Contribution Of Lattice Vibrations Research Articles

EVERAL NOTES ON THE LATTICE THERMALCONDUCTIVITY OF FRACTAL-SHAPED NANOPARTICLES

Using the additive technologies in the production of nanoparticle-fabricated three-dimensional materials has become one of the most promising ways of obtaining effective and low-cost thermoelectric energy converters. Nanostructuring provides a route to modifying selectively the transport properties which determine the materials thermoelectric efficiency. In this paper, we have shown one more effect which consists in a significant dependence of the contribution of lattice vibrations to the thermal conductivity coefficient of a nanoparticle (its reducing is required in practice) on its morphology for nanoparticles of a pure substance. The particle morphology has been specified by the values of its effective diameter, fractal dimension and surface roughness.Using nanoparticles of pure bismuth at low temperatures as an example, we have demonstrated a notable decrease in the lattice thermal conductivity in “complicating” the particle morphology. In the final section, we have presented methods of calculating characteristics of nanoparticle ensembles, the methodology of measuring the fractal dimension experimentally also being discussed.

Relative stability of Cu, Ag, and Pt at high pressures and temperatures from ab initio calculations

The paper presents ab initio studies into the relative stability of the crystalline structures of copper, silver, and platinum up to high pressures at $T\ensuremath{\ge}0$ K. Our calculations in quasiharmonic approximation suggest that not the fcc structure of Cu and Ag, but the body-centered cubic one, is thermodynamically most favorable at $P\ensuremath{\gtrsim}100$ GPa and $T>3$ kK. The shock Hugoniot of Cu and Ag crosses the fcc-bcc phase boundary and the calculated transition pressures agree well with the result of recent laser shock experiments by Sharma et al. The advantage of the bcc structure comes from its softer low-frequency phonon modes and the smaller contribution of lattice vibrations to free energy at high temperatures, as compared to close-packed structures. The compression of platinum crystal also causes the $\mathrm{fcc}\ensuremath{\rightarrow}\mathrm{bcc}$ transition but at much higher pressures, $P>1.4$ TPa.

Ab initio calculations of the phase diagrams of tin and lead under pressures up to a few TPa

The paper studies relative structural stability for various crystal phases of tin and lead from first principles with the full-potential all-electron full-potential all-electron linear muffin-tin orbital method to pressures of a few TPa both at zero temperature and at T > 0. Using data from our calculations we construct phase diagrams for the two metals in the region of very high compressions and obtain their melting curves. For tin at pressures <100 GPa and zero temperature, we did not find the region of stability of the body-centered orthorhombic (bco) phase, as it was earlier observed in experiments by Salamat et al [2013 Phys. Rev. B 88 104104]. Our calculations suggest that one structural transition from the tetragonal to cubic phase, bct → bcc, occurs in perfect Sn crystal at T = 0 K in the pressure range of about 27–32 GPa. But any deviation from perfection may cause an orthorhombic distortion of its tetragonal phase. At pressures above 100 GPa, the bcc → hexagonal close-packed (hcp) transition exists in both metals, and the phase boundary has a domed shape and does not rise in temperature above 2 kK. This behavior of the phase boundary with the increasing temperature is caused by the softer phonon modes of the bcc structure and the smaller contribution of lattice vibrations to the free energy of the crystal compared to the hcp phase. At pressures above 2.5 TPa and T ≲ 1 kK, lead can also undergo another structural transition, hcp → fcc, but at T > 1.5 kK there must exist the more energetically preferable bcc → fcc transition.

Ammonia borane structural study by temperature through high-resolution Raman spectroscopy and principal component analysis.

The structural change by temperature of the ammonia borane has been studied through Raman spectroscopy and principal components analysis (PCA). Its phase transition, from tetragonal to orthorhombic, was observed at 212K. In addition, between 99 and 83K, a zone of structural stability was observed. These behaviors could be observed thanks to the application of the PCA technique to the Raman data. Here, we can find a characteristic Raman signal which can also be associated with the orthorhombic structure. Also using the PCA technique, we have observed two Raman bands in the low frequency region, below 100cm-1, which could be associated with the lattice vibrational contribution.

