Study of the electric field gradient at <Superscript>111</Superscript>Cd in the <Emphasis Type="Italic">R</Emphasis>Al<Subscript>3</Subscript> compounds synthesized under high pressure

The electric quadrupole interaction parameters for impurity ¹¹¹Cd nuclei in intermetallic RAl₃ compounds (R = La, Ce, Sm, Gd, Tb, Dy, Ho, Er, Yb, or Lu) synthesized under high (8 GPa) pressure at high (1800–1900°C) temperatures have been measured using the method of perturbed angular γγ correlations. It has been established by X-ray diffraction analysis that with an increase in the atomic number of the R element the obtained high-pressure phases are crystallized successively in the orthorhombic, hexagonal, and cubic structures. In the compounds with R = La, Ce, Sm, and Gd, deviation from the known structural types and formation of new ones due to the change in the stoichiometric composition have been observed. The data obtained by the method of perturbed angular γγ correlations have confirmed deviation from the specified stoichiometric composition 1R: 3Al for the LaAl₃, CeAl₃, SmAl₃, and GdAl₃ compounds and verified the correctness of the stoichiometric composition and the presence of the Cu₃Au structural type for the remaining RAl₃ high-pressure phases.     

稀土金属间化合物RFe2Zn20-xInx的结构属性研究

RFe₂Zn₂₀（R代表稀土元素）型稀土金属间化合物因其低稀土含量和良好的铁磁性,已成为铁磁材料的研究热点之一.添加第四组元对该系列化合物的晶体结构和材料性能会产生一定影响.利用晶格反演方法获得了一系列有效的原子间相互作用势,对三元RFe₂Zn₂₀和四元RFe₂Zn_20-xIn_x化合物进行原子级模拟计算.研究表明,随着稀土元素原子量的增加,三元体系的晶格参数和体积呈线性下降,第四组元引入与否对该线性关系无直接影响.第四组元In替代Zn时,择优占据16c晶位,占满16c后选择占据96g晶位,始终不占据48f晶位.择优占位的结论符合实验观测,并与晶格反演势分析的结果一致. 关键词： 原子间相互作用势 择优占位 晶体结构     

Magnetic properties of some RCo2B2 and RCo4B4 compounds

RCo₂B₂ (R=Pr, Nd, Sm, Gd) and RCo₄B₄ (R=Pr, Nd, Sm) compounds have been investigated by X-ray and magnetometry techniques in the temperature range 4.2-300 K. These compounds crystallize in tetragonal crystal structures of the types CeAl₂Ga₂ and CeCo₄B₄, respectively, with lattice parameters decreasing for increasing atomic weight of the rare earth ion. All compounds order magnetically with the Curie temperatures much below room temperature. Anomalous magnetic behavior is observed for RCo₄B₄ alloys.     

Study on the oscillatory behaviour of the lattice parameter in ternary iron–nitrogen compounds

The structural properties of the XFe₃N (X=Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn) cubic ternary iron based nitrides as well as the preferential occupation site of X in the structure were studied using Full Potential Linearized Augmented Plane Wave method, within the Density Functional Theory formalism, Wien2k code, the exchange-correlation potential described with the Perdew–Burke–Ernzerhof expression, based in the Local Spin Density Approximation and Generalized Gradient Approximation. According the calculations, the Sc, Co, Ni, Cu and Zn, atoms preferred the corner sites of the cubes, while Ti, V, Cr and Mn occupy the centre of the faces of the equilibrium structures. The equilibrium structure lattice parameters have an oscillatory behaviour with the atomic number of X, with decreasing amplitude as the atomic number of X increases. This trend do not correlated with the atomic radii of X.     

Hard X-ray emission by clusters in an intense femtosecond laser field during collective recombination

We consider the new mechanism of X-ray generation by clusters under irradiation by femtosecond laser pulses, the so-called collective photorecombination. We develop the theory of the photo-recombination of electrons that pass from atomic clusters at the outer ionization to the ground level of a homogeneously charged cluster. Such a cluster is considered to be a quantum potential well. The dipole approximation is inapplicable for this process. We conclude that X-ray photons in collective photorecombination on a charged cluster as a whole have an energy that is much larger than that for photorecombination on separate atomic ions inside the cluster. For a typical cluster of 2.25 × 10⁶ electrons, with a radius R = 300 Å, and a number density of plasma electrons n _e = 2 × 10²² cm^?3, we find that at a 5% outer ionization of this cluster, the energy of hard X-ray photons is 7.2 keV.     

