Phonon DOS Research Articles

Phasego 3.0: Automatic analysis of synthesis and decomposition conditions for compounds

In this paper, we present a revised version of Phasego 2.0 toolkit which is a package for automatic phase diagram analysis and anharmonic effect corrections. In this new version, the automatic synthesis/decomposition conditions analysis functionality is added to predict the stable field of compounds based on the quasi-harmonic approximation (QHA) or the anharmonic free energy corrections. The analysis about the synthesis/decomposition conditions provides the vital information for new materials design under high pressure and temperature. New version program summaryProgram Title:Phasego3Program Files doi:http://dx.doi.org/10.17632/xp7gvmrxrg.1Licensing provisions: GNU General Public License, version 3Programming language: Python (versions 2.4 and later)Journal reference of previous version: Computer Physics Communications, 197 (2015) 341Does the new version supersede the previous version?: YesExternal libraries: Numpy [1], Scipy [2], Matplotlib [3]Nature of problem: A compound can be synthesized from different components and at different pressure and temperature conditions. It can also decompose into several possible products at specific pressure and temperature conditions according to the thermodynamic equilibrium relations. There are many possibilities in products when pressure and/or temperature vary. However, it is very complicated to analyze which chemical process can really proceed according to the Gibbs free energy criteria.Solution method: All kinds of possible combinations of reactants/products before/after the synthesis/decomposition of a compound can be automatically analyzed according to the atom number equality of each atom species on the two sides of the chemical equation. The comparison and analysis of the Gibbs free energies among all the possible combinations of reactants/products are automated in the updated version. The boundaries between the compound and the possible combinations of reactants/products can also be plotted and labeled automatically. The reactants for a compound or the decomposition products of a compound can be predicted and located accurately.Reasons for the new version: We have improved the package considerably to perform the synthesis/decomposition conditions analysis for compounds. In the new version, the phase boundaries between the compound and the possible combinations of reactants/products can also be plotted automatically. The reactants for a compound and the decomposition products of a compound can be predicted and located accurately. These functionalities are very helpful to the researchers in the new materials design community.Summary of revisions:• The functionality of synthesis/decomposition conditions analysis is added.• The automatic chemical equations search functionality has been added according to the atom number equality of each atom species on the two sides of a chemical equation.• The phase diagram can be analyzed by taking into account the synthesis/decomposition conditions.• The energy and volume now are treated with per formula unit (f.u.) instead of per atom.• The previously calculated phonon Helmholtz free energies can be imported for further calculations.• Some parameters are renamed or added.• A number of bugs have been corrected.Restrictions: The restrictions are from the phonon DOS calculations. The accuracy of the previously calculated phonon DOS also affects that of the final results.Unusual features: The synthesis/decomposition conditions of compounds can be analyzed and plotted automatically.Additional comments: The new version of this package can perform the synthesis/decomposition conditions analysis based on the free energies calculated from the phonon density of states data from many methods, e.g. the molecular dynamics (MD) simulations [4], the self-consistent ab initio lattice dynamics (SCAILD) calculations [5], or other simulation methods.Running time: The examples provided in the distribution take less than 5 minute to run.

DFT Study of Carbon Crystals from Nested K4 Lattice to BC‐8 Structure

AbstractCarbon crystals of a nested K4 lattice and BC‐8 structure are studied. We propose a unified relationship between the nested K4 lattice and the BC‐8 structure. The atomic positons of both structures can be given by Wyckoff position 16(c) of space group No. 206 and its shifted lattice. We can obtain continuously changing configurations from the nested K4 lattice to the BC‐8 structure with one parameter. DFT calculation based on plane waves is performed in order to estimate the total energy and phonon DOS. The structure near the BC‐8 structure is found to have a stable state. We consider the nested K4 lattice a saddle point on the energy landscape and it is in a metastable state.