TEMPERATURE-DEPENDENT PHOTOLUMINESCENCE STUDY OF POROUS GAP

This paper reports on the temperature-dependent photoluminescence of porous GaP under the 532-nm excitation. Porous GaP formed by electrochemical anodization of (111)-oriented bulk material exhibits green emission at 550 nm (2.25 eV) and red emission at 770 nm (1.65 eV) at room temperature. In the temperature range from 25 K to 275 K intensity from the green emission gradually decreases when the temperature increases. Additionally, peak position of the green luminescence band shifts to lower energy with increasing temperature and the same the GaP band gap narrowing with temperature. This means a contribution of lattice vibrations.

Experimental Determination of Momentum-Resolved Electron-Phonon Coupling.

We provide a novel experimental method to quantitatively estimate the electron-phonon coupling and its momentum dependence from resonant inelastic x-ray scattering (RIXS) spectra based on the detuning of the incident photon energy away from an absorption resonance. We apply it to the cuprate parent compound NdBa_{2}Cu_{3}O_{6} and find that the electronic coupling to the oxygen half-breathing phonon branch is strongest at the Brillouin zone boundary, where it amounts to ∼0.17 eV, in agreement with previous studies. In principle, this method is applicable to any absorption resonance suitable for RIXS measurements and will help to define the contribution of lattice vibrations to the peculiar properties of quantum materials.

The incredible excess entropy in high entropy alloys

The increased excess entropy in face-centered-cubic high entropy alloys is confirmed by quantitatively measuring heat capacities of pure Ni together with equiatomic NiCo, NiCoCr, and NiCoFeCr alloys with increasing configurational entropy. Surprisingly, the incredible excess entropy in the NiCoFeCr alloy can be even up to 0.9R, much larger than that in traditional alloys. The temperature-dependent measurement of the excess entropy indicates the major contribution of lattice vibrations. The excess magnetic entropy dominates in the temperature of 300 to 1000 K. The abnormal excess entropy reveals extraordinary lattice dynamic effects in high entropy alloys, which is a new feature in these alloys.

Temperature dependence of the energy band gap of CuSi2P3 semiconductor using PSOPW method

Abstract Theoretical formalism based on the orthogonalized plane wave method supplemented by a potential scaling scheme was used to predict the temperature dependence of energy gap of CuSi2P3 semiconductor. A computer code in Pascal was used to perform the variation of fundamental energy gap with temperature in the range of 150 K to 800 K. The dependence of energy gap on temperature for lattice dilation contribution, lattice vibration contribution and total temperature effect were performed separately. The results revealed that, as temperature increases, the top of the valence band and the bottom of the conduction band increase, while the energy band gap decreases. Generally, at low temperatures, the energy gap varies slowly and exhibits a nonlinear dependence and approaches linearity as temperature increases. The calculated energy gap of CuSi2P3 at T = 300 K is 0.4155 eV. The temperature coefficients in the linear region due to lattice dilation contribution, lattice vibration contribution and total temperature effect were calculated as –1.101 × 10−5 eV/K, –1.637 × 10−4 eV/K and –1.7523 × 10−4 eV/K, respectively. Also, the ratio of temperature coefficient of the energy gap due to LV contribution to its value and LD contribution in the linear region is equal to 14.868. That ratio is compared to those of CuGe2P3 and III-V compounds, where those of the latter show a systematic change with Eg. Moreover, the Eg of all the compounds shows a quadratic dependence on the inverse of mean bond length.

