Structures and physical properties of R_2TX_3 compounds

Rare-earth compounds have been an attractive subject based on the unique electronic structures of the rare-earth elements. Novel ternary intermetallic compounds R2TX3 (R = rare-earth element or U, T = transition-metal element, X = Si, Ge, Ga, In) are a significant branch of this research field due to their complex and intriguing physical properties, such as magnetic order at low temperature, spin-glass behavior, Kondo effect, heavy fermion behavior, and so on. The unique physical properties of R2TX3 compounds are related to distinctive electronic structures, crystal structures, micro-interaction, and external environment. Most R2TX3 compounds crystallize in AlB2-type or derived AlB2-type structures and exhibit many similar properties. This paper gives a concise review of the structures and physical properties of these compounds. Spin glass, magnetic susceptibility, resistivity, and specific heat of R2TX3 compounds are discussed.     

Structures,microstructures, physical and chemical properties of different rice grains

Herein, we demonstrated a well-rounded exploration of the universal structures, microstructures and physical properties of different rice grains. We show that these rice grains are nanocrystalline in nature, have the standard pattern of type A rice crystalline, and can be attributed to the hexagonal crystal structure with space group of P6. These rice grains have uneven surfaces, clusters of the granules, or schistose structure. Carbon, nitrogen and oxygen elements are presented in the rice grains. No significant difference is observed in FTIR spectra in these rice grains, indicating the fundamental chemical structures of these rice grains are analogical. Their microstructural and physical properties were investigated in detail. Moreover, chemical properties (e.g. thermal stability) of the rice grains were explored. This research provides an in-depth understanding on the physical and structural properties on the atomic and molecular level, as well as guidance for food and industry applications.     

若干In2Se3化合物的晶体结构与电子特性

In_2Se_3是一种常见的A_2~ⅢB_3~Ⅵ型半导体,在不同温度下可表现出不同的物相,对应的晶格参数与物理性质也会有所不同.在过去几十年间,In_2Se_3的多相性引起了人们的关注,各物相的晶体结构被广泛地研究.近年来,研究发现α-In_2Se_3具有优异的光电、压电性能以及独特的铁电性能,可以预见它将在未来的半导体电子器件中发挥出重要的作用.本文综述了一系列In_2Se_3化合物的晶体结构与电子特性,我们首先简要介绍了这些In_2Se_3化合物的基本知识,接着详细讨论了它们的晶体结构与电子特性,最后对In_2Se_3化合物的研究前景进行了展望.     

Volume properties and compressibility of the graphite-potassium-mercury compounds

Abstract

The volume properties of graphite intercalation compounds (GIGS) C4KHg and of the initial intermetallic compounds KHg and KHg₂ have been investigated in the piston-cylinder apparatus, using the direct volumetric technique, under pressures up to 25 kbar. The compounds, average compressibility K₊, was determined to be 3.8×10^?3 kbar^?1 for C₄KHg, 3.0×10^?3 kbar-^?1 for C₈KHg, 4.8×10^?1 kbar^?1 for KHg, and 4.0 ×10^?1 kbar^?1 for KHg₂ at pressures of 0-20 kbar. The compressibility of the “two-dimensional” KHg layer in the GIC under various pressure conditions has been estimated. These estimates permit comparison of KHg properties in the “three dimensional” and “quasi-two dimensional” states. It was concluded that the influence of the graphite matrix on the intercalant is insignificant for this type ternary GIC.     

La0.1Bi0.9-xEuxFeO3化合物的结构与性能研究

利用固相反应烧结技术制备La0.1Bi0.9-xEuxFeO3系列化合物.利用X射线粉末衍射进行物相鉴定和结构分析,确定了材料的相关系:x≤0.05,材料为R3c结构相;0.08≤x≤0.12,材料为赝R3c结构相;x≥0.15是Pbnm相,其中0.15≤x≤0.20区域Pbnm相存在畸变.磁测量结果表明,材料具有弱铁磁性,对于x≤0.20材料,磁矩在x=0.12成分存在极值.利用阻抗分析仪测量了室温介电常数随成分的变化关系.讨论了材料的结构与弱铁磁性和室温介电常数间的关系.     

