Mössbauer spectroscopic study of half-Heusler compounds

The family of half-Heusler compounds offers a variety of half-metallic ferromagnetic materials. We have applied the Mössbauer spectroscopy to study the atomic order, local surroundings and hyperfine fields to several half-Heusler compounds. ¹²¹Sb Mössbauer study of the compound CoMnSb revealed the presence of two nonequivalent antimony positions in the elementary cell and enabled to identify the structure. ^119mSn, ¹⁵⁵Gd and ¹⁹⁷Au Mössbauer spectroscopic studies were used to characterize the properties of ferromagnetic granular material based on the half-Heusler ferromagnet MnAuSn in the antiferromagnetic GdAuSn matrix.     

Half-Heusler合金NiFeSb和NiMnSb的磁性及电子结构的第一性原理研究

用基于密度泛函理论的第一性原理方法,计算了half-Heusler合金NiFeSb和NiMnSb的晶体结构、磁性及电子结构。计算结果表明,磁性原子Fe和Mn在两种合金的总磁矩中贡献最大,在NiFeSb总磁矩中,Ni原子贡献比例接近在NiMnSb中的2倍,而Sb原子的贡献比例是在NiMnSb中的1/5;两种合金的自旋向上能带都具有明显的金属特征,而自旋向下能带有明显的差别;两种合金费米能级以下的总态密度(DOS)主要由Ni-3d 和Fe-3d（Mn-3d）态决定,费米能级以上主要由Fe-3d（Mn-3d）自旋向下部分决定。     

三元化合物ZnVSe2半金属铁磁性的第一性原理计算

采用基于密度泛函理论(DFT)框架下的第一性原理平面波赝势(PWP)方法,结合广义梯度近似(GGA),对三元化合物ZnVSe₂晶体的电子结构进行了计算,分析了ZnVSe₂晶体自旋极化的能带结构、电子态密度、电荷布居、磁矩等.计算结果表明,三元化合物ZnVSe₂会产生自旋极化状态,能带结构和态密度显示为半金属特征,表现出显著的铁磁性行为,具有高达近100%的传导电子自旋极化率,其半金属能隙为0.443eV,理论预测其可能是一种具有一定应用潜能 关键词： 2')" href="#">ZnVSe₂ 平面波赝势方法 半金属铁磁性 第一性原理     

MgCu4Sn型稀土镁合金相力学和热力学性质的第一性原理研究

采用基于密度泛函理论的第一性原理计算方法,系统研究了典型立方结构合金相LaMgX4(X=Co, Ni, Cu)的力学特性和热力学特性。根据广义胡可定律计算了合金相的单晶弹性常数;根据单晶弹性常数和Hershey’s averaging方法计算了合金相的多晶弹性模量、泊松比、Zener各向异性因子和德拜温度。采用基于准谐近似的Gibbs2代码计算了LaMgX4(X=Co, Ni, Cu)合金相的吉布斯自由能、熵和等体热容与温度的关系。LaMgNi4的计算结果与其他文献计算结果及实验结果符合的很好。结果表明：LaMgX4 (X= Co, Ni, Cu)合金均为延展性、塑性和弹性各向异性材料。德拜温度按以下顺序递减：LaMgNi4>LaMgCu4>LaMgCo4。     

LiH的热力学性质及分子内部运动对体系热力学性质的影响

利用基于密度泛函理论的第一性原理平面波超软赝势方法,计算了LiH在零温零压下的晶格常数、体弹模量,计算结果与实验值和其他理论计算值符合得较好.通过准谐德拜模型计算了LiH在压强为0-80GPa、温度为0-2000K范围内,体积膨胀率、热涨系数、德拜温度及定容热容随压强和温度的变化关系.最后,以代数方法（AM）和势能变分法(PVM)为基础,运用统计热力学理论计算了LiH分子内部运动对体系热力学性质的影响.     

Co_(2~-)基Heusler合金Co_2FeAl_(1–x)Si_x(x=0.25,x=0.5,x=0.75)的结构、电子结构及热电特性的第一性原理研究

运用基于密度泛函理论的第一性原理方法,对Co_2FeAl_(1–x)Si_x(x=0.25, 0.5, 0.75)系列Heusler合金的电子结构、四方畸变、弹性常数,声子谱以及热电特性进行了计算研究.结果显示, Co_2FeAl_(1–x)Si_x系列合金的电子结构均为半金属特性,向下自旋态(半导体性)均呈现良好的热电特性,并且随着硅原子浓度的增加功率因子随之增加.计算的声子谱不存在虚频,均满足动力学稳定性条件,弹性常数均满足玻恩稳定性条件,机械稳定性均良好.随着晶格常数c/a的比值变化,体系的能量最低点均出现在c/a=1处,即结构稳定性不随畸变度c/a的变化而变化,说明不存在马氏体相变.此系列合金薄膜的电子结构呈现较高的自旋极化率,在替代浓度x=0.75时自旋极化率达到100%,且当x=0.75时薄膜在畸变度c/a=1.2时存在马氏体相变.随着晶格畸变度的改变,总磁矩也发生变化,且主要由Fe和Co两种过渡金属原子的磁矩变化所决定.     

