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1.
The thermodynamic properties of LiBC are investigated by using the full-potential linearized muffin-tin orbital method (FP-LMTO) within the frame of density functional theory (DFT) and using the quasi-harmonic Debye model. The dependencies of the normalized lattice parameters a/a0 and c/c0, the ratio (c/a)/2, the normalized primitive volume V/V0 on pressure and temperature are successfully obtained. It is found that the interlayer covalent interactions (Li-B bonds or Li-C bonds) are more sensitive to temperature and pressure than intralayer ones (B-C bonds), as gives rise to the extreme lattice anisotropy in the bulk hcp LiBC. 相似文献
2.
This paper investigates the electronic structure and thermodynamic properties of LiBC in the hexagonal structure by using the generalized gradient approximation (GGA) and local density approximation correction scheme in the frame of density functional theory. The geometric structure of LiBC under zero pressure, and the dependences of the normalized lattice parameters a/ao and c/co, the ratio e/a, the normalized primitive volume V/Vo on pressure are given. The thermodynamic quantity (including the heat capacity Cv, Debye temperature 6~D, thermal expansion a and Grfineisen parameter -y) dependences on temperature and pressure are obtained through the GGA method and the quasi-harmonic Debye model. The band structures and density of state of LiBC under different pressures have also been analysed. 相似文献
3.
Using ab initio plane-wave pseudopotential density functional theory method, we have studied the structural and thermodynamic properties of the wurtzite boron nitride (w-BN). Through the quasi-harmonic Debye model, in which the phononic effects are considered, the dependencies of the normalized lattice parameters a/ao and c/co, the axial ratio c/a, and the normalized primitive cell volume V/Vo on pressure P and temperature T are investigated. Furthermore, the Debye temperature θ, the variation of thermal expansion α, as well as the heat capacity Cv as functions of P and T are studied systematically. 相似文献
4.
The lattice parameter, bulk modulus and its pressure derivative of
the wurtzite-type aluminium nitride (w-AlN) are investigated by using
the Cambridge Serial Total Energy Package (CASTEP) program in the
framework of Density Functional Theory (DFT). The calculated results
are in good agreement
with the available experimental data and other theoretical results.
Through the quasi-harmonic Debye model, the dependences of the
normalized lattice parameters $a/a_{0}$ and $ c/c_{0}$, axial ratio
$c/a$, normalized primitive-cell volume $V/V_{0}$, Debye
temperature ${\it\Theta} _{\rm D} $ and heat capacity $C_{\rm V}$ on
pressure $P $ and temperature $ T$ are obtained. It is found that the
interlayer covalent interactions (Al-N bonds) are more (even a
little) sensitive to temperature and pressure than intralayer ones
(N--N bonds), which gives rise to a little lattice anisotropy in the
w-AlN. 相似文献
5.
We have measured the magnetoresistance ρ(H,T 0) of the magnetic Kondo lattice CeAl 2 at temperatures T 0 in the range 0.035 – 1.3 K, well below the Néel temperature T N=3.8 K, in magnetic fields H up to 145 k0e. The ρ vs H curve exhibits a rapid decrease between H=45–65 kOe corresponding to the metamagnetic transition at H=H M. The resistivity then levels out to a value which depends only weakly on field and temperature. For H M a small positive magnetoresistance was observed with the derivative dρ/dH increasing as the temperature is lowered. The present results are compared with the ρ(H,T0) data obtained for the CePb3 magnetic Kondo lattice, where the decrease in the resistivity at HHM was considered earlier to be evidence of field induced superconductivity. 相似文献
6.