Crystal growth,structural and physical properties of the 5d noncentrosymmetric LaOsSi_3

LaOsSi3 single crystals are synthesized for the first time by an arc melting method. The crystal features a tetrag- onal BaNiSn3-type structure （space group 14mm） which lacks inversion symmetry along the crystallographic c axis and is isostructural with the intensively studied Rashba-type noncentrosymmetric superconductors LaRhSi3 and LaIrSi3. Un- like LaRhSi3 and LaIrSi3 displaying superconductivity, LaOsSi3 shows only metallic behavior over the measured temper- ature range of 2 K-300 K. The Sommerfeld coefficient ]/derived from specific heat is 5.76 mJ.mol-1 -K-2, indicating that LaOsSi3 has a weak electronic correlation effect. The absence of superconductivity in LaOsSi3 may lie in the Os 5d bands in the electronic structure. If it is true, it would be useful to provide complementary knowledge in understanding the relation between conduction and the d bands of M in LaMSi3 compounds （M = transition metals）.     

LiF分子在外电场中的物理性质研究

本文采用量子力学从头算方法,运用密度泛函B3LYP方法在6-311基组水平上, 对不同外加电场(--0.15---0.15a.u.) 作用下LiF分子基态的稳定电子结构进行了计算,研究了外电场对LiF分子键长、能量、电荷分布、能级分布、能隙及红外光谱的影响规律.结果表明,随着Z方向外电场的增加,分子键长、偶极矩和能隙递增,原子电荷也递增,总能量降低, 频率及其红外强度递减, LiF基态分子势能曲线降低,离解能减小.     

Magnetic properties of lithium intercalation compounds

Magnetic experiments are powerful tools to study fundamental properties and to check the qualities of samples. Temperature, stress, and impurities of materials can all affect magnetic properties and play an important role in the utilization of these materials for engineering applications. The estimation and analysis of the spontaneous magnetization can reveal ferromagnetic particles as impurities in samples. The shape of the temperature dependence of magnetization is indicative of the origin of the magnetic properties. However, it is necessary to correlate the χ _m (T) curves and isothermal M(H) plots to achieve a complete analysis of the electronic properties of the materials. Highlights of magnetic properties of lithium intercalation compounds are briefly described. Intrinsic and extrinsic properties are considered as useful parameters to determine the purity of electrode materials for rechargeable Li-ion batteries.     

Gd3Co29-xCrx新相化合物的结构与磁性

制备出具有室温单轴磁晶各向异性的非间隙型Co基Gd3Co29-xCrx化合物(x=65和70)，x射线衍射和磁性测量表明所有单相化合物均属于单斜晶系，Nd3(Fe,Ti)29型结构和A2/m空间群.Gd3Co29-xCrx化合物的居里温度在x=65时为412 K，x=70时为359 K. Gd3Co29-xCrx化合物在x=65 时磁化强度随温度的变化曲线表明，在居里温度以下的某一温度处有一补偿点，在补偿点处求得晶格分子场系数nRT=33 T f.u./μB. 关键词： Gd3Co29-xCrx化合物 x射线衍射 磁晶各向异性     

Structural,electronic, optical and bonding properties of strontianite,SrCO3: First-principles calculations

The first-principles calculations are performed within the density functional theory to investigate the crystal structure, energy band structure, density of states, optical properties, and bonding properties of strontianite. The optimized structure parameters and bonding results with the generalized gradient approximation (GGA) functional and the localized density approximation (LDA) functional are in good agreement with the earlier experimental data. The band structure, density of states and chemical bonding of strontianite have been calculated and analyzed. The indirect band gap of strontianite is estimated to be ~4.45 eV (GGA) or ~4.24 eV (LDA). The absorption, reflectivity, refractive index and extinction coefficient have been calculated using the imaginary part of the dielectric function. The calculated results of the optical properties show that strontianite has an optical anisotropy along [100] (or [010]) and [010] polarization directions of incoming light. Furthermore, the calculated results of the density of states and Mulliken population indicate that the interactions among atoms are both ionic and covalent bonding in strontianite.     

