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1.
Phase transition and thermodynamic properties of ThO2:Quasi-harmonic approximation calculations and anharmonic effects
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The thermodynamic properties and the phase transition of ThO2 from the cubic structure to the orthorhombic structure are investigated using the first-principles projector-augmented wave method. The vibrational contribution to Helmholtz free energy is evaluated from the first-principles phonon calculations. The anharmonic contribution to quasi-harmonic free energy is accounted for by using an effective method(2010 Phys. Rev. B 81 172301). The results reveal that at ambient temperature, the phase transition from the cubic phase to the orthorhombic phase occurs at 26.45 GPa, which is consistent with the experimental and theoretical data. With increasing temperature, the transition pressure decreases almost linearly. By comparing the experimental results with the calculation results, it is shown that the thermodynamic properties of ThO2 at high temperature improve substantially after including the anharmonic correction to quasi-harmonic free energy. 相似文献
2.
在相图研究中, 严格计算一个真实系统在特定温度、压强下的自由能是近年来该领域理论方法发展的前沿. 自Mermin提出有限温度密度泛函理论后, 在电子结构层面, 弱关联系统中人们就其在对自由能贡献的描述已相对完善, 但在原子核运动的描述上, 热运动与量子运动的非简谐项却总被忽视. 本文将路径积分分子动力学与热力学积分结合, 对300 GPa下氢晶体Cmca 结构中原子核热涨落与量子涨落对自由能的影响进行了分析. 发现在100 K核量子涨落非简谐项的贡献约为15 meV每原子, 远大于不同结构间静态焓的差别. 该研究提醒人们简谐近似在核量子效应描述中可能存在的不准确性(即使在低温下). 同时, 我们采取的方法 也为人们进行自由能的准确计算提供了一个简单有效的手段. 相似文献
3.
We report high-temperature thermodynamics for fcc silver by combining ab initio phonon dynamics to empirical quadratic temperature-dependent term for anharmonic part of Helmholtz free energy. The electronic free energy is added through an interpolation scheme, which connects ambient condition free electron gas model to Thomas-Fermi results.The present study shows good agreement with experimental and reported findings for several thermal properties, and the discrepancy observed in some caloric properties is addressed. The decreases in the product of volume thermal expansion coefficient and isothermal bulk modulus and in the constant volume anharmonic lattice specific heat at high temperature are the clear evidences of proper account of anharmonicity. The present study also reveals that T~2-dependent anharmonic free energy is sufficient for correct evaluation of thermal pressure and conventional Grüneisen parameter. We observe that the intrinsic phonon anharmonicity starts dominating above characteristic temperature, which is attributed to higher order anharmonicity and can be related to higher order potential derivatives. We conclude that the uncorrelated and largeamplitude lattice vibrations at high temperature raise dominating intrinsic thermal stress mechanism, which surpasses the phonon-anharmonism and requires future consideration. 相似文献
4.
Despite ab initio computational techniques have opened new possibilities to interpret experimental results and predict the properties of new materials, their applications are limited by the adopted approximative schemes. Consequently, the first-principles calculation of many physical properties and phenomena is hindered and ab initio methods need to be further developed to overcome such limits. For example, the standard harmonic approximation used to assess the vibrational properties of materials often completely breaks down, so that the vibrational properties need to be calculated including strong anharmonic effects. The harmonic approximation has also intrinsic failures as it cannot estimate the lattice thermal conductivity of materials nor the temperature dependence of the phonon frequencies, crucial to account for temperature driven second-order phase transitions. Several methods developed in the last years to account for anharmonicity in the non-perturbative regime and overcome such difficulties are briefly reviewed in this colloquium paper. In particular, the stochastic self-consistent harmonic approximation, a variational method that allows calculating vibrational properties in strongly anharmonic systems, is described in further detail. Applications of the latter method to superconducting palladium, platinum, and sulfur hydrides are discussed, where anharmonicity has a huge impact on their vibrational and superconducting properties. 相似文献
5.
The pseudopotential plane-wave approach in the framework of the density functional theory, and the density functional perturbation theory with the generalized gradient approach for the exchange-correlation functional has been used to calculate the structural phase stability, elastic constants and thermodynamic properties of boron-arsenide (BAs) compound. The BAs compound transforms from the zincblende phase to rock-salt structure; the phase transition pressure was found to be 141.2 GPa with a volume contraction of around 8.2%. The thermodynamic properties under high pressure and temperature up to 125 GPa and 1200 K respectively were also determined, analyzed and discussed in comparison with other data of the literature. The systematic errors in the static energy were corrected using the bpscal EEC method. Our results agree well with those reported in the literature, where for example, our calculated melting temperature (2116 K) deviates from the theoretical one (2132.83 K) with only 0.8%, and the deviation between our result (1.86) of the Grüneisen parameter and the theoretical one (1.921) is only around 3.2%. 相似文献
6.