Based on density functional first-principles calculations and anisotropic Eliashberg equations, we have investigated the electronic structure, lattice dynamics, and phonon-mediated superconductivity in newly synthesized layered compound Sr BC under pressure. Different from Li BC and Mg B2, our calculations surprisingly reveal that Sr BC is isotropic in compressibility, due to the accumulation of substantial electrons in the interstitial region. We find that the Sr phonons strongly couple with B-2 pz orbital and the interstitial states, giving rise to a two-gap superconductivity in Sr BC, whose transition temperature shows an inverted V-shaped dependence on pressure. The maximal transition temperature is about 22 K at50 GPa. On both sides of 50 GPa, the transition temperature exhibits quasi-linear variation with positive and negative slopes, respectively. Such a variation of transition temperature is infrequent among phonon-mediated superconductors. The competition between enhanced electron–phonon matrix element and hardened phonons plays an essential role in governing the behavior of the critical temperature. 相似文献
7.
The structural compression mechanism and compressibility of gallium oxyhydroxide, α-GaOOH, are investigated by in situ synchrotron radiation x-ray diffraction at pressures up to 31.0 GPa by using the diamond anvil cell technique. Theα-GaOOH sustains its orthorhombic structure when the pressure is lower than 23.8 GPa. The compression is anisotropic under hydrostatic conditions, with the a-axis being most compressible. The compression proceeds mainly by shrinkage of the void channels formed by the coordination GaO_3(OH)_3 octahedra of the crystal structure. Anomaly is found in the compression behavior to occur at 14.6GPa, which is concomitant with the equatorial distortion of the GaO_3(OH)_3 octahedra. A kink occurs at 14.6 GPa in the plot of finite strain f versus normalized stress F, indicating the change in the bulk compression behavior. The fittings of a second order Birch–Murnaghan equation of state to the P–V data in different pressure ranges result in the bulk moduli B_0= 199(1) GPa for P 14.6 GPa and B_0= 167(2) GPa for P 14.6 GPa. As the pressure is increased to about 25.8 GPa, a first-order phase transformation takes place, which is evidenced by the abrupt decrease in the unit cell volume and b and c lattice parameters. 相似文献
8.
研究高压下NH_4ClO_4的结构和性质对于NH_4ClO_4在固体推进剂和炸药的安全应用具有重要意义.采用基于色散校正密度泛函理论的第一性原理方法,研究了0—15 GPa静水压力下NH_4ClO_4的晶体结构、分子结构、电子性质和弹性性质,计算结果与实验值具有较好的一致性.在压强为1,4和9 GPa时,NH_4ClO_4的晶体参数、键长和分子构型等均出现不连续变化,说明了在压强作用下结构发生变化.随着压强增加,氢键增多且作用增强,由分子内氢键向分子内和分子间的氢键转变;导带态密度峰值增加,电子局域性增强,晶体内N-H和Cl-O共价键作用增强,带隙增大,不同相变区域内带隙呈线性关系.0—15 GPa条件下NH_4ClO_4的弹性常数满足力学稳定性标准,采用Voigt-Reuss-Hill方法计算了体积模量B,剪切模量G和杨氏模量E,根据Cauchy压力和B/G值,说明NH_4ClO_4属于韧性材料,随着压强增加韧性增强. 相似文献
9.
This is a study of the equilibrium statistical mechanics of a mixed system of planar spins and Ising spins. The Ising spins σ r are situated (one on each of the bonds connecting the planar spins sr so that the planar spins interact only via Ising spins. For a one-dimensional lattice the partition function and the two-spin correlation functions (σ rσ r1 and Sr· Sr1) are calculated ex actly in terms of modified Bessel functions. For a two-dimensional anisotropic generalization of the model an equation is given for the critical temperature and the equation is shown to have one and only one solution Tc such that 0<Tc<∞. 相似文献
10.
The elastic constants of superconducting MgB 2 are
calculated using a molecular dynamics method (MD) with shell model. The
lattice parameters, five independent elastic constants, equations of state
(EOS), Debye temperature, and bulk modulus of MgB 2 are obtained.
Meanwhile, the dependence of the bulk modulus B, the lattice parameters a and c, and the unit cell volume V on the applied pressure are presented. It is
demonstrated that the method introduced here can well reproduce the
experimental results with a reasonable accuracy. 相似文献
11.