The mixed-spin ternary-alloy in the form of ABpC1−p on the Bethe lattice

The AB_pC_1−p type of mixed ferromagnetic-ferrimagnetic ternary-alloy with A (spin-3/2), B (spin-1) and C (spin-5/2) ions was studied on the Bethe lattice with the odd numbered shells containing only A ions, while the even numbered shells either containing B or C ions randomly. The phase diagrams were obtained on the (R=|J_AC|/J_AB,kT_c/J_AB) and (p, kT_c/J_AB) planes for given values of p and R, respectively, with the coordination numbers z=3, 4, 5 and 6. The explicit dependence of the phase diagrams on z and each shell of the Bethe lattice having only one type of ion lead to some differences when compared with the previous works. The model presents one or two compensation temperatures for appropriate values of the system parameters.     

An effective atomic charge model for infrared intensities

Infrared intensities, in particular the dipole derivatives with respect to internal (symmetry) coordinates derived from the intensities, can be explained in terms of an effective atomic charge model which includes both the equilibrium charges and their first-order fluxes. For diatomic molecules it is found that most of the intensity arises from the equilibrium charges in the case of the hydrogen halides, but conversely, in CO, most of the intensity is due to the charge flux. The parameters found can be nicely related to elementary bonding theory.In small symmetrical polyatomic molecules the number of parameters is sufficiently restricted by symmetry and charge conservation that the parameters would be uniquely determinate except for the ambiguity in sign of the experimental dipole derivatives. The examples of AB₂ (D_∞h and C_2v), AB₃ (D_3h and C_3v), and AB₄ (Td) are discussed in detail; a simple generalization for molecular ions is included.For nonpolar molecules, the effective equilibrium charges are determined by the motion (not the equilibrium value) of the electronic centroid.     

High pressure X-ray diffraction studies on UAI2, UAI2, and UAI4 systems

Abstract

The UAI₂, UAI₂, AND UAI₄ compounds have been studied by high pressure X-ray diffraction up to a maximum pressure of ～ 35 GPa. The compressibility behaviour of UAI₂ has been found to be consistent with the itinerant 5f states, whereas that of UAI₂ and UAI₄, indicate a more localized nature. Further, UAI₂ has been found to undergo a structural transition at ～ 11 GPa and the structure of the high pressure phase has been identified to be of MgNi₂ type with space group P6₃/mmc. The structure of UAI₂ at ambient pressure is of MgCu₂ type with space group Fd3m. From the electronic considerations, for instance, free electrons per atom ratio e/a, it is anticipated that it may transform back to MgCu₂ type structure at still higher pressures. On similar considerations, it is expected that most of the AB₂ type Laves phase compounds of the ‘f’ electron systems may undergo the structural sequence: MgCu₂ – MgZn₂ (or MgNi₂) – MgCu₂ due to increased delocalization of their ‘f’ electron states.     

Some ABO3 oxides with defect pyrochlore structure

Studies on the formation, structures and properties of the ternary oxides TlNbO₃, TlTaO₃ and BiVO_3+x occurring in A₂B₂O_7-y defect pyrochlore structure are reported. The reaction of Tl₂ CO₃ with Nb₂O₅/Ta₂O₅ initially leads to the formation of TlNbO_3+x [x ～ 0.3] and TlTaO_3+x [x ～ 0.4]. On further heating, the phases lose oxygen forming TlTaO₃ and TlNbO₃. The occurrence of ABO₃ oxides having Tl⁺, Pb²⁺ or Bi³⁺ as the A cation in defect pyrochlore structure in preference to the perovskite structure is attributed to the formation of a “trap-mediated” A-A bond through 6s² electrons.     

On the vacancy in a metal

The self-consistent calculations of spatial distributions of electrons and potentials in an isolated spherical cavity (with atomic radius R _WS) of the Wigner-Seitz cell, the energy of the vacancy formation, and the vacancy contribution to the electrical resistance have been performed in terms of the modified Kohn-Sham method and the stabilized jellium model. The energy shift of the ground state of electrons in the Wigner-Seitz cell with radius R _v ? R _WS (where R _v is the average distance between vacancies), the contribution of vacancies to the electron work function, and the effective mass of electrons have been found using the calculated phase shifts of the scattering wave functions and the representation of a system of vacancies as a “superlattice” in a metal.     

