Electronic topological transition and phase transition in rhodium and iridium based inter metallic compounds

Of the Rh₃Y, Rh₃La, Ir₃Y and Ir₃La inter-metallic compounds, the compound Rh₃Y exists in hexagonal structure, Ir₃Y and Ir₃La exist in rhombohedral structure, whereas the compound Rh₃La exists in both hexagonal and rhombohedral structures. Based on our tight binding-linear muffin tin orbital (TB-LMTO) study of other rhodium and iridium-based Rh₃X and Ir₃X (where X=Ti, Zr, Hf, V, Nb, Ta and Sc) inter-metallic compounds of AuCu₃ type cubic structure, an attempt is made to examine whether the compounds Rh₃Y, Rh₃La, Ir₃Y and Ir₃La will undergo a structural phase transition to cubic structure from their experimentally reported structures. From our study, it is observed that the compounds Rh₃Y and Rh₃La undergo a structural phase transition to cubic phase at 4.5 and 10.1 GPa, respectively, from their experimentally reported hexagonal and rhombohedral phases. Further it is predicted that both the compounds Ir₃Y and Ir₃La can exist in the cubic phase itself at ambient condition, in contrary to the experimental observation. From the band structure outputs that have been plotted for the compounds under compression, it is observed that the compounds Rh₃La, Ir₃Y and Ir₃La undergo the Lifshitz type of transition which may change the Fermi surface topology and hence the physical properties of these compounds.     

Phase transitions in TiNi(Rh) alloys

In this paper we report the results of x-ray structure analysis of how gamma rays and slight plastic strain affect phase transitions in the alloy Ti₅₀Ni₄₈Rh₂. Analysis of the published data on the temperature dependences of the rhombohedral angle in the R phase during the B2-R phase transition showed that not a single rhombohedral angle curve reflects the distortion relative to the cubic lattice of the B2 phase from the reduced temperature in the alloys TiNi and Ti₅₀Ni_50–xMe_x (Me=Fe, Co, Rh).Tomsk State Architectural-Construction Academy. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 67–75, October, 1995.     

FeAl(B2) 合金La, Ac, Sc 和 Y 元素微合金化的第一性原理研究

采用基于密度泛函理论的第一性原理计算方法, 研究了稀土元素(La, Ac, Sc 和 Y) 微合金化对FeAl (B2) 有序金属间化合物合金晶体结构、 弹性和电子性能的影响. 计算结果表明: 稀土元素Y 易于取代Fe位, 而Sc, La和Ac易于取代Al位, 其中Ac元素的加入使晶格点阵发生最大的变形. 弹性性能的计算表明La, Ac, Sc 和 Y 元素的加入可以改善FeAl (B2) 的塑性, 其中Fe₇Al₈Sc具有最好的塑性和硬度. 稀土元素对合金性能的影响, 主要是由于稀土原子的加入改变了Fe和Al电子之间的杂化作用. 计算结果与已有的试验结果和理论结果相一致.     

The hyperfine fields in ferromagnetic rare-earth zinc compounds with CsCl structure

A detailed analysis of the hyperfine interactions in a series of binary and pseudobinary ferromagnetic rare-earth zinc compounds (REZn; RE_{1 –x} A _x Zn; Gd_{1 –x} RE _x Zn; A: La, Y, Sc) is reported. With the help of the NMR spin echo technique the distance dependence of the hyperfine interaction is determined, without using a special model. The easy directions of magnetization of the pseudobinary compounds are deduced as well as the isotropic and anisotropic nearest neighbour hyperfine interaction. The data derived are discussed in terms of the electronic structure and the magnetic order in these compounds.D 17, Darmstädter Dissertation 1975.     

The superconductivity of Sc5T4Si10 (T = Co,Rh, Ir) and isomorphous compounds

Superconductivity is reported for a new class of ternary compounds of general formula R₅T₄X₁₀ where R is Sc, Y or a rare earth, T a transition metal of the platinum or iron groups and X either Si or Ge. Critical temperatures up to 8.6 K are observed. The relationship with non-superconducting compounds of similar composition and structure is discussed.     