The thermoelastic properties of CaO over a wide range of pressure and
temperature are studied using density functional theory in the
generalized gradient approximation. The transition pressure taken
from the enthalpy calculations is 66.7GPa for CaO, which accords
with the experimental result very well. The athermal elastic moduli
of the two phases of CaO are calculated as a function of pressure up
to 200GPa. The calculated results are in excellent agreement with
existing experimental data at ambient pressure and compared
favourably with other pseudopotential predictions over the pressure
regime studied. It is also found that the degree of the anisotropy
rapidly decreases with pressure increasing in the B1 phase, whereas
it strongly increases as the pressure increases in the B2 phase. The
thermodynamic properties of the B1 phase of CaO are predicted using
the quasi-harmonic Debye model; the heat capacity and entropy are
consistent with other previous results at zero pressure. 相似文献
7.
Zhong-Li Liu Xiu-Lu Zhang Ling-Cang Cai Qiang Wu 《Journal of Physics and Chemistry of Solids》2008,69(11):2833-2840
We performed molecular dynamics simulations with the extended Finnis-Sinclair (EFS) potential to investigate thermal equation of state (EOS), and melting and thermoelastic properties of tantalum. The agreement of the obtained thermal EOS with experiments at ambient conditions is reasonably good. The EFS potential with the two-phase method also reproduced very satisfyingly the high-pressure melting curve, excellently consistent with both the experiments of melting temperature at ambient pressure and shock melting at high pressure. From molecular dynamics simulations, we also obtained the thermoelastic properties of Ta for temperatures up to 3000 K at ambient pressure. Fully including anharmonic effects in molecular dynamics, our calculated elastic constants are in excellent agreement with experimental data. Shear modulus G decreases quickly with increasing temperature. 相似文献
8.
Katayama I Aoki H Takeda J Shimosato H Ashida M Kinjo R Kawayama I Tonouchi M Nagai M Tanaka K 《Physical review letters》2012,108(9):097401
The ferroelectric soft mode in a SrTiO(3) thin film was impulsively driven to a large amplitude using intense picosecond terahertz pulses. As the terahertz electric field increased, the soft-mode absorption peak exhibited blueshifting and spectral narrowing. A classical anharmonic oscillator model suggests that the induced displacement is comparable to that of the ferroelectric phase transition. The spectral narrowing indicates that the displacement exceeds that induced by any inhomogeneities in the film, demonstrating that the method can be used to explore intrinsic quartic anharmonicity. 相似文献
9.
Wilfried B. Holzapfel 《高压研究》2013,33(1):209-216
Equations of State (EOS) for solids under strong compression and wide ranges in temperature are most commonly represented by "parametric" EOS forms using temperature dependent parameters for the volume V 0 , the bulk modulus K 0 , and its pressure derivative K_{0}^{\prime} for the given ambient (or zero) pressure. Therefore, various common "parametric" EOS forms are compared at first with the recently proposed [1,2] A dapted P olynomical expansion AP2, and in the second part with the Mie-Grüneisen approach, which uses one of the common EOS forms for the pressure of the static lattice or for the zero temperature isotherm, p ZT ( V ), and a detailed modelling of the additional thermal pressure, p th ( V , T ), in the form p(V,T) = p_{\rm ZT}(V) + p_{\rm th}(V,T) . Thereby, it is shown, that "intrinsic" anharmonicity effects have to be taken into account and between the two schemes differences of the order of a few percent in pressure are noticed for regular solids, like Cu, Ag, and Au. These differences are discussed with respect to the present uncertainties in a practical pressure scale for wide ranges in pressure (up to several TPa) and in temperature (up to 1500 K and above). 相似文献
10.
Phase transition and thermodynamic properties of BiFeO3 from first-principles calculations
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The first-principles projector-augmented wave method employing the quasi-harmonic Debye model,is applied to investigate the thermodynamic properties and the phase transition between the trigonal R3c structure and the orthorhombic Pnma structure.It is found that at ambient temperature,the phase transition from the trigonal R3c phase to the orthorhombic Pnma phase is a first-order antiferromagnetic-nonmagnetic and insulator-metal transition,and occurs at 10.56 GPa,which is in good agreement with experimental data.With increasing temperature,the transition pressure decreases almost linearly.Moreover,the thermodynamic properties including Grneisen parameter,heat capacity,entropy,and the dependences of thermal expansion coefficient on temperature and pressure are also obtained. 相似文献
11.
Theory of pseudopotential has been used in the present study to carry out computation of various thermodynamic parameters of barium. The role of anharmonic effect due to vibrations of lattice ions has been accounted by coupling local pseudopotential with mean field potential which has been computed using second-order perturbation theory. Contribution due to thermally excited electrons has been accounted by Mermin functional. The excellent agreement of presently computed pressure with experimental result has also been observed at which body centered cubic to hexagonal close packed structure phase transition occurs. Such success leads to conclude that the s-p-d hybridization and anharmonic effects are included properly in the presently used conjunction scheme with additional advantage of its computational simplicity. 相似文献
12.