Site preference and thermodynamic properties of R3Ni13-xCoxB2（R=Y,Nd and Sm）

This paper investigates the structural stability of intermetallics R3Ni13-xCoxB2(R=Y,Nd and Sm) with Nd 3 Ni 13 B 2-type structure and the site preferences of the transition element Co by using a series of interatomic pair potentials.The space group remains unchanged upon substitution of Co for Ni in R3Ni13-xCoxB2 and the calculated lattice constants are found to agree with reports in literatures.The calculated cohesive energy curves show that Co atoms substitute for Ni with a strong preference for the 3g sites and the order of site preference is 3g,4h and 6i.Moreover,the total and partial phonon densities of states are first evaluated for the R 3 Ni 13 B 2 compounds with the hexagonal Nd 3 Ni 13 B 2-type structure.     

Computer simulation of the structural phase transitions in ferroelectrics of barium titanate type

A molecular-dynamic model of an ABO₃ crystal with a perovskite-type structure has been developed, which implements dense packing of soft ions. A large number of different crystalline modifications, including polar ones, can be obtained within this model by insignificant variation in the radii of AandB ions. The model can be used to describe the structural phase transitions occurring with a change in temperature.     

Ferromagnetic ordering in the MMnF6 series (M = Ni,Cu, Zn,Pd)

Neutron diffraction experiments performed on MMnF₆ compounds (M = Ni, Cu, Zn, Pd) have shown that both NiMnF₆ and ZnMnF₆ crystallize in the rhombohedral R$\text{3}$ space group with a cationic ordering, whereas PdMnF₆ shows cationic disordering (R$\text{3}$c, a = 5.46 Å, α = 54.45°). CuMnF₆ has a monoclinic distortion derived from the MnF₃ structural type (a = 8.57 Å, b = 4.85 Å, c = 13.46 Å, β = 92.12°). The magnetic properties can be correlated with the structure: the R$\text{3}$ ordered compounds show a ferromagnetic behavior (T_C(NiMnF₆) = 39 K; T_C(ZnMnF₆) = 9.5 K; T_C(CdMnF₆) = 8 K), while the other two have an antiferromagnetic ordering.     

A new ternary carbide Dy2Mn2−xC5 (x=0.6): Preparation,crystal structure,and physical properties

The crystal structure, magnetic and electrical transport properties of the new ternary compound Dy₂Mn_2−xC₅ (x=0.6) have been investigated. According to X-ray powder diffraction the carbide crystallizes in its own structure type, space group I4/mmm, a=3.6421(2), c=15.7713(9) Å, R_B=0.062, R_p=0.134. The crystal structure contains isolated carbon atoms and C₂ dimers in square-bipyramidal holes and distorted bicapped square anti-prisms, respectively. Manganese atomic positions in the structure were found to be not fully occupied. Physical properties were studied in the temperature range down to 0.4 K. The electrical resistivity of Dy₂Mn_2−xC₅ (x=0.6) reveals its basically metallic nature with a positive temperature coefficient above about 30 K. A resistive anomaly at around 20 K indicates the appearance of an antiferromagnetic superzone boundary gap at low temperatures. A phase transition towards long range antiferromagnetic magnetic ordering below 19 K is further revealed by heat capacity and ac susceptibility data. Magnetization data refer to a non-trivial nature of the magnetic ground state which may be caused by the intrinsic structural disorder associated with random vacancies at the Mn site.     