Stress/strain aging mechanisms in Al alloys from first principles

First-principles based calculations were carried out to explore the possible mechanisms of stress/strain aging in Al alloys. Potential effects of temperature and external stress/strain were evaluated on the solvus boundary of Al3Sc in Al-Sc alloy, and the interface energy of Al/θ″ in Al-Cu alloys. Results show that applying tensile strain/stress during conventional aging can significantly decrease the solubility entropy, by red-shifting the phonon DOS at high states. The resulted solvus boundary would shift up on the phase diagram, suggesting a reduced solubility limit and an increased maximum possible precipitation volume of Al3Sc in Al-Sc alloy. Moreover, the applied strain/stress has different impacts on the formation energies of different orientated Al/θ″ interfaces in Al-Cu alloys, which can be further exaggerated by the Poisson effect, and eventually affect the preferential precipitation orientation in Al-Cu alloy. Both mechanisms are expected to play important roles during stress/strain aging.

Ab initio systematic study of chirality effects on phonon spectra, mechanical and thermal properties of narrow single walled carbon nanotubes

Pseudo-potential method under frame work of density functional perturbation theory (DFPT) has been utilized to calculate the phonon spectrum, phonon DOS, specific heat capacity and mechanical properties of narrow armchair and zigzag single walled carbon nanotubes (SWCNTs). Our calculations show that although their Young modulus are about 1TPa, but armchair SWCNTs have greater compressive modulus than the zigzag tubes while the situation would be vice versa for tensile modulus. On the other hand, it was found out that specific heat capacity of narrow tubes shows chirality independence and decreases by increasing the radius of nanotube whereas, this is not the case for the wider ones. In addition, chirality dependence of the radial breadth mode (RBM) frequency of narrow SWCNTs has been shown.

Puzzling phonon dispersion curves and vibrational mode instability in superconducting MgCNi3

A first principles calculation of the lattice dynamical properties of superconducting MgCNi3 has been performed using density functional perturbation theory (DFPT). The calculated phonon dispersion curves and phonon density of states have been compared with inelastic x-ray scattering (IXS) and inelastic neutron scattering (INS) measurements. We show for the first time that phonon dispersion curves for MgCNi3 in whole Brillouin zone are positive (stable phonon modes) and in good agreement with the experimental data. The phonon DOS shows absence of phonon density of states at zero energy unlike earlier calculations. There is a good agreement between calculated and experimental electron-phonon parameter and superconducting transition temperature. The Eliasberg function is quantitatively as well as qualitatively different from the phonon density of states. The lattice specific heat and Debye temperature do not show any anomalous behaviour.

Grüneisen parameter of hcp-Fe to 171 GPa

individual gvib data points with gvib = gvib,0(V/V0) q ,a common parameterization, we found an ambient pressure gvib,0 = 2.0 ± 0.1 for the range q = 0.8 to 1.2. We also determined the Debye sound velocity (vD) from the low‐energy region of the phonon DOS and our in situ measured volumes, and used the volume dependence of vD to determine the commonly discussed Debye Gruneisen parameter (gD). Comparing our gvib(V )a ndgD(V), we found gvib to be ∼10% larger than gD at any given volume. Finally, applying our gvib(V )t o aM ie ‐Gruneisen type relationship and an approximate form of the empirical Lindemann melting criterion, we predict the vibrational thermal pressure and estimate the high‐pressure melting behavior of ‐Fe at Earth’s core pressures. Citation: Murphy, C. A., J. M. Jackson, W. Sturhahn, and B. Chen (2011), Gruneisen parameter of hcp‐Fe to 171 GPa, Geophys. Res. Lett., 38, L24306, doi:10.1029/ 2011GL049531.

Stable structure of platinum carbides: A first principles investigation on the structure, elastic, electronic and phonon properties

A comprehensive first principles study of structural, elastic, electronic, phonon and thermodynamical properties of novel metal carbide, platinum carbide (PtC) is reported within the density functional theory scheme. The ground state properties such as lattice constant, elastic constants, bulk modulus, shear modulus and finally the enthalpy of PtC in zinc blende (ZB) and rock-salt (RS) structures are determined. The energy band structure and electron density of states for the two phases of PtC are also presented. Of these phases zinc blende phase of PtC is found stable and phase transition from ZB to RS structure occurs at the pressure of about 37.58 GPa. The phonon dispersion curves and phonon DOS are also presented. All positive phonon modes in phonon dispersion curves of ZB-PtC phase indicate a stable phase for this structure. Within the GGA and harmonic approximation, thermodynamical properties are also investigated. All results reveal that the synthesized PtC would favor ZB phase. The compound is stiffer and ductile in nature.