Ga填充n型方钴矿化合物的结构及热电性能

用熔融退火结合放电等离子烧结(SPS)技术制备了具有不同Ga填充含量的Ga_xCo₄Sb₁₂方钴矿化合物,研究了不同Ga含量对其热电传输特性的影响规律. Rietveld结构解析表明,Ga占据晶体学2a空洞位置,Ga填充上限约为0.22,当Ga的名义组成x≤0.25时,样品的电导率、室温载流子浓度N_p随Ga含量的增加而增加,Seebeck系数随Ga含量的增加而减小. 室温下霍尔测试表明,每一个Ga授予框架0.9个电子,比Ga的氧化价态Ga³⁺小得多. 由于Ga离子半径相对较小,致使Ga填充方钴矿化合物的热导率κ及晶格热导率κ_L较其他元素填充的方钴矿化合物低. 当x＝0.22时对应的样品在300K时的热导率和晶格热导率分别为3.05Wm^-1·K^-1和 2.86Wm^-1·K^-1.在600K下Ga_0.22Co_4.0Sb_12.0样品晶格热导率达到最小,为1.83Wm^-1·K^-1,最大热电优值Z,在560K处达1.31×10^-3K^-1. 关键词： skutterudite化合物 Ga原子填充 结构 热电性能     

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.     

Structural and magnetic properties of Dy2Co17—xMnx compounds

Using x-ray diffraction and magnetic measurements, we have studied the structural and magnetic properties of Dy₂Co_17-xMn_x (x=0∽5) compounds with a rhombohedral Th₂Zn₁₇-type structure. With an increasing Mn concentration x, the unit-cell volume V was found to increase linearly. The Curie temperature T_c decreases linearly, and the saturation magnetization M_s at 5K first increases slightly for x<1, then decreases rapidly for x>1 with a further increase of Mn concentration x. In compounds for x=1～3, a spin reorientation was found. A magnetic diagram of the compounds is given.     

第一性原理研究钨掺杂对锐钛矿物性的影响

在实验上, W掺杂量在0.02083–0.04167的范围内时, 有关掺杂体系的电导率影响的研究有两种相悖的结论. 为解决这个问题, 本文采用第一性原理平面波模守恒赝势方法, 首先构建了两种Ti_0.97917W_0.02083O₂ 和Ti_0.95833W_0.04167O₂ 超胞模型, 分别对这两种模型进行了几何结构优化、能带结构分布和态密度分布计算. 同时还计算了掺杂体系的电子浓度、有效质量、迁移率和电导率. 计算结果表明, 在电子自旋极化或电子非自旋极化的条件下, W掺杂浓度越大、掺杂体系的电子浓度越大、有效质量越小、迁移率越小、电导率越大、导电性能越强. 由电离能和Bohr半径分析进一步证实了Ti_0.95833W_0.04167O₂ 超胞的导电性能优于Ti_0.97917W_0.02083O₂ 超胞. 为了研究掺杂体系的结构稳定性和形成能, 又分别构建了Ti_0.96875W_0.03125O₂, Ti_0.9375W_0.0625O₂两种超胞模型, 几何结构优化后进行了计算, 结果表明, 在电子自旋极化或电子非自旋极化的条件下, 在W掺杂量为0.02083–0.04167的范围内, W掺杂浓度越大、掺杂体系的总能量越高、稳定性越差、 形成能越大、掺杂越困难. 将掺杂体系的晶格常数与纯的锐钛矿TiO₂相比较, 发现沿a轴方向的晶格常数变大、沿c轴方向的晶格常数变小、掺杂体系的体积变大, 计算结果与实验结果相符合. 在电子自旋极化的条件下, 掺杂体系形成了半金属化的室温铁磁性稀磁半导体.     

Y替代La2/3Ca1/3MnO3体系的结构与输运行为

系统研究了(La1-xYx)2/3Ca1/3MnO3(0.0≤x≤0.3)体系的结构和输运行为.结果表明，实验样品具有很好的单相结构，随Y掺杂浓度的增加，金属—绝缘体（M—I）转变温度T-MI向低温区移动，对应的峰值电阻率ρp升高，对x=0.3样品，较未替代样品（x=0.0）增幅达8个数量级.在外加磁场下，材料表现出很强的磁电阻效应.同时，从实验结果出发，直接给出了输运特性与晶体结构之间的关联，并从双交换模型和可变程跃迁理论出发，对实验结果进行了初步讨论. 关键词： La2/3Ca1/3MnO3锰氧化物 Y替代 晶体结构 输运行为     