Phase transition,lattice dynamical and thermodynamic properties of yttrium antimonide with the rock salt structure under high pressure: first-principles investigations

We investigate the structural, electronic, first-order pressure-induced phase transition, lattice dynamical, and thermodynamic properties of yttrium antimonide (YSb) with the rock salt structure at high pressures and high temperatures using the projector-augmented wave method based on the density-functional theory. By the usual condition of equal enthalpy, we find that the rock salt-structured YSb is stable up to 31.10 GPa, and then transforms to the CsCl-type structure, this is consistent with the experiment result which begins transform from 26 GPa then ends at 36 GPa. The phonon dispersion curves of the rock salt-structured YSb are calculated under high pressure for the first time using a linear-response approach to density-functional perturbation theory successfully. Within the calculated phonon density of state and the quasi-harmonic approximation, we predict further the thermal physical properties of YSb under high temperature and high pressure systematically.     

Magnetic properties of several potential rocksalt half-metallic ferromagnets based on the first-principles calculations

Several rocksalt Sr₄X₃N (X=,O, S, Se, and Te) are predicted to be potential half-metallic ferromagnets free of transition-metal and rare-earth elements by performing the first-principles calculations. Then their magnetic properties, such as the half metallicity and the crystal-cell magnetic moments are investigated. The Sr₄X₃N possibly have higher Curie temperatures and have more stable half metallicity than the Sr₄X₃C. Their crystal-cell magnetic moments are all 1.00 μ_B. The crystal-cell magnetic moments and the half metallicity arise mainly from the N ions. The main mechanism is the strong covalent interaction leading to the sp² hybridized orbitals in the Sr₄X₃N. Then two Sr-5s and three N-2p electrons enter into three sp² hybridized orbitals. Among these five electrons, four electrons are paired and one is unpaired, so there are three spin-up electrons and two spin-down electrons in these sp² hybridized orbitals.     

GGA+U方法研究不同浓度Cr掺杂对TiO2的磁性和光学性质的影响

本文采用基于第一性原理的GGA+U方法,计算研究了本征态锐钛矿TiO2和不同浓度Cr掺杂锐钛矿TiO2(1/8、1/16、1/32)的电子结构、磁性及光学性质。计算结果表明：所有掺杂体系中Ti0.9375Cr0.0625O2的结合能最小,因此Ti0.9375Cr0.0625O2体系的稳定性要高于Ti0.875Cr0.125O2、Ti0.96875Cr0.03125O2体系;Cr元素的掺入导致掺杂后体系发生晶格畸变,这有利于光生空穴和电子对的分离,提高其光催化性能;同时,由于Cr-3d和O-2p电子相互作用,使得掺杂体系呈现出铁磁性质,并且随着掺杂浓度的增加会使体系具有更好的铁磁性质;掺杂体系与本征TiO2相比,掺杂后吸收带边均发生红移,光谱响应范围变大;并且随掺杂浓度的增加,光响应范围也在增大,从而有效增强了体系对于可见光的吸收能力。     

铟钇金属间化合物力学性能的第一性原理计算

本文基于密度泛函理论的平面波超软赝势方法,对铟钇(In-Y)金属间化合物的力学结构稳定性、弹性性质和热力学性能进行了研究.通过结构优化得到了In_3Y、InY、InY_2三种金属间化合物的晶格常数,发现与实验值比较吻合.弹性常数的计算结果表明In-Y金属间化合物的结构是稳定的,由弹性常数推算出In_3Y、InY、InY_2三种合金的体积模量、杨氏模量、剪切模量、泊松比和各向异性等力学性质,发现InY_2合金的体积模量、杨氏模量、剪切模量要比其它两种的值大,其抗形变能力更强.本文还预测了In-Y金属间化合物的热力学性质,如德拜温度、热导率,通过第一性原理计算得到的In-Y金属间化合物的力学和热力学性质为In-Y合金材料的实际应用和材料设计提供了参考.     

Phase transition and thermodynamic properties of SrS via first-principles calculations

The phase transition of SrS from NaCl structure （B1） to CsCl structure （B2） is investigated by means of ab initio plane-wave pseudopotential density functional theory, and the thermodynamic properties of the B1 and the B2 structures are obtained through the quasi-harmonic Debye model. It is found that the transition phase from the B1 to the B2 structures occurs at 17.9 GPa, which is in good agreement with experimental data and other calculated results. Moreover, the thermodynamic properties （including specific heat capacity, the Debye temperature, thermal expansion and Griineisen parameter） have also been obtained successfully.     