X-ray single-crystal structural characterization of MgCr2O4, a post-spinel phase synthesized at 23 GPa and 1600 °C

The crystal structure and chemical composition of a crystal of MgCr₂O₄ post-spinel phase synthesized in the model system Mg₃Cr₂Si₃O₁₂–Mg₄Si₄O₁₂ at 23 GPa and 1600 °C have been investigated. Electron microprobe analysis confirmed the MgCr₂O₄ stochiometry of the studied phase. The compound was found to crystallize with the orthorhombic calcium-titanate (CaTi₂O₄) structure type, space group Bbmm, with lattice parameters a=9.468(1), b=9.670(1), c=2.845(1) Å, V=260.5(1) Å³, and Z=4. The structure was refined to R1=0.046 using 286 independent reflections. Magnesium was found to fully occupy the eightfold-coordinated A site (with a mean bond distance of 2.289 Å) and Cr the octahedral B site (mean: 1.986 Å). The successful synthesis of MgCr₂O₄ with (CaTi₂O₄)-type structure and its structural characterization demonstrate the stability of the new post-spinel phase. The absence of MgCr₂O₄ compounds with spinel structure coexisting with the post-spinel phase in the investigated run is discussed.     

Polytypisme dans le systeme titane-soufre

Polytypism has been observed among non stoichiometric compounds. in the titanium-sulphur system. This phenomenon, studied at 800 and 900°C, appears near the composition TiS_1,70. Among a lot of varieties which commonly exhibit one-dimensional disorder, four structures have been worked out (12H, 48R, 8H, 10H) and four others identified (12H_a, 24H_b, 40H, 696R).A method is given for accurate evaluation of composition and z coordinates of sulphur atoms. Thus it appears that in these polytypes, atomic planes are equally spaced. Several modes of coexistence of two types present in a single crystal have been studied.     

Crystal structure of the quaternary compound CuTa2InTe4 from X-ray powder diffraction

The crystal structure of the new quaternary compound CuTa₂InTe₄ was studied using X-ray powder diffraction data. The powder pattern refined by the Rietveld method indicates that this material crystallizes in the tetragonal system with space group I-4¯2m (No. 121), Z=2, and unit cell parameters a=6.1963(2) Å, c=12.4164(4) Å, c/a=2.00 and V=476.72(3) Å³. The structural and instrumental refinement of 28 parameters led to R_p=10.4%, R_wp=11.1%, R_exp=6.8% and χ²=2.7 for 96 independent reflections.     

First-principles study of the structural properties and magnetism of R2CoIn8 (R=Y,Pr, Nd,and Dy) intermetallic compounds

The electronic structure of R₂CoIn₈ (R=Y, Pr, Nd, and Dy) intermetallic compounds is calculated from first principles based on the density functional theory (DFT). The Kohn–Sham single-particle equations of the DFT are solved using two independent computational methods, namely APW+lo and FPLO. First the structural properties of Y₂CoIn₈ are studied. Good agreement of calculated equilibrium volume and c/a ratio with the experimental data is found. Also we minimize the forces at equilibrium volume and calculate the symmetry-free structural parameters of the space group P4/mmm for Y₂CoIn₈. In Y₂CoIn₈, the Co 3d states are almost fully occupied and situated below the Fermi level. We applied the fixed-spin-moment method and a stable paramagnetic ground state for Y₂CoIn₈ was found. Finally, the crystal field (CF) parameters were calculated for R=Pr, Nd and Dy from first principles. The microscopic tetragonal CF Hamiltonian was diagonalized and the obtained eigenvalues and eigenfunctions were used to predict the anisotropy of the magnetic properties of R₂CoIn₈ single crystals.     

EXAFS study of size dependence of atomic structure in palladium nanoparticles

Dependence of atomic structure of Palladium nanoparticles on supports Al₂O₃ and SiO₂ upon their size, changed from 1.3 to 10.5 nm, was studied by Pd K-edge EXAFS. To determine the structure of the interior (core) and the near surface regions of nanoparticle, the fitting technique of the Fourier-transforms F(R) of spectra was used, which enabled to overcome instabilities of the obtained structural parameters values. The processing of experimental data was performed using results of the study of features formation in │F(R)│ of Pd K-EXAFS in Pd foil. By this approach it was revealed that the local structure of Pd atoms in the core is similar to fcc structure of bulk Pd, irrespective of size. The percentage of Pd atoms, which can be attributed to the core, upon the particles size was determined and the obtained dependence was described by the “cluster size equation”. In the near surface region of nanoparticles, nearest-neighbors Pd–Pd distances show a large Debye–Waller parameters and the mean bond length slightly contracted for nanoparticles of sizes less than ~2 nm. The effect of small structural distortions in the vicinity of absorbing Pd atom in the near surface region was studied using the cluster model of nanoparticle.