Computational study on thermal conductivity of defective carbon nanomaterials: carbon nanotubes versus graphene nanoribbons

Carbon-based nanostructures possess exceptional thermal properties and are being intensively explored as thermal management materials. In this paper, the thermal conductivities of carbon nanotubes (CNTs) and graphene nanoribbons (GNRs) with vacancy defects are compared by using non-equilibrium molecular dynamics simulations. The results show that the vacancy defects markedly decrease the thermal conductivities of CNTs and GNRs. By analyzing the vibrational density of states and contributions of lattice vibrations, we demonstrate that the lower thermal conductivities of defective CNTs and GNRs are attributed to the depressions of longitudinal acoustic and transverse acoustic modes, respectively. In the temperature range of 200–600 K, the vacancies always have a weaker effect on heat transfer of GNR than that of CNT, which stems from the mode localization induced by the phonon scattering on GNR edges. In addition, the percentage change in thermal conductivity of CNT decreases with increasing temperature, while that of GNR is generally independent of temperature. This phenomenon is related to the sensitivity of lattice vibration contributions to temperature. The present work expands our understanding of vacancy effect on heat conduction in carbon-based nanostructures.

First-principles calculations of the thermodynamics of wurtzite and zincblende ZnO1-xSx alloys

A theoretical study on the thermodynamics of wurtzite (WZ) and zincblende (ZB) ZnO1-xSx was carried out using the first-principles method and the cluster expansion method. The effective cluster interaction of the cluster expansion was calculated, which constructs the formation energy of the disordered alloys. Excluding the contribution of lattice vibrations, the critical temperature of phase separation for the WZ structure is 3946K and for the ZB structure is 3894K. Including lattice vibrations, a reduction of the critical temperature by 17.1% was found for the WZ structure, thus increasing the solid solubility of WZ-ZnO1-xSx. In contrast, an increase of the critical temperature by 4.4% was found for the ZB structure when considering lattice vibrations. With analyses of the formation energy and phase diagram, the ZB structure was found thermodynamically more stable than the WZ structure at high temperatures.

Theoretical studies on phonon density of states in nonstoichiometric palladium hydrides

We investigate the phonon density of states in nonstoichiometric palladium hydrides PdHx, in order to clarify the contribution of lattice vibrations to superconductivity. We employ the Burger-MacLachlan model, which considers a one-dimensional triply-connected diatomic chains with randomly distributed vacancies, focusing on the <111> direction in the rock-salt lattice of PdHx. The phonon density of states is calculated by the Forced Oscillator Method suitable for random systems. We find that phonon modes at high frequencies change from localized to collective modes with increasing x and the crossover stoichiometry is consistent with that of the appearance of superconductivity. These results support the optical phonon mechanism for superconductivity in PdHx.

Ab initio calculations of the elastic and thermodynamic properties of gold under pressure

The paper presents first-principles FP-LMTO calculations on the relative stability of fcc, bcc, hcp and dhcp gold under pressure. They were done in local density approximation (LDA), as well as in generalized gradient approximation (GGA) with and without spin–orbit interaction. Phonon spectra for the considered gold structures were obtained from LDA calculations within linear response theory and the contribution of lattice vibrations to the free energy of the system was determined in quasiharmonic approximation. Our thorough adjustment of FP-LMTO internal parameters (linearization and tail energies, the MT-sphere radius) helped us to obtain results that agree well with the available experimental phase relation Dubrovinsky et al (2007 Phys. Rev. Lett. 98 045503) between fcc and hcp structures of gold under pressure. The calculations suggest that gold compressed at room temperature successively undergoes the following structural changes: . The paper also presents the calculated elastic constants of fcc, bcc and hcp Au, the principal Hugoniot and the melting curve. Calculated results were used to construct the PT-diagram which describes the relative stability of the gold structures under study up to 500 GPa.