Epitaxial growth of Nb3Ge on Nb3Ir and Nb3Rh

This is a report on a cooperative research carried out in Stanford University to investigate the possibility of using epitaxy to prepare the high T_c superconductor Nb₃Ge in an A15 crystalline structure at the 3:1 stoichiometry.Nb₃Ir polycrystalline films with the A15 structure deposited on sapphire were used as substrates for the epitaxial growth of Nb₃Ge because of the favorable lattice parameter match. The experimental results clearly show that epitaxial growth indeed occurs and helps to extend the range of homogeneity of the A15 phase up to 26.3 at.% Ge as compared with the thermodynamic equilibrium boundary at 19 at.% Ge. We also used Nb₃Rh films as substrates and found them inferior to Nb₃Ir because of the multiphase nature of the films.In addition to extending the A15 phase boundary epitaxy results in a considerable rise in the superconducting transition temperature for Ge-rich samples together with a reduction in the transition width. The work suggests that polycrystalline epitaxy can be an important tool in the synthesis of thin-film intermetallic compounds.     

Magnetic and magneto-optical properties of heusler alloys based on aluminium and gallium

The authors have studied a number of alloys of the type X₂YZ, where X represents a 3d transition metal (or Cu), Y a second 3d transition metal or one of the elements La, Th, Zr, Hf, Ta, Nb, W or Mo, and Z either Al or Ga. It was found that only a limited number of these combinations leads to the cubic L2₁ Heusler-type compounds. The lattice constants of these compounds were determined. The formation of the Heusler L2₁-type phase was compared with model predictions. A study to the magnetic properties and the room temperature Kerr rotation of the L2₁-type Heusler compounds showed that there is hardly any correlation between the Kerr rotation in the ferromagnetic Heusler alloys and the corresponding magnetization. In the Co₂YAl and Co₂YGa series the moment per Co atom and the electronegativity difference between Co and the Y component showed a linear dependence.     

Impurity doping into Mg2Sn: A first-principles study

We studied the formation energy and atomic structure of impurities in Mg₂Sn using first-principles plane-wave total energy calculations. Twenty elements, namely H, Li, Na, K, Rb, Sc, Y, La, Cu, Ag, Au, B, Al, Ga, In, N, P, As, Sb, and Bi, were selected as the impurity species. We considered structural relaxation of the atoms within the second nearest neighbors of the impurity atom in the 48-atom supercell. The results of the formation energy calculations suggested that Sc, Y, La, P, As, Sb, and Bi are good n-type dopants whereas Li and Na are good p-type dopants. The electrical properties of Li-, Na-, and Ga-doped Mg₂Sn and La-doped Mg₂(Si, Sn) composites reported previously can be explained by the low formation energies of Li, Na, Ga, and La in Mg₂Sn.     

Complex magnetic behavior in Ce5Rh4Sn10 doped with La

Ce₅Rh₄Sn₁₀ is known as a heavy-fermion antiferromagnet with magnetic-ordering temperature T _N2?=?4.3?K. We report a new antiferromagnetic phase transition at T _N1?=?1.4?K. In the series of Ce_1?x La _x Rh₄Sn₁₀ compounds both magnetic phase transitions at T _N1 and T _N2 shift towards lower temperatures. For Ce₃La₂Rh₄Sn₁₀ and Ce_3.5La_1.5Rh₄Sn₁₀ we found only one transition. In the temperature region T _N1<T<T _N2, the magnetization M measured as a function of magnetic field H exhibits a large jump at very low fields of ～0.03?T with a very small hysteresis loop. This abnormal behavior could be due to spin-flip transitions. Thermodynamical properties of the series of Ce_1?x La _x Rh₄Sn₁₀ compounds are discussed in terms of the antiferromagnetic Kondo lattice.     