S. V. Lepeshkin M. V. Magnitskaya N. L. Matsko E. G. Maksimov 《Journal of Experimental and Theoretical Physics》2012,115(1):105-111
The melting and lattice dynamics of sodium are studied by quantum molecular dynamics simulation, i.e., with allowance for anharmonicity, at pressures up to 1 Mbar and temperatures up to 1000 K. The simulation results agree well with the experimental data and our earlier calculation performed ab initio in the quasi-harmonic approximation. The simulation results demonstrate that anharmonic interactions weakly affect the melting curve and the phonon frequencies of Na up to near-melting temperatures. 相似文献
13.
Shalibo Y Rofe Y Barth I Friedland L Bialczack R Martinis JM Katz N 《Physical review letters》2012,108(3):037701
We measure the state dynamics of a tunable anharmonic quantum system, the Josephson phase circuit, under the excitation of a frequency-chirped drive. At small anharmonicity, the state evolves like a wave packet-a characteristic response in classical oscillators; in this regime, we report exponentially enhanced lifetimes of highly excited states, held by the drive. At large anharmonicity, we observe sharp steps, corresponding to the excitation of discrete energy levels. The continuous transition between the two regimes is mapped by measuring the threshold of these two effects. 相似文献
14.
We investigate the low temperature properties of the recently discovered clathrates Ba 6 Ge 25 and Na 2 Ba 4 Ge 25 by tuning both materials with hydrostatic pressure. At ambient pressure, Ba 6 Ge 25 undergoes a two-step structural phase transition between 230 K and 180 K from metallic behavior to a high-resistivity state. A superconducting transition occurs at T_{C}\approx 0.24\,\hbox{K} out of the resulting bad metal ( \rho_{0}\approx 1.5\,\hbox{m}\Omega\;\hbox{cm} ). With increasing pressure, the structural phase transition is shifted to lower temperature but T C increases drastically. T C reaches a maximum value of 3.85 K at the critical pressure p_{C}\approx 2.8\,\hbox{GPa} , where the structural distortion is completely suppressed and the system exhibits metallic behavior. On replacing 1/3 of the Ba atoms with Na (Na 2 Ba 4 Ge 25 ), no structural transformation is observed below room temperature, and the superconducting transition temperature is higher (T_{C}(p = 0) \approx 1.05\,\hbox{K}) than in the undoped case at ambient pressure but decreases slightly with increasing pressure. 相似文献
15.
The thermodynamic properties of the cerium dioxide (CeO2) are studied using the statistical moment method, including the anharmonicity effects of thermal lattice vibrations. The free energy, linear thermal expansion coefficient, bulk modulus, specific heats at the constant volume and those at the constant pressure, CV and CP, are derived in closed analytic forms in terms of the power moments of the atomic displacements. The temperature dependence of the thermodynamic quantities of cerium dioxide is calculated using three different interatomic potentials. The influence of dipole polarization effects on the thermodynamic properties and thermodynamic stability of cerium dioxide have been studied in detail. 相似文献
16.
17.
Thomas SA Uhoya WO Tsoi GM Wenger LE Vohra YK Chesnut GN Weir ST Tulk CA dos Santos AM 《J Phys Condens Matter》2012,24(21):216003
Neutron diffraction and electrical transport measurements have been made on the heavy rare earth metal holmium at high pressures and low temperatures in order to elucidate its transition from a paramagnetic (PM) to a helical antiferromagnetic (AFM) ordered phase as a function of pressure. The electrical resistance measurements show a change in the resistance slope as the temperature is lowered through the antiferromagnetic Néel temperature. The temperature of this antiferromagnetic transition decreases from approximately 122 K at ambient pressure at a rate of -4.9 K GPa(-1) up to a pressure of 9 GPa, whereupon the PM-to-AFM transition vanishes for higher pressures. Neutron diffraction measurements as a function of pressure at 89 and 110 K confirm the incommensurate nature of the phase transition associated with the antiferromagnetic ordering of the magnetic moments in a helical arrangement and that the ordering occurs at similar pressures as determined from the resistance results for these temperatures. 相似文献
18.
《Current Applied Physics》2014,14(3):496-507
The structural high pressure and temperature investigation of narrow-gap semiconductors (lead chalcogenides) has been performed in the present article. A realistic approach for room temperature and high temperature study of narrow-gap semiconductors has been used. It is examined that the present compounds are more stable in NaCl-phase and they transform to CsCl-phase at high pressure. In the present article, the phase transition pressures and volume collapses of lead chalcogenides have been investigated at room and high temperatures. Phase transition pressures have been reported at high temperature range from 0 to 1200 K. Elastic and anharmonic constants have also been reported at room temperature. A structural study of the narrow-gap semiconductors have been carried out using the realistic model including temperature effect. The temperature and pressure behaviour of elastic constants for the present compounds have also been discussed. Furthermore, various mechanical and thermo dynamical properties like modulus of elasticity, Debye temperatures etc. are also presented. 相似文献
19.
20.
Bruno Rousseau 《高压研究》2013,33(1):30-34
Aluminum hydride has been predicted to be a superconductor with a transition temperature of 24 K at 110 GPa, in disagreement with the experimental observation. In this work, it is shown that the bulk of the electron–phonon coupling can be associated with modes that are highly anharmonic according to frozen phonon calculations. This large anharmonicity could partially explain the origin of the disagreement between previous predictions and experiments. 相似文献