温度和压强对金属化合物Co_2Zr和Co_2Ti的结构和热力学性质影响的研究

基于平面波赝势密度泛函理论(DFT)和广义梯度近似(GGA)的第一性原理计算了高温高压下金属化合物Co_2Zr和Co_2Ti的结构和热力学性质.Co_2Zr和Co_2Ti均为立方晶体,且结构类型为Cu2Mg结构.通过计算得出的晶格参数跟实验值符合较好.用准谐德拜模型计算了Co_2Zr和Co_2Ti的热力学性质.在0~100 GPa的压强和0~1500 K的温度作用下,两种物质的德拜温度Θ,热容CV,热膨胀系数α随压强和温度的变化而变化.随着压强的增大,德拜温度单调的增加;相同压强下,Co_2Ti的德拜温度始终大于Co_2Zr.热容CV和热膨胀系数α均随温度的增加而增加,增大压强时反而减小,说明减小温度和增大压强对CV以及α有相同的影响.     

First principles study of structural,electronic, mechanical and thermal properties of A15 intermetallic compounds Ti3X (X=Au,Pt, Ir)

The structural, electronic, elastic, mechanical and thermal properties of Ti₃Au, Ti₃Pt and Ti₃Ir intermetallic compounds crystallizing in A15 structure have been studied using density functional theory within generalized gradient approximation (GGA) for the exchange correlation potential. Elastic properties such as Young's modulus (E), rigidity modulus (G), bulk modulus (B), Poisson's ratio (σ) and elastic anisotropic factor (A) have been calculated. From the present study it is noted that Ti₃Ir is the hardest compound among the three materials studied due to its larger bulk modulus. Also, it is more ductile in nature.     

Thermoelectric properties and electronic structure of Te-doped CoSb3 compounds

Co₄Sb_12−xTe_x compounds were prepared by mechanical alloying combined with cold isostatic pressing, and the effects of Te doping on the thermoelectric properties were studied. The electronic structure of Te-doped and undoped CoSb₃ compounds has been calculated using the first-principles plane-wave pseudo-potential based on density functional theory. The experimental and calculated results show that the value of the solution limit x of Te in Co₄Sb_12−xTe_x compounds is between 0.5 and 0.7. The Fermi surface of CoSb₃ is located between the conduction band and the valence band, and its electrical resistivity decreases with increasing temperature. The density of states is mainly composed of Co 3d and Sb 5p electrons for intrinsic CoSb₃.The Fermi surface of Te-doped compounds moves to the conduction band and its electrical resistivity increases with increasing temperature, exhibiting n-type degenerated semiconductor character. Under the conditions of the experiment, the maximum value 2.67 mW/m K² of the power factor for Co₄Sb_11.7Te_0.3 is obtained at 600 K; this is about 14 times higher than that of CoSb₃.     

Cohesive energy and physical properties of nanocrystals

Recently, a lattice-type-sensitive model, free of any adjustable parameter, for the size dependence of the cohesive energy of nanocrystals (nanodisks, -films, -wires and -particles) has been developed, taking into account the effects of the averaged structural and energetic properties of their surface and volume. These effects are related to the first- and second-nearest-neighbor atomic interactions. Now, considering the intimate relation between cohesive energy and other physical properties of materials, the recently obtained formula for the cohesive energy of nanocrystals has been applied to the cases of melting point (In, Bi, Si and Ag), evaporation temperature (Ag and Au), vacancy formation energy (Au), diffusion activation energy (Au), surface energy (Au, Al and Na), liquid–vapor interfacial energy (Al and Na), Curie temperature (Pb), Debye temperature (Au and Fe) and band gap energy (Si) of nanocrystals. In general, good agreement between the present model and the data has been obtained. Moreover, the surface-area-difference (SAD) model has been derived as a first-order approximation of the present model.     

Synthesis and physical properties of alkaline earth metal graphite compounds

We report synthesis and search for superconductivity of Ba, Sr, Ca and Mg-GIC. We adopted conventional vapor phase reaction in order to prepare high quality GICs. No superconducting transition was found for Ba and Mg-GIC. As for Sr, Sr-GIC showed a sharp superconducting transition at T_c=1.65 K. Sr graphite compound SrC_x prepared from powder graphite by thermal treatment at higher temperature showed ferromagnetic character.