Preparation and characterization of Li2.9Sc1.9−y Y y Zr0.1(PO4)3 (where y = 0, 0.1) solid electrolyte ceramics

The solid electrolyte Li_2.9Sc_{1.9? y} Y _y Zr_0.1(PO₄)₃ (where y = 0, 0.1) compounds belong to monoclinic symmetry (space group P2₁/n) at room temperature. The Zr 3d, Sc 2p, P 2p, Y 3d, O 1s, and Li 1s core level X-ray photoelectron spectra (XPS) were fitted. The Li ions in ceramics without Y occupy two different positions and in the ceramics with Y they occupy one position in the lattice. The deconvolutions of the Zr 3d, P 2p, Sc 2p, and Y 3d core level XPS are associated with different valence states on the surfaces of the investigated ceramics. Anomalies of enthalpy, change of activation energy of ionic conductivity, anomalies of dielectric permittivity in the temperature range 420–520 K of investigated compounds were found. The phenomena are related to diffuse structure phase transition in the compounds. At temperatures 600 and 900 K, the compounds belong to orthorhombic symmetry (space group Pbcn).     

High‐pressure high‐temperature synthesis of Rh2O3‐II‐type In2O3 polymorph

The metastability of the bixbyite‐ and corundum‐type In₂O₃ polymorphs up to 33 GPa (at room temperature) is shown. While compressed (in diamond anvil cells) and laser‐heated, both polymorphs undergo a phase transition to the Rh₂O₃‐II‐type structure (space group Pbcn, No. 60). The direct transition from bixbyite to Rh₂O₃‐II structure has not yet been observed for any other oxide.

     

High pressure-high temperature synthesis of distorted perovskite-type Cu-based oxides

Abstract

We have synthesized the rhombohedrally distorted perovskite phase of LaCuO₃ by reacting mixtures of La₂O₃ and CuO in an oxygen rich atmosphere at 1500°C and 6.5 GPa. We find this phase to be metastable; at 410°C and ambient pressure, it undergoes an irreversible transition to a tetragonal structure. By selective replacement of some or all of the La or Cu with one or more of the following elements: Ba, Ca, Cr, Ni, Pb, Sc, Sr, Ti, Y, Zn, and Zr, over 150 different alloys have been formed. Magnetic susceptibility measurements have failed to reveal the presence of superconductivity in any of these new polymorphs.     

Ionic size effect on magnetic and electronic properties of La0.70A0.05Sr0.25CoO3 (A=La,Nd, Gd,Ho, Y)

The structural, magnetic and electronic transport properties of La_0.70A_0.05Sr_0.25CoO₃ (A=La, Nd, Gd, Ho, Y) have been investigated by X-ray diffraction and the magnetization and resistivity measurements. All the samples crystallize into hexagonal lattice with the space group R-3C (A167) at room temperature. Small doped ions shrink the lattices almost linearly and isotropically, and bend the Co–O–Co bond angle. Although the Co atoms in all compounds are stabilized in intermediate-spin electronic configuration, the (residual) resistivity (T→0$T\to 0$) increase significantly with doping of the small ions. At low temperatures, the compounds change from metallic state of La_0.70A_0.05Sr_0.25CoO₃ (A=La, Nd, Gd) to insulated state of La_0.70A_0.05Sr_0.25CoO₃ (A=Ho, Y). The 5–7% magnetoresistance (MR) near Curie temperature can be interpreted in terms of the external field suppression of the spin fluctuation, but the relative large MR in the insulator state of La_0.70A_0.05Sr_0.25CoO₃ (A=Ho, Y) (∼20% up to 50-kOe field at 5 K) probably results from the field-induced transfer integral and the tunneling effect.     

ARF4(A=Na,K;R=La,Gd,Y,Lu)中Eu2+的光谱结构及其价态稳定性

Eu2+在ARF4(A=Na,K;R=La,Gd,Y,Lu)中的荧光光谱结构是由特征的d→f跃迁宽带发射和f→f跃迁尖峰发射组成,低温下线/带强度比明显增强;高纯氩气流中合成的样品,Eu2+的价态是稳定的。     

Structural stability and electronic properties of Co2N, Rh2N and Ir2N

The structural stability and electronic properties of Co₂N, Rh₂N and Ir₂N were studied by using the first principles based on the density functional theory. Two structures were considered for each nitride, orthorhombic Pnnm phase and cubic Pa3¯ phase. The results show that they are all mechanically stable. Co₂N in both phases are thermodynamically stable due to the negative formation energy, while the remaining two compounds are thermodynamically unstable. The calculated properties show that they are all metallic and non-magnetic. Ir₂N at Pnnm phase is a potentially hard material. The bonding behavior is analyzed.     