Lattice Dynamics of Ferroelectric SbSBr Crystal

The temperature dependence of phonon dispersion, total and partial phonon DOS of SbSBr crystal have been determined in paraelectric phase at T = 30 K and in ferroelectric phase at T = 11 K using FP—LAPW method in the framework of DFT along with the GGA approximation. Temperature dependence of total and partial phonon DOS for Sb, S and Br atoms where used for calculation of the vibration free energy of the unit cell Fvib and contribution from Sb, S and Br atoms to the free energy Fi,μ along c(z) —axis in quasiharmonic approximation in paraelectric and ferroelectric phases in temperature range from 11 K to 294 K. For investigation of softening effect nature of phonons along c(z) —axis the mean-squared displacements of Sb, S and Br atoms in x -, y -, z - direction have been calculated in quasiharmonic approximation in ferroelectric and paraelectric phases.

Ab initio lattice dynamics of CoH and NiH

Ground state parameters and lattice dynamics of CoH and NiH phases are studied by means of the state-of-the-art ab initio pseudopotentials methods. Calculated ground state parameters and phonon DOS are in rather good agreement with available experimental data. No phase transformation for CoH and NiH was found in a wide pressure and temperature range.

Studies of high-temperature electron–phonon interactions with inelastic neutron scattering and first-principles computations

Several recent studies of phonons combining inelastic neutron scattering and first-principles calculations are summarized. Inelastic neutron scattering was used to measure the phonon densities of states of the A15 compounds V3Si, V3Ge, and V3Co at temperatures from 10 K to 1273 K. It was found that phonons in V3Si and V3Ge, which are superconducting at low temperatures, exhibit an anomalous stiffening with increasing temperature, whereas phonons in V3Co have a normal softening behavior. Additional measurements of the phonon DOS of BCC V alloys were performed, and it was found that a stiffening anomaly present in pure V is suppressed upon introduction of extra d-electrons by alloying. First-principles calculations of the electronic and phonon densities of states show that in both these systems, the anomalous phonon stiffening originates with an adiabatic electron–phonon coupling mechanism. The anomaly is caused by the thermally-induced broadening of sharp peaks in the electronic density of states, which tends to decrease the electronic density at the Fermi level. These results illustrate how the combined use of first-principles calculations and inelastic neutron scattering provides powerful insights into couplings of excitations in condensed-matter.

Lattice Dynamics of LaFeAsO1-xFx and PrFeAsO1-y via Inelastic X-Ray Scattering and First-Principles Calculation

The lattice dynamics of LaFeAsO 1- x F x ( x =0,0.1) and PrFeAsO 1- y ( y ∼0.1) are investigated using inelastic x-ray scattering and ab-initio calculation. Measurements of powder samples provide an approximation to the phonon DOS, while dispersion is measured from a single crystal of PrFeAsO 1- y . A model that agrees reasonably well with all of the data at room temperature is built from results of ab-initio calculations by reducing the strength of the Fe–As bond by 30%.

First-Principles Calculations on Crystal Structure and Thermodynamic Properties of Ceramics

First-principles calculations have been widely used to describe the ground state properties of materials over almost 20 years. Recently, a great progress was made in the first-principle calculations. Thermodynamic properties can also be gotten by calculations of the phonon densities of states (phonon DOS) and phonon dispersions of materials, which show widely potential applications in material researches. In the present work, the energetics and bonding properties of interfaces between ZrO2 and Ni metal were given by first-principles calculations. The results show that alloy element impurities (Al, Cr and Y) influence remarkably the adhesion of the ceramic and metal. On the other hand, the phonon densities of states and phonon dispersions of ZrO2 were calculated with density functional perturbation theory. From the phonon DOS, the thermodynamic properties were derived and the phase transformation of ZrO2 was discussed. By this method, the thermodynamic properties of material can be gotten from atom and electron levels without any experiment data. It is a new approach to design and study the thermodynamic properties in new material system.

