First-principles study of electronic structure and optical properties of Sr(Ti,Zr)O3

Electronic and optical properties of Sr(Ti,Zr)O₃ crystals in the cubic (Pm-3m) and tetragonal (I4/mcm) phase were calculated by the first-principles calculations using the density functional theory and the local density approximation. The band structure of cubic and tetragonal phases show an indirect band gap at (R-Γ) point and at (M-Γ) point in the Brillouin zone, respectively. The linear photon-energy dependent dielectric functions and some optical properties such as the absorption coefficient, energy-loss function and reflectivity are calculated for both phases. The optical properties of tetragonal phase of Sr(Ti,Zr)O₃ were investigated by theoretical methods for the first time. We have also made some comparisons with the available related experimental and theoretical data.     

A first-principle study of Os-based compounds: Electronic structure and vibrational properties

The electronic structure, elastic, and phonon properties of OsM (M=Hf, Ti, Y and Zr) compounds are studied using first-principles calculations. Elastic constants of OsY and specific heat capacity of OsM (M=Hf, Ti, Y, and Zr) are reported for the first time. The predicted equilibrium lattice constants are in excellent agreement with experiment. The calculated values of bulk moduli are considerably high but are much smaller than that of Osmium, which is around 400 GPa. The phase stability of the OsM (M=Hf, Ti, Y and Zr) compounds were studied by DOS calculations and the results suggest that OsY is unstable in the B2 phase. The brittleness and ductility properties of OsM (M=Hf, Ti, Y and Zr) are determined. OsM (M=Hf, Ti, Y and Zr) compounds are predicted to be ductile materials. The electronic structure and phonon frequency curves of OsM (M=Hf, Ti, Y and Zr) compounds are obtained. The position of Fermi level of these systems was calculated and discussed in terms of the pseudo gaps. The finite and small DOS at the Fermi level 0.335, 0.375, 1.063, and 0.383 electrons/eV for OsHf, OsTi, OsY, and OsZr, respectively, suggest that OsM (M=Hf, Ti, Y and Zr) compounds are weak metals.     

Narrow band in the intermetallic compounds MNiSn (M=Ti,Zr, Hf)

The presence of a narrow band below conduction band of nonmagnetic compounds MNiSn (M=Ti, Zr, Hf) is assumed to explain their low temperature properties such as the heat capacity, IR optics, electronics transport. A computation of the Seebeck coefficient supplies support for this assumption.     

Electronic structure of ZrTiO4 and HfTiO4: Self-consistent cluster calculations and X-ray spectroscopy studies

Total and partial densities of states of the constituent atoms of ZrTiO₄ and HfTiO₄ titanates have been calculated using a self-consistent cluster method as incorporated in the FEFF8 code. The calculations reveal the similarity of the electronic structure of both titanates and indicate that the valence band of the compounds under consideration is dominated by contributions of O 2p states. These states contribute throughout the whole valence-band region; however their maximum contributions occur in the upper portion of the band. Other significant contributors in the valence-band region are Ti 3d and Zr 4d states in ZrTiO₄ and Ti 3d and Hf 5d states in HfTiO₄. All the above d-like states contribute throughout the whole valence-band region of the titanates; however maximum contributions of the Ti 3d states occur in the upper portion, whilst those of the Zr 4d (Hf 5d) states are in the central portions of the valence band. The FEFF8 calculations render that the bottom of the conduction band of ZrTiO₄ and HfTiO₄ is dominated by contributions of Ti 3d^? states, with also smaller contributions of Zr 4d^?/Hf 5d^? and O 2p^? states. To verify the above FEFF8 data, the X-ray emission bands, representing the energy distributions of mainly O 2p, Ti 3d and Zr 4d states, were measured and compared on a common energy scale. These experimental data are found to be in agreement with the theoretical FEFF8 results for the electronic structure of ZrTiO₄ and HfTiO₄ titanates. Additionally, X-ray photoelectron valence-band and core-level spectra were recorded for the constituent atoms of the titanates under study.     