Structural,elastic, and vibrational properties of phase H:A first-principles simulation

Phase H(MgSiO_4H_2), one of the dense hydrous magnesium silicates(DHMSs), is supposed to be vital to transporting water into the lower mantle. Here the crystal structure, elasticity and Raman vibrational properties of the two possible structures of phase H with Pm and P2/m symmetry under high pressures are evaluated by first-principles simulations. The cell parameters, elastic and Raman vibrational properties of the Pm symmetry become the same as the P2/m symmetry at～ 30 GPa. The symmetrization of hydrogen bonds of the Pm symmetry at ～ 30 GPa results in this structural transformation from Pm to P2/m. Seismic wave velocities of phase H are calculated in a range from 0 GPa to 100 GPa and the results testify the existence and stability of phase H in the lower mantle. The azimuthal anisotropies for phase H are A_(P0)= 14.7%,A_(S0)= 21.2%(P2/m symmetry) and A_(P0)= 16.4%, A_(S0)= 27.1%(Pm symmetry) at 0 GPa, and increase to A_(P30)= 17.9%,A_(S30)= 40.0%(P2/m symmetry) and A_(P30)= 19.2%, A_(S30)= 37.8%(Pm symmetry) at 30 GPa. The maximum V P direction for phase H is [101] and the minimum direction is [110]. The anisotropic results of seismic wave velocities imply that phase H might be a source of seismic anisotropy in the lower mantle. Furthermore, Raman vibrational modes are analyzed to figure out the effect of symmetrization of hydrogen bonds on Raman vibrational pattern and the dependence of Raman spectrum on pressure. Our results may lead to an in-depth understanding of the stability of phase H in the mantle.     

高压下CaPo弹性性质和热力学性质的第一性原理研究

利用密度泛函理论的平面波赝势方法预测研究了CaPo从岩盐结构(B1结构)到氯化铯结构(B2结构)的相变以及B1结构CaPo高压下的弹性性质以及热力学性质等.通过等焓原理发现B1→B2的相变压力为22.8GPa. 同时计算了B1结构CaPo高压下的弹性常数以及剪切模量、杨氏模量等相关弹性参数,结果发现当压力超过20GPa时,B1结构CaPo开始不稳定了,这和等焓原理所得结果相符合. 最后通过Debye模型成功获取了B1结构C 关键词： 相变 弹性性质 热力学性质 CaPo     

Mechanical and thermodynamic properties of cubic YH_2 under high pressure:Prediction from first-principles study

First-principles calculations are used to investigate the mechanical and thermodynamic properties of cubic YH2 at different pressures and temperatures. The generalized gradient approximation （GGA） with Perdew-Burke-Ernzerhof （PBE） method is used to describe the exchange-correlation energy in the present work. The calculated equilibrium lattice constant a and bulk modulus B are in good accordance with the available experimental values. According to the Born-Huang criteria for mechanical stability, elastic constants are calculated from the strain-induced stress method in a pressure range from 0 to 67.1 GPa. Isotropic wave velocities and sound velocities are discussed in detail. It is found that the Debye temperature decreases monotonically with the increase of pressure and that YH2 has low anisotropy in both longitudinal and shear-wave velocities. The calculated elastic anisotropic factors indicate that YH2 has low anisotropy at zero pressure and that its elastic anisotropy increases as pressure increases. Through the quasi-harmonic Debye model, in which phononic effects are considered, the thermodynamic properties of YH2, such as the relations of （V-Vo）/Vo to the temperature and the pressure, the dependences of heat capacity Cv and thermal expansion coefficient a on temperature and pressure ranging from 0 to 2400 K and from 0 to 65 GPa, respectively, are also discussed.     

Elastic and thermodynamic properties of c-BN from first-principles calculations

The elastic constants and thermodynamic properties of c-BN are calculated using the first-principles plane wave method with the relativistic analytic pseudopotential of the Hartwigen, Goedecker and Hutter (HGH) type in the frame of local density approximation and using the quasi-harmonic Debye model, separately. Moreover, the dependences of the normalized volume V/V₀ on pressure P, as well as the bulk modulus B, the thermal expansion α, and the heat capacity C_V on pressure P and temperature T are also successfully obtained.     