Point-contact spectroscopy of the borocarbide superconductor YNi2B2C

Point-contact (PC) spectroscopy measurements on YNi2B2C single crystals in the normal and superconducting (SC) state (Tc≃15.4K) for the main crystallographic directions are reported. The PC study reveals the electron–phonon interaction (EPI) function with a dominant maximum around 12meV and a further weak structure (kink or shallow broad maximum) at higher energy at about 50meV. Other phonon maxima at 20, 24 and 32meV specified in the phonon DOS of YNi2B2C by neutron measurements [F. Gompf et al., Phys. Rev. B 55 (1997) 9058] are not resolved in the PC spectra pointing out to the main role of the low energy phonon modes in EPI. Directional study of the SC gap results in Δ[100]≈1.5meV for the a-direction and Δ[001]≈2.4meV along the c-axis which may point to anisotropic and/or multiband behavior. Noteworthy, the critical temperature Tc in all cases corresponds to that of bulk samples. The value 2Δ[001]/kBTc≈3.6 is close to the BCS one of 3.52, and the temperature dependence Δ(T) is BCS-like, while for the a-direction Δ(T) deviates from mean-field BCS behavior above Tc/2. The directional variation in Δ can be attributed to the multiband nature of the SC state in YNi2B2C predicted 10 years ago [S.V. Shulga et al., Phys. Rev. Lett. 80 (1998) 1730].

Pressure-dependent phonon properties ofLa0.7Sr0.3MnO3

In this work, we present the calculated results on the pressure dependent phonon properties of rhombohedral $(R\overline{3}c)$ ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}$ manganite by using a lattice dynamical model theory. The effect of internal pressure determined by the average $A$-site atomic radius is also investigated and compared with the effect of applied pressure. The computed zone center phonon frequencies at ambient pressure agree fairly well with the experimental results and the modes related to the octahedral distortion exhibit hardening with the increase in pressure. The Raman active ${A}_{1g}$ mode is most sensitive to pressure with a slope of $1.0\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}∕\mathrm{GPa}$ and shows unusual couple of slope change. Both internal and external pressures harden the ${A}_{1g}$ phonon mode and reduce the electron-phonon interactions. Change in the frequency of the Raman active ${E}_{g}$ modes with frequency is also observed. Different effect is observed for the low and high average $A$-site atomic radius. The phonon dispersion curves in high symmetry directions of the Brillouin zone and phonon density of states are also calculated. The pronounced shift of the peak positions in phonon DOS is observed with the increase in pressure. The analysis of the reduction of the high frequency phonon peak width and effective mode-Gr\"uneisen parameter allows us to draw a conclusion for the decrease in lattice distortion and to some extent Jahn-Teller (JT) distortion, a signature of rhombohedral structure for the manganites. The role of pressure on the lattice specific heat is also discussed.

Thermal Expansion of the Perovskite‐Type NdGaO3.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Thermal expansion of the perovskite-type NdGaO3

The structure of NdGaO3 was investigated by means of powder diffraction using synchrotron radiation in the temperature range 12–1173K. The NdGaO3 structure can be assigned to the GdFeO3-type (space group Pbnm) in the whole temperature range investigated. The thermal behaviour of the cell parameters was measured and found to be highly anisotropic. NdGaO3 shows practically the same thermal expansion along [100] and [001] directions, whereas the expansion along [010] is about 40% lower. In order to calculate the NdGaO3 thermal expansion three independent simulation techniques were applied: lattice dynamics, molecular dynamics and the Debye model. Using the GULP code and interatomic potentials described in the Buckingham form the Nd–O and Ga–O interactions have been derived. The total phonon DOS, its projections onto atomic species, the heat capacity Cv, the Grüneisen parameter γ and the thermal expansion coefficient α were determined using quasiharmonic lattice dynamics. Molecular dynamic simulations were applied to model the thermal expansion at different pressures and temperatures. The structure, its elastic properties and melting temperature are close to the experimentally observed values, which indicate the correct choice of the interatomic potential form and parameters. Comparison between the total DOS and its projections with the Debye DOS and Grüneisen parameter values calculated for different approaches allows to conclude that NdGaO3 cannot be considered as a good Debye-like solid.