The first principle study of magnetic properties of Mn2WSn,Fe2YSn (Y=Ti,V), Co2YSn (Y=Ti,Zr, Hf,V, Mn) and Ni2YSn (Y=Ti,Zr, Hf,V, Mn) heusler alloys

The spin polarized electronic band structures, density of states (DOS) and magnetic properties of Mn₂WSn, Fe₂YSn (Y=Ti, V), Co₂YSn (Y=Ti, Zr, Hf, V, Mn) and Ni₂YSn (Y=Ti, Zr, Hf, V, Mn) huesler compounds are reported. The calculations are performed by using full-potential linearized augmented plane wave method (FP-LAPW) within density functional theory. The magnetic trend in these compounds is studied using values of magnetic moments, exchange interaction and calculated band gap. The results reveal that Mn₂WSn and Ni₂VSn show 100% spin polarization, Co₂YSn (Y=Ti, Zr, Hf, Mn), Fe₂YSn (Y=Ti, V), and Ni₂MnSn exhibit metallic nature and Ni₂YSn (Y=Ti, Zr, Hf) and Co₂VSn show semi-conducting behavior.     

快速烧结合成Zr1-xMxW2O8(M=Hf,Sn,Y)固溶体的Raman光谱研究

首次采用快速烧结合成技术制备了Zr1-xMxW2O8 (M=Hf,Sn,Y)系列固溶体.制备的合适条件为:温度1523～1553 K,时间30 min～1 h,同传统固相反应烧结相比,该方法合成时间和能耗显著降低.XRD和Raman光谱分析表明,Zr1-xMxW2O8 (M=Hf,Sn,Y)固溶体具有α-ZrW2O8的...     

平均键能Em的物理内涵探讨

在面心立方(fcc)、体心立方(bcc)和六角密堆积(hcp)3种不同结构晶体的自由电子能带模型中,发现4个最低能带与5个次低能带本征值的平均能量(称为平均键能,E_m)与费米能级(E_F)相当接近;并进一步在hcp结构的钛(Ti)、锆(Zr)和铪(Hf)以及bcc结构的铁(Fe)等金属中,采用从头赝势能带计算方法和平均键能计算方法,证实在这些金属的实际能带中,平均键能(E_m)值仍然非常接近于费米能级(E_F)值.该发现有助于进一步了解平均键能(E_m)的物理内涵. 关键词： 平均键能 费米能级 能带结构     

Insight into the structural,elastic, electronic,thermoelectric, thermodynamic and optical properties of MRhSb (M = Ti,Zr, Hf) half-Heuslers from ab initio calculations

A detailed theoretical investigation on the structural, elastic, electronic, thermoelectric, thermodynamic and optical properties of half-Heusler MRhSb (M = Ti, Zr, Hf) compounds is presented. The computations are carried out using the full potential linear augmented plane wave method (FP-LAPW) within density functional theory (DFT). The optimized lattice parameters are in fairly good agreement with available experimental data. The computed elastic constants (C_ij) and their related elastic moduli confirm the stability of the studied compounds in the cubic phase and highlight their ductile nature. Analysis of band structures and densities of states (DOS) profiles reveal the semiconducting nature with an indirect energy band gap (Γ-X). The bonding nature discussed via the electron charge density plot shows a mixture of covalent and ionic character. The evaluation of Seebeck coefficient leads to thermopower S ≥ 500 µeV which is very benefic for thermoelectric applications. Estimated thermodynamic characteristic within the quasi-harmonic approximation shows similar behavior for the three compounds. Finally, some optical spectra such as the complex dielectric function, refractive index, reflectivity, energy loss function and absorption are presented.     

三元层状陶瓷M-Al-N(M=Ti,Zr,Hf)的结构及力学性质的第一性原理模拟

基于第一性原理方法，探索M-Al-N（M=Ti，Zr，Hf）结构的稳定性，计算其力学性质.计算M-Al-N化合物的能量，发现除实验已知的结构Ti₂AlN和Ti₄AlN₃、Zr₂AlN、Hf₂AlN外，还存在两种新的热力学稳定结构Zr₄AlN₃、Hf₄AlN₃.弹性常数和声子谱的计算，表明这两个结构是力学稳定和晶格动力学稳定的.计算M₂AlN和M₄AlN₃的力学性质，发现它们具有高的体模量、剪切模量、弹性模量、维氏硬度等；分析其力学性质随组分比例、组成元素的变化规律，为该类材料的选择和应用提供理论依据.最后计算M₂AlN和M₄AlN₃的电子态密度和分态密度、电子密度分布、Mulliken群分析等.     