A comparative study of electronic structure and magnetic properties of SrCrO3 and SrMoO3

A comparative study of electronic structure and magnetic properties of SrCrO₃ and SrMoO₃ has been carried out using FPLAPW method with density-functional theory. The calculated results suggest that both compounds are nonmagnetic (NM) metal in cubic structures at room temperature, and they exhibit very similar band structure and electronic properties except more extend Mo 4d orbitals than Cr 3d electronic states. However, the electronic structure and magnetic properties exhibit remarkable differences between them in the low temperature phases. SrCrO₃ is with a C-AFM ground state with magnetic moment of 1.18μ_B/Cr in the tetragonal structure, while SrMoO₃ is with a NM ground state in the orthorhombic structure. It is assumed that the extend 4d orbitals may be the reason which results in NM solution at low temperature phase of SrMoO₃.     

Band structures of Ni3Pt and NiPt3

The results of self-consistent, spin-polarized LMTO band structure calculations are shown for the compounds Ni₃Pt and NiPt₃, ofL12 (Cu₃Au) structure. Lattice constants are reported together with bulk moduli, and the electronic structure is studied in relation to magnetism in both cubic compounds. Covalent magnetism is shown to act against the magnetization in Ni₃Pt.     

Ba0.5Sr0.5TiO3有序构型的第一性原理研究

采用第一性原理计算了Ba_0.5Sr_0.5TiO₃三种有序构型的晶格结构和对应的电子结构,晶格结构的详细分析结果表明BST{100}有序构型为四方相,Ti-O八面体中Ti原子和Ba-Sr平面上的O原子沿[100]方向分别偏心位移0.040 Å和0.065 Å,八面体畸变导致反平行自发极化出现,构型处于反铁电态. BST{110}构型也是四方相,并且(110)和(1 关键词： 钛酸锶钡 第一性原理 有序结构 铁电性     

Phase transition in (NH4)3FeF6

By use of Mössbauer spectroscopy we have found that the transition from tetragonal to cubic structure in (NH₄)₃FeF₆ takes place at 263 K. The phase transition exhibits a hysteresis of 0.5 K. The experimental data indicate that the tetragonal deformation found at low temperatures diminishes gradually as the transition temperature is approached. The spectra are influenced by electronic relaxation.     

Structural distortion in the primitive cubic phase of the superconducting/magnetic ternary rare-earth rhodium stannides

A structural distortion in the primitive cubic phase of the SnM₃Rh₄Sn₁₂ compounds with M = La, Ce, Pr, Nd, Sm, and Gd has been detected by electron diffraction and studied by X-ray diffraction. On the contrary, no distortion has been detected for the compounds with M = Eu, Yb, Ca, Sr, and Th. The La, Yb, Ca, Sr, and Th compounds become superconductors with T_c ranging between 8.7 K and 1.9 K, whereas those of Eu and Gd have a magnetic transition at about 11 K. Long exposure (200 hrs.) precession photographs revealed the existence of superstructure spots which can be indexed either on a body-centered cubic cell with a_I = 2a_cp or on a tetragonal cell with a_T = ∫2 a_cp and c_T = c_cp. In this latter case the sample must be considered as twinned and the extra spots would come from three different individuals each having the tetragonal axis along one of the three cubic fourfold axes. However, electron microscope photographs have failed so far to reveal the existence of domains. From the systematic absences it has been determined that the cubic distortion belongs to space group I2₁3 while the tetragonal one to space group P4₂22. The main effect of the distortion is the lowering of the site symmetries which favors the disorder between the cationic tin and the M atoms. The size of these latter atoms does not seem to be an important factor for the distortion. The only feature which separates the distorted compounds from the undistorted ones is the valence of the M atoms. However, no explanation can be offered why the valence plays an important role for the distortion.