. The lattice dynamics (LD) method and the MEAM potentials were adopted to calculate the phonon dispersion properties. The calculation results show better agreement with the experimental data than the previous calculations by using the unimproved embedded atom model.

Generation of the TSL for Zirconium Hydrides from Ab Initio Methods

Zirconium hydride (ZrHx) is a moderator material used in TRIGA and other reactors that may exist in multiple phases with varying stoichiometry, which include the δ phase and the ϵ phase. Current ENDF/B-VIII.0 ZrHx thermal scattering law (TSL) evaluations do not distinguish between phases. These sub-libraries were generated with the LEAPR module of NJOY using historic phonon spectra derived from a central force model and assume incoherent elastic scattering for both bound hydrogen and zirconium, which neglects the effects of crystal structures important for scattering from zirconium bound in ZrHx. In this work, the TSLs for hydrogen and zirconium bound in δ-ZrHx and ϵ-ZrH2 were generated from phonon spectra derived from modern ab initio lattice dynamics methods and ab initio molecular dynamics. Subsequently, TSLs for hydrogen and zirconium in ZrHx and ZrH2 were generated using the Full Law Analysis Scattering System Hub (FLASSH) code. The built-in generalized coherent elastic routine was used to generate the previously neglected elastic contribution from zirconium for this material. The present TSLs provide both a re-evaluation of the current ZrH sub-libraries and expansion of the set of TSLs available for the examination of neutrons in systems with zirconium hydride, permitting explicit treatment of δ and ϵ phases.