First-Principles Calculations on Electronic,Chemical Bonding and Optical Properties of Cubic Hf_3N_4

Electronic,chemical bonding and optical properties of cubic Hf 3N4(c-Hf3N4) are calculated using the firstprinciples based on the density functional theory(DFT).The optimized lattice parameter is in good agreement with the available experimental and calculational values.Band structure shows that c-Hf3N4 has direct band gap.Densities of states(DOS) and charge densities indicate that the bonding between Hf and N is ionic.The optical properties including complex dielectric function,refractive index,extinction coefficient,absorption coefficient,and reflectivity are predicted.From the theory of crystal-field and molecular-orbital bonding,the optical transitions of c-Hf3N4 affected by the electronic structure and molecular orbital are studied.It is found that the absorptive transitions of c-Hf3N4 compound are predominantly composed of the transitions from NT22p valence bands to HfT2(dxy,dxz,dyz) conduction bands.     

XPS valence bands of Ti,Zr, Nb,Mo and Hf

The XPS valence band spectra of polycrystalline Ti, Zr, Nb, Mo and Hf have been measured with a resolution of 0.6 eV. The spectra were compared with theoretical densities of states. Good agreement is achieved if the partial densities of states are considered and a many body line shape function and life time broadening are taken into account. The sigularity index determined at various core lines as well as the experimentally determined relative valence band cross sections are reported.     

Pb(Zr_(0.4)Ti_(0.6))O_3电子结构的第一性原理研究

采用基于密度泛函理论的赝势平面波方法计算了Pb(Zr0.4Ti0.6)O3五层超晶胞的顺电相和铁电相的电子结构.由态密度、电子密度和能带结构的计算结果发现顺电相下钛氧八面体Ti-O6和锆氧八面体Zr-O6在铁电相中分裂为由1个O1离子和4个O2离子组成的金字塔结构Ti-O5和Zr-O5;与顺电相相比,铁电相中钛离子的3d电子和氧离子的2p电子存在更强的轨道杂化,这种杂化降低了离子间的短程排斥力,使得具有铁电性的四方结构更为稳定,而且钛离子与氧离子的相互作用对于铁电相Pb(Zr0.4Ti0.6)O3沿c轴自发极化的贡献大于锆离子与氧离子的相互作用;由电子密度的分布可推断立方结构的Pb-O键呈现离子键特征,而铁电相下Pb-O键则有较大的共价成分,铅离子与氧离子的这种轨道杂化对Pb(Zr0.4Ti0.6)O3的铁电性起重要作用.所得结果对深入理解Pb(Zr0.4Ti0.6)O3铁电性的微观机理具有参考价值.     

Metal-encapsulated caged clusters of germanium with large gaps and different growth behavior than silicon

Metal ( M)-encapsulated caged clusters of Ge are studied using the ab initio pseudopotential plane-wave method and the generalized gradient approximation for the exchange-correlation energy. Depending upon the size of the M atom, we find Frank-Kasper polyhedral M@Ge(16) for M= Ti, Zr, Hf, and capped decahedral or cubic M@Ge(14) and M@Ge(15) clusters for several M atoms. The growth behavior differs from the one found in M@Si(n) clusters. The highest-occupied-lowest-unoccupied molecular orbital gaps are, however, similarly large or even higher in some cases. Cr@Ge(16) and Fe@Ge(15) are magnetic. The weak interaction between the clusters makes such species attractive for cluster assembled materials.     

环带钾长石、榍石和锆石的显微结构与微区组成特征分析

南秦岭沙河湾石英二长岩中的钾长石、榍石和锆石具有明显的成分环带。利用电子探针(EPMA)波谱仪(WDS)、扫描电子显微镜(SEM)及其附带的能谱仪(EDS)和激光剥蚀电感耦合等离子体质谱(LA-ICP-MS)对造岩矿物环带钾长石、榍石和锆石的显微结构与微区组成进行表征。结果表明：钾长石中K+被Na+,Ca2+,Ba2+,Fe2+和Ce3+不同程度替代。榍石中Ca2+被V3+,Ce3+和Ba2+等替代,Ti4+被Fe2+和Al3+等替代。锆石含Fe,Th,U,Nb,Ta,Y,Hf,Yb和Pb等稀土和微量元素。钾长石中元素浓度由高到低为Si,Al,K,Ca,Na,Mg和Ba,其中K和Na互补,较亮处Ba含量高,越靠近边缘,Si升高、K升高与Na降低趋势越明显。榍石中元素浓度由高到低为Ca,Si,Ti,Ba,V,Ce,Al和Fe,较亮处Fe含量较高。锆石中元素浓度由高到低为Zr,Si,Nd,Ce,Hf,U,Pb和Th,Hf与Zr呈明显的互补关系,核部Zr含量较边部高,核部Hf、U和Th含量较边部低。     