Assuming that the energy of a 2D Wigner lattice, which has been frequently observed on the CuO 2 plane in layered high- Tc cuprates, is composed of Coulomb energy, kinetic energy, and charge-transfer energy which is necessary to transfer electrons or holes onto the CuO 2 plane (the magnitude of the energy is expected to be of the order of the work function of the compound constituting the plane from which the electrons are transferred, if the CuO 2 plane easily accepts the electrons or holes, as if the CuO 2 plane were crystal surface), we find in the mean-field approximation, that the total energy for a unit lattice, E0, can be given by a function of the charge-transfer energy Eφ as E0= ( Eφ− Eφ0+O(( Eφ− Eφ0) 2)(>0), where the value of Eφ0(1.5 in the ground state, for example) is independent of the lattice configuration. This relation implies that the upper bound to the charge-transfer energy is given by Eφ0. The smallness of Eφ0, compared with the work function of ordinary metals and alloys (with the work function around 4 eV), is necessary for the realization of the 2D Wigner lattice in many layered high- Tc cuprates. 相似文献
12.
Polycrystalline lithium-ammonium sulphate samples were subjected to X-ray diffraction analysis for determining the lattice parameters of the prepared mixed crystals. The compositions of the samples were determined using X-ray microanalysis. The Li0.5(NH4)0.5SO4 samples were irradiated with different doses of γ-radiation in order to investigate the effect of this ionizing radiation on the phase transition temperatures Tc1 and Tc2. The temperature dependence of the d.c. resistivity ρd.c. and dielectric constant of irradiated samples showed (i) a shift of Tc1 to higher temperatures; (ii) a shift of Tc2 to lower temperatures; and (iii) the appearance of a new anomaly near 128°C at moderately high γ-doses. The mechanisms giving rise to this behaviour are discussed. 相似文献
13.
The lattice parameter bulk modulus and pressure derivative of BeB2 are calculated by using the Cambridge Serial Total Energy Package (CASTEP) program in the frame of density function theory. The calculated results agree well with the average experimental data and other theoretical results. Through the quasi-harmonic Debye model, the dependences of the normalized lattice parameters a/ao, c/c0 and the normalized primitive cell volume V/Vo on pressure P, the variation of the thermal expansion coefficient ~ with pressure P and temperature T, as well as the dependences of the heat capacity Cv on pressure P and temperature T are obtained systematically. 相似文献
14.
采用基于密度泛函理论的第一性原理平面波赝势方法研究ZrV 2的晶体结构和弹性,利用准谐Debye模型计算在不同温度( T=0~1200 K)和不同压强( P=0~20 GPa)下ZrV 2的热力学性质,包括弹性模量与压强,热熔与温度,以及热膨胀系数与温度和压力的关系.结果表明:计算的ZrV 2晶格常数与实验值符合较好,晶体材料的弹性常数随着压力增加而增加;在一定温度下,相对体积、热熔随着压强的增加而减小,德拜温度、弹性模量随着压强的增加而增加,且高压下温度对ZrV 2热膨胀系数的影响小于压强的影响. 相似文献
15.
Spectral features of a large-angle stimulated Raman scattering (LA SRS) of a short electromagnetic pulse in an underdense plasma, which are caused by the presence in a plasma of a given linear long-wavelength electron plasma wave (LW EPW), are investigated. It is shown that the LW EPW, whose phase velocity coincides with a group velocity of a pulse and a density perturbation normalized to a background electron density δn LW/n 0 exceeds the ratio of the electron plasma frequency to the laser frequency ω pe/ω 0 suppresses the well-known Stokes branch of the weakly coupled LA SRS. Under the same condition, the anti-Stokes band appears in the spectrum of the scattered radiation. Variation of a scattering angle and an electron temperature do not significantly modify qualitative features of the effect. In the case of strongly coupled LA SRS, the maximum of the increment is decreased by nearly one-half for δn LW/n 0~(a 0ω pe /ω 0) 2/3≫ω pe /ω 0, where a 0 is an amplitude of an electron quiver velocity in the laser field normalized to a speed of light c, and it decreases further with an increase in plasma density perturbation in LW EPW 相似文献
16.