密度泛函理论研究高温高压下UO2弹性与热力学性能

采用第一性原理与准谐德拜模型研究UO2在高温高压条件下的弹性与热力学性能。UO2在高温高压下仍属离子型晶体,并且弹性性能计算表明,四角方向剪切常数在高温与高压下均保持稳定。高温下弹性常数C44没有明显变化,而高压下C44迅速增大。体积模量、剪切模量与杨氏模量均随压强增加而增大;高温条件下,体积模量、剪切模量与杨氏模量也未出现明显的降低,表明UO2在高温度高压下均可保持良好的力学性能。不同压强下,UO2定容热容均随温度迅速增大,并在1000 K 附近趋近于杜隆-佩蒂特极限。德拜温度则随温度降低,随压强升高。在低于室温条件下,热膨胀系数随温度急剧增加;温度继续增加,系数的增加趋势则逐渐变缓。计算结果还表明,UO2的热膨胀系数在相同条件下,远小于其他核材料。     

LiNH2 的晶格动力学、介电性质和热力学性质第一性原理研究

采用基于密度泛函理论的平面波赝势方法,在局域密度近似下采用线性响应的密度泛函微扰理论计算了LiNH₂的晶格动力学、介电性质和热力学性质,得到了布里渊区高对称方向上的声子色散曲线和相应的声子态密度,分析了 LiNH₂的红外和拉曼活性声子频率,同时给出它的介电张量和玻恩有效电荷张量. 研究表明,LiNH₂存在小的各向异性,计算所得结果与实验值和其他理论值符合较好.最后,利用得到的声子态密度进一步预测了LiNH₂的热力学性质 关键词： 密度泛函理论 晶格动力学 热力学性质 第一性原理计算     

Structural,electronic, magnetic and vibrational properties of half-Heusler NaZrZ (Z = P,As, Sb) compounds

The structural, electronic, magnetic and vibrational properties of NaZrP, NaZrAs and NaZrSb half-Heusler alloys have been investigated on the basis of density functional theory and generalized gradient approximation. There are three types of structures for these compounds where type a is the most stable one. It is found that all of these materials are half-metallic ferromagnets with a magnetic moment of $2{\mu }_{\mathrm{B}}$. The half-metallic gaps are estimated to be 0.16, 0.35 and 0.55 eV for Z = P, As and Sb, respectively. The hybridization between s and $t_{2g}$ orbitals of Zr with s and p orbitals of Z leads to half-metallic ferromagnetism in these compounds. The effect of strain on the half-metallic property is also investigated, and we notice that the half-metallicity is conserved up to the lattice compressions of 54.43%, 48.29% and 47.55% for NaZrP, NaZrAs and NaZrSb, respectively. The dynamical stability of these compounds is confirmed using dispersion curves. The Curie temperatures are also estimated to be 501.29 K, 855.49 K and 1348.88 K for NaZrP, NaZrAs and NaZrSb, respectively. Therefore, it seems that NaZrZ (Z = P, As, Sb) could be suitable materials for spin-injector applications.     

Effect of Zr concentration on the mechanical and thermodynamic properties of NbIr3 intermetallic compounds from theoretical estimations

ABSTRACT

The iridium is an important metal which has excellent resistance to corrosion at high temperature. L1₂ intermetallic compounds i.e. Ir₃Nb and Ir₃Zr, with similar lattice parameters are ideal for working at high temperature. They are fully soluble due to their low lattice misfit. A first-principle investigation into the effect of doping Zr with different concentrations on the electronic structure, mechanical and thermodynamic properties of NbIr₃ has been studied to prompt the development of novel high-temperature materials. Nine Zr_xNb_8?xIr₂₄ compounds are carefully considered. The results show that adding Zr into these compounds can strengthen their structural stability and ductility. Nevertheless, it reduces the elastic modulus and elastic stiffness. Simultaneously, with the increase of Zr content, the thermodynamic properties of these compounds decrease. It is also found that the changes of elastic modulus are mainly attributed to the variations of bonds in these compounds.     

Electronic Structure and Magnetic Properties of Cu[C(CN)3]2 and Mn[C(CN)3]2 Based on First Principles

The electronic structure and the magnetic properties of the molecule-based ferromagnets Cu[C(CN)₃]₂ and Mn[C(CN)₃]₂ are studied accordingto first principles within density-functional theory (DFT) and the full potential linearized augmented plane wave (FP-LAPW) method. The total energy, atomic spin magnetic moments, and density of states (DOS) of Cu[C(CN)₃]₂ and Mn[C(CN)₃]₂ are all calculated. The calculations reveal that the compounds have a stable ferromagnetic ground state and half-metallic properties. The total spin magnetic moment is 1.0 μ_B for Cu[C(CN)₃]₂ and 5.0 μ_B for Mn[C(CN)₃]₂ per molecule, the magnetic moment mainly comes from metal atoms, although there is a slight contribution from N and C atoms.