Ordered orthorhombic phases of titanium monoxide

Symmetry analysis is carried out for the ordered phases of cubic monoxide TiO_y with relative oxygen contents y<1 and y>1. It is established that a partially ordered orthorhombic phase (space group Immm)—a derivative of the orthorhombic M₃X₂□ superstructure (at y<1.0) or the inverse superstructure M₂■X₃ (at y>1)—may arise in TiO_y. The distribution of Ti and O atoms, oxygen vacancies □, and titanium vacancies ■ in unit cells of the orthorhombic ordered phases is determined. The phases are formed through the order-disorder transition channel along two rays of a non-Lifshitz star {k ₄}, and the ordering proceeds as a first-order phase transition. The distribution functions of Ti atoms over the sites of metallic and O atoms over the sites of nonmetallic sublattices are calculated for the orthorhombic superstructures of cubic titanium monoxide TiO_y.     

The superconductivity and structure of equiatomic ternary transition metal pnictides

Recent superconductivity and crystallographic data for the MM′X ternary compounds with M = Ti, Zr, or Hf, M′ = Ru or Os, and X = P or As are presented. Moderate to high superconducting transition temperatures (T _c 's)are exhibited by the ZrRuSi-type hexagonal phase which is found to be metastable at low temperatures. The low-temperature phase of ZrRuP has the TiNiSi-type orthorhombic structure and exhibits superconductivity at 3.9K. The low-temperature phase of HfRuAs has the TiFeSi-type orthorhombic superstructure and remains normal at 1.0K. TiRuAs exists only with the superstructure and remains normal at 0.35K. The details of these various structural modifications point to the importance of undistorted zig-zag chains of metal atoms for the occurrence of superconductivity in these compounds. The lowT _c for TiRuP and the absence of superconductivity for TiOsP above 0.35K are not accounted for by these structural modifications.     

The electronic,optical,and thermodynamical properties of tetragonal,monoclinic,and orthorhombic M_3N_4(M=Si,Ge,Sn):A first-principles study

A detailed study of the M_3N_4(M = Si,Ge,Sn) nitrides in their tetragonal,monoclinic and orthorhombic phases has been performed with the plane-wave pseudo-potential method combined with the quasi-harmonic approximation,including the phononic effects.We rationalize the main puzzle,i.e.,the fundamental properties of these phases are unclear,by calculating the crystal structures,density of states,and optical properties.The direct band gaps of t-Ge_3N_4,m-Si_3N_4,and o-Ge_3N_4 benefit the opto-electrical properties,t-,m-,and o-Si_3N_4 can be used as refractive materials while m-M_3N_4(M = Si,Ge,Sn) are optically transparent in the visible light region.Our results improve the understanding of the detailed electronic structures of all compounds,as well as the influences of electronic structure on their stabilities.Furthermore,we find that thermodynamic quantities are sensitive to structures and,therefore,depend on various temperature and pressure conditions.     

M@C28(M=Ti,Zr,Hf)内裹配合物的电子结构及其稳定性

用B3-LYP赝势模型的密度泛函理论,对M@C₂₈(M=Ti,Zr,Hf)内裹配合物电子结构进行从头计算研究.计算包括确定稳定的几何结构参量的分子几何结构进行优化和自然键轨道分析.结果发现Ti与C₂₈基团的相互作用明显不同于Zr(Hf)与C₂₈的相互作用,表现在原子的电子组态、成键特征和电子态分布等诸多方面.另外,结合能估算表明,这三种内裹配合物均能稳定存在,但Zr@C₂₈和Hf@C₂₈比Ti@C     

Structural dependence of the superconducting transition temperature in liquid-quenched alloys based on the group IV B elements

The structural modifications and superconducting transition temperatures in TiPd, ZrPd, ZrMo, and HfPd thin-foil alloys (containing up to ～95 at.% of the group IV B elements) obtained from the liquid state have been investigated. Contrary to some previously reported results, a ‘single phase’ of the body-centered cubic β structure cannot be retained in alloys containing more than 90 at.% of the IV B elements at room temperature. The present findings provide a more reliable assessment of the transition temperatures for the cubic β phase.