A series of new compounds Ba4(Pb 1−xBi x) 3O 10 with 0≤x<0.3 has been prepared and characterized by X-ray and electron diffraction. The d.c. resistivity vs. temperature data for different x values are presented and structural features are compared to those of the superconducting phase Ba(Pb 1−xBi x)O 3. The “mother-compound” Ba 4Pb 3O 10 is tetragonal, with lattice constants A=0.4280(1) and C=3.017(1) nm. Its structure can be regarded as a stacking of Ba(PbBi)O 3 triple-layers, separated by single BaO layers. Electron diffraction reveals the presence of a weak superstructure with the a-axis related to that of the perovskite by a=√2a p. Frequent intergrowth of this phase with the perovskite structure is observed. The conductivity of the samples Ba 4Pb 3O 10 is nearly independent of temperature, while Bi-doped samples exhibit semiconductor-like behaviour at low temperature. No superconducting transition is observed down to 2K. 相似文献
17.
We employ a first-principles plane wave method with the
relativistic analytic pseudopotential of Hartwigsen, Goedecker and Hutter
(HGH) scheme in the frame of DFT to calculate
the equilibrium lattice parameters and the thermodynamic properties of
AlB 2 compound with hcp structure. The obtained lattice parameters are in
good agreement with the available experimental data and those calculated by
others. Through the quasi-harmonic Debye model, obtained successfully are
the dependences of the
normalized lattice parameters a/a 0 and c/c 0 on pressure P, the normalized
primitive cell volume V/V 0 on pressure P, the variation of the thermal
expansion α with pressure P and temperature T, as well as the Debye
temperature \Theta D and the heat capacity C V on pressure P and
temperature T. 相似文献
18.
We introduce a new parameterization of four-fermion operator matrix elements which does not involve quark masses and thus allows a reduction of systematic uncertainties. In order to simplify the matching between lattice and continuum renormalization schemes, we express our results in terms of renormalization group invariant B-parameters which are renormalization-scheme and scale independent. As an application of our proposal, matrix elements of Δ I=3/2 and SUSY Δ S=2 operators have been computed. The calculations have been performed using the tree-level improved Clover lattice action at two different values of the strong coupling constant (β=6/ g2=6.0 and 6.2), in the quenched approximation. Renormalization constants and mixing coefficients of lattice operators have been obtained non-perturbatively. Using lowest order χPT, we also obtain ππ| O7| KNDRI=2=(0.11±0.02) GeV 4 and ππ| O8| KNDRI=2=(0.51±0.05) GeV 4 at μ=2 GeV. 相似文献
19.
采用第一性原理计算对Ce_(0.8)La_(0.1)Th_(0.1)在高压下fcc-bct的结构相变、弹性性质及热力学性质进行了研究讨论.通过对计算结果的分析,发现了合金在压力下的相变规律,压强升高到31.6 GPa附近时fcc相开始向bct相转变,到34.9 GPa时bct相趋于稳定.对弹性模量的计算结果从另一角度反映了结构相变的信息.最后,利用准谐德拜模型对两种结构的高温高压热力学性质进行了理论预测. 相似文献
20.
The lattice dynamical properties of the Invar alloys Fe 65Ni 35 and Fe 72Pt 28 are discussed. The experiments on the lattice vibration by inelastic neutron scattering have shown an apparent correlation of phonon anomalies with the ferromagnetic long range order. The elastic softening below the Curie temperature is attributed to the dynamical aspects, at least for the [ςς0]TA 1 mode. The lattice dynamical features are derived from the electronic structure of the Invar alloys. They are consistent with the theoretical model that Invar characters are closely related to the detailed band structure of the d electrons near the Fermi level. The present studies indicate significant contributions of phonon anomalies to the Invar problem. 相似文献
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