Effect of external Gaussian random field on phonon spectrum of linear atomic chains

We present the theoretical study of the effect of external random field characterized by a Gaussian probability distribution function on the continuous phonon spectrum of one-dimensional (1D) chain, based on the Jacobian matrix method. The cumulative effect of the random field and simple isotopic defect is studied analytically and numerically. The Gaussian random field removes a square-root divergence appearing in the phonon spectrum of ideal 1D chain. The impurity phonon DOS shows strong dependence on the variance and the mean of the random field and exhibits very different behavior from the non-random case: the continuous spectrum is expanded and the δ-peak, describing discrete impurity vibrations in the non-random chain with the impurity, falls into a continuous zone.

The crystal structure and thermal expansion of the perovskite-typeNd0.75Sm0.25GaO3: powder diffraction and lattice dynamical studies

The structure of Nd0.75Sm0.25GaO3 was studied by high-resolution powder diffraction methods using conventionalx-ray and synchrotron radiation in the temperature range 85–1173 K. TheGdFeO3 structure typewas confirmed for Nd0.75Sm0.25GaO3 in the temperature region investigated and no structural transitions were observed. The cellparameters show a monotonic and anisotropic increase with temperature. The interatomicpotential was fitted using the GULP code. Using this potential, a self-consistentapproximation following the Debye model was constructed from the elastic constants of thecrystals. The total phonon DOS, its projections onto atomic species, heat capacityCv, Grüneisen parameterγ and thermalexpansion coefficient α were considered in the framework of quasiharmonic lattice dynamics and the Debye model.The shape of the phonon DOS calculated from lattice dynamics differs significantly fromthe respected Debye DOS. The rare earth, gallium and oxygen atoms dominate in differentfrequency regions of the phonon spectrum. The heat capacity is well reproduced by theDebye model below 100 K, where acoustic phonons play an important role and above800 K when the classical limit is reached. Predicted values of Grüneisen parameterand thermal expansion coefficients in the frame of the Debye model are too low. Therefore, the thermal properties ofNd0.75Sm0.25GaO3 cannot be explained by acoustic phonons only and hence,Nd0.75Sm0.25GaO3 cannot be described perfectly as a Debye-like solid with respect to its thermodynamicproperties.

Phonons in nanocrystalline fcc nickel

The enhancement in the phonon DOS of nanocrystalline fcc Ni at low and high energies is due to the anisotropic changes in the interatomic force constants as compared to the bulk phase fcc Ni. The specific heat and vibrational entropy of nanophase is found to be greater than that of the bulk phase. There occurred a peak in the excessive specific heat of nanocrystalline fcc Ni at low temperatures. The excessive entropy of nanocrystalline Ni also has a peak at low temperatures but it becomes constant at high temperatures. The free energy of nanophase is lower than that of the bulk phase. The difference in the free energies of bulk phase and nanophase Ni increases linearly with temperature.

Characterization of perovskite related oxides by nuclear resonance inelastic scattering of synchrotron radiation

Resonant nuclear inelastic scattering (NIS) spectra of (Sr,Ca)(Fe,Co)O3- and (Ba,Ca)(Fe,Co)O3- were measured with the energy resolution of 3.5 MeV by detecting the 6.3 keV Fe K X-rays following after Mossbauer effect on 57 Fe transition excited by the monochromatized 14.41 keV synchrotron radiation. Both series of oxides annealed in air and in CO2 were examined. Soft phonon peak arising at ~8 MeV in NIS spectra of (Sr0.5Ca0.5)(Fe0.5Co0.5)O3- and (Ba0.5Ca0.5)FeO3- was found to be correlated with the oxygen ordering into brownmillerite structure, whereas more complex multi-phonon structure was observed in the spectra of less oxygen-deficient oxides such as (Sr0.95Ca0.05)(Fe0.5Co0.5)O3- and (Sr0.7Ca0.3)(Fe0.5Co0.5)O3- showing rather random distrubution of the oxygen vacancies. The changes in phonon DOS after CO2 absorbtion were also detected. Lam-Mossbauer factors of these oxides were calculated on the base of Debye Model.