Thermal conductivity of silicene calculated using an optimized Stillinger-Weber potential

Silicene, the silicon-based counterpart of graphene with a two-dimensional honeycomb lattice, has attracted tremendous interest both theoretically and experimentally due to its significant potential industrial applications. From the aspect of theoretical study, the widely used classical molecular dynamics simulation is an appropriate way to investigate the transport phenomena and mechanisms in nanostructures such as silicene. Unfortunately, no available interatomic potential can precisely characterize the unique features of silicene. Here, we optimized the Stillinger-Weber potential parameters specifically for a single-layer Si sheet, which can accurately reproduce the low buckling structure of silicene and the full phonon dispersion curves obtained from ab initio calculations. By performing equilibrium and nonequilibrium molecular dynamics simulations and anharmonic lattice dynamics calculations with the new potential, we reveal that the three methods consistently yield an extremely low thermal conductivity of silicene and a short phonon mean-free path, suggesting silicene as a potential candidate for high-efficiency thermoelectric materials. Moreover, by qualifying the relative contributions of lattice vibrations in different directions, we found that the longitudinal phonon modes dominate the thermal transport in silicene, which is fundamentally different from graphene, despite the similarity of their two-dimensional honeycomb lattices.

Phase equilibrium of a CuInSe2–CuInS2 pseudobinary system studied by combined first-principles calculations and cluster expansion Monte Carlo simulations

We report the phase diagram of a CuInSe2–CuInS2 pseudobinary system calculated by a combination of first-principles calculations based on density functional theory, cluster expansion, and Monte Carlo simulations. All formation energies of CuIn(Se1−xSx)2 (CISS) alloys are positive, indicating that CISS alloy is a miscibility gap system and has a tendency to phase separation. The phase diagram computed with conventional cluster expansion shows a miscibility gap with consolute temperature TC=170K. The contribution of lattice vibrations lowers TC to 130K. The miscibility gaps for the CuInSe2–CuInS2 system are predicted to be asymmetric. The effect of lattice vibrations on the miscibility gap is found to be large, and the size mismatch mechanism can be used to explain the large vibrational effect in the CuInSe2–CuInS2 system.

Thermal Conductivity of Zeolitic Imidazolate Framework-8: A Molecular Simulation Study

While metal–organic frameworks have been extensively investigated for gas storage and separation, their thermal properties are scarcely studied and largely unknown. We report here a molecular simulation study to examine the thermal conductivity of zeolitic imidazolate framework-8 (ZIF-8). From equilibrium molecular dynamics simulation, the thermal conductivity of ZIF-8 is predicted to be about 0.165 W/mK. The low thermal conductivity is due to the short mean free path of the phonon in ZIF-8, which is estimated to be less than two unit cells. With increasing temperature from 300 to 1000 K, the thermal conductivity increases from 0.165 to 0.190 W/mK. The temperature effect is attributed to the enhanced overlap in the vibrational density of states between Zn and N atoms. The contributions of lattice vibrations in different directions to heat flux are examined from nonequilibrium molecular dynamics simulation. It is found that the longitudinal vibration contributes 60% to thermal transport in ZIF-8; in contrast, transverse vibration contributes 40%. Furthermore, the contributions from different forces to heat flux are analyzed. While the stretching and Lennard-Jones components have 31% contribution each, the bending and Coulombic components contribute 17% and 21%, respectively. However, the contribution of the torsional component is nearly zero. This simulation study provides quantitative understanding of the thermal conductivity of ZIF-8 and microscopic insight into the mechanism of heat transfer.

Cr segregation at C11b/C40 interface in MoSi2-based alloys: A first-principles study

Based on the first-principles calculation, we investigated Cr segregation behavior at the C11b/C40 lamellar interface for MoSi2-based alloys. We first elucidated the energetically stable interface structure among possible contacts between C11b and C40, where the interface energy is 1.7 meV/Å2. Without inclusion of lattice vibrational contribution to segregation energy, Cr composition at the interface was lower than that at C40 phase, which was inconsistent with the early experimental result. Inclusion of the vibrational contribution successfully resulted in strong Cr segregation at the interface over that at C11b as well as C40 phase. This indicates that Cr segregation at the most stable interface is a thermodynamically stable state, in which lattice vibration plays a significant role.

Thermal rectification at silicon/horizontally aligned carbon nanotube interfaces

Non-equilibrium molecular dynamics simulations were performed to investigate the thermal rectification effect of a system composed of a 400 nm long horizontally aligned single-walled (10, 10) carbon nanotube (CNT) and Si substrate. By imposing a series of positive and negative heat currents across the interface, a thermal rectification effect was observed. The maximum thermal rectification is about 184% when the interfacial heat flux is 60 W/m, which is very promising for thermal rectifier applications. By phonon-related analysis, we found that for heat flowing from Si to CNT, the increase of the interfacial thermal conductance with heat flux is due to the better match of phonon density of states between CNT and Si substrate at broad moderate frequencies, while for heat flowing from CNT to Si, the low-frequency phonon modes excited at large heat fluxes dominate the interfacial heat transfer and such low-frequency phonon mode mechanism is responsible for the thermal rectification effect. Moreover, we proposed a simple yet very useful method to quantify the directional contributions of lattice vibrations to the total interfacial heat flux and we demonstrated that the out-of-plane lattice vibrations at the interface dominate the heat transfer across the silicon/horizontally aligned carbon nanotube interfaces.

First-principles studies of electronic, optical, and vibrational properties of LaVO4 polymorph

First-principles calculations of electronic, optical, and vibrational properties of LaVO4 polymorph were performed with the density functional theory plane-wave pseudopotential method. The results of the electronic structure reveal that the different coordinated structure for monoclinic LaVO4 leads to an indirect band gap, while tetragonal LaVO4 has a direct band gap. Besides, the analysis of the electronic structure shows ionic nature in La–O bonds and covalent nature in V–O bonds. From further study in chemical bonding behavior, we find that the V–O covalent bonds have four types: σ bonding, π bonding, π∗ antibonding, and σ∗ antibonding states. Various optical properties, including the dielectric function, reflectivity, absorption coefficient, refractive index, and the energy-loss spectrum as functions of the photon energy were calculated. Our calculations indicate that monoclinic LaVO4 has excellent dielectric properties along [0 0 1] direction. In the optical-frequency (ω→∞) contributed from electrons the optical properties of tetragonal LaVO4 show the isotropy, while the diagonal components of static dielectric tensors ε(0) of tetragonal LaVO4 have the εxx=εyy≠εzz relation by adding the lattice vibration contribution (ω→0) to the electronic dielectric tensor. The vibrational spectra of LaVO4 polymorph have also been calculated from first principles by the linear response method. The calculated frequencies are in good agreement with the experimental data available for these crystals obtained by the methods of infrared and Raman spectroscopies. The vibrational spectra of monoclinic and tetragonal LaVO4 crystal exhibit three groups of frequencies: the low-frequency (&amp;lt;240 cm−1), middle-frequency (270–450 cm−1), and high-frequency region (850–970 cm−1), according to the vibration dominated by translation of La atoms, the bending vibration of O–V–O bonds, and stretching vibration of O–V–O bonds, respectively. Our studies report on microstructure of LaVO4 polymorph, and provide useful information for the potential application of this material.

A Molecular Dynamics Study of Thorium Nitride

AbstractFor Abstract see ChemInform Abstract in Full Text.

A molecular dynamics study of zirconium nitride

A molecular dynamics (MD) calculation was performed for zirconium nitride (ZrN) in the temperature range from 300 to 2800 K to evaluate the thermophysical properties, viz., the lattice parameter ( a), linear thermal expansion coefficient ( α lin), compressibility ( β), heat capacity ( C P ), and thermal conductivity ( λ). A Morse-type potential function added to the Busing–Ida type potential was employed as the potential function for interatomic interactions. The interatomic potential parameters were semi-empirically determined by fitting to the experimental values of variations of the lattice parameter with temperature and pressure. In case of the heat capacity, the lattice vibration contribution ( C V) and lattice dilational contributions ( C d) were evaluated, and in case of the thermal conductivity, only the phonon contribution ( λ lat) was evaluated by the MD calculation.