Structural,magnetic and electrical properties of some uranium ternary germanides: UM2Ge2 (M = Rh,Ir) and U4M7Ge6 (M = Ru,Os)

The crystal structures and the physical properties of U₄M₇Ge₆ (M = Ru, Os) and UM₂Ge₂ (M = Rh, Ir) have been investigated. The former crystallize in the cubic structure of U₄Re₇Si₆; U₄Ru₇Ge₆ orders ferromagnetically at T_C ≈ 10–13 K whereas U₄Os₇Ge₆ remains paramagnetic down to 4.2 K. The latter whise structures derive from the tetragonal ThCr₂Si₂ or CaBe₂Ge₂ types display limited homogeneity ranges; URh₂Ge₂ exhibits a dense-Kondo behaviour at low temperatures; UIr₂Ge₂ shows polymorphism and its physical properties are strongly influenced by its crystal structure.     

Magnetic properties,electronic structure,and optical properties of the filled skutterudite BaFe4Sb12

The magnetic properties, electronic structure, and optical properties of the filled skutterudite BaFe₄Sb₁₂ are calculated by the first-principles full-potential linearized augmented plane wave (FPLAPW) plus local orbital method. It is found that the local spin density approximation (LSDA) method appears more accurate than the generalized gradient approximation (GGA) method in calculating the electronic structures and optical properties of this compound. Furthermore, our calculated lattice constant and spin magnetic moments with the LSDA method are in overall better agreement with experiment. In contrast with recent experiment, our calculations are in good agreement with experimental reflectivity spectra and optical conductivity spectrum.     

Superconductivity in the Nb3 (Al,Ge) compound system

Superconducting samples of the nominal composition Nb _x (Al_1?y , Ge _y )_1?x withx reaching from 0.72 to 0.78 andy from 0.175 to 0.30 were prepared and the superconducting transitions of as-cast and of annealed samples were measured. The onset of superconductivity for samples with a suitable composition and annealing procedure has been observed to be at about 21.05 K. By the help of anodic oxydation process and by multichannel semiconductor x-ray spectroscopy with scanning electron microscope it has been possible to carry out the phases distribution and detailed phase-analysis in the samples.     

Ca3Al2M3O12（M=Si,Ge）石榴石中Ce^3＋离子的荧光光谱

本文研究了Ｃａ３Ａｌ２Ｍ３Ｏ１２（Ｍ＝Ｓｉ，Ｇｅ）石榴石中Ｇｅ＾３＋离子的荧光光谱。在这两种石榴石中，Ｃｅ＾３＋的发射光谱均为Ｃｅ＾３＋的５ｄ→４ｆ能级跃迁发射宽谱带，但有很大差异。在锗酸盐中，Ｃｅ＾３＋的发射强度很弱，而在硅酸盐中发射强，２９５和７７Ｋ下在Ｃａ３Ａｌ２Ｓｉ３Ｏ１２中Ｃｅ＾３＋的荧光寿命分别为４６和４９ｎｓ。Ｇｄ＾３＋和助溶剂的引入可提高Ｃｅ＾３＋的发射强度。     

Magnetic and electrical properties of the equiatomic cerium germanides CeMGe (M = Rh, Ir, Pd, Pt)

From X-ray Guinier powder data the crystal structures of the ternary germanides Ce(Rh or Ir)Ge and Ce(Pd or Pt)Ge were confirmed to be of the TiNiSi-type and the CeCu₂-type, respectively. Magnetic, electrical and thermodynamic properties of these materials have been studied in the temperature range 1.5 K T 300 K. CeRhGe orders antiferromagnetically below 9.3 K, whereas in CeIrGe cerium is in an intermediate valence state and no magnetic transition is observed down to 1.5 K. Resistivity and specific heat measurements reveal a magnetic transition near 3.4 K for both CePdGe and CePtGe. According to the present investigations the compounds CeMGe, with M = Rh, Pd, Pt are classified as magnetically ordered Kondo systems.     

Features of the local structure of rare-earth dodecaborides RB12 (R = Ho, Er, Tm, Yb, Lu)

The parameters of the local crystal structure of dodecaborides RB₁₂ (R = Ho, Er, Tm, Yb, Lu) have been determined by extended X-ray absorption fine structure (EXAFS) spectroscopy. It has been shown that the vibrations of the rare-earth ion with respect to the boron cage are well described in the harmonic approximation. At the same time, the displacement of rare-earth ions from equilibrium positions of the crystal structure should be taken into account to determine the length of the R-B bond. The analysis of EXAFS spectra has revealed the displacement of 1–6% of rare-earth ions by about 0.2–0.3 Å in all compounds under investigation; this displacement at low temperatures results in the formation of a cage glass phase.     

Optical pseudogap from iron states in filled skutterudites AFe4Sb12 (A=Yb, Ca, Ba)

Optical investigations are presented of the filled skutterudites AFe4Sb12 with divalent cations A=Yb, Ca, Ba. For each of these compounds a very similar pseudogap structure in the optical conductivity develops in the far-infrared spectral region at temperatures below 90 K. Highly accurate local-density approximation electronic band structure calculations can consistently explain the origin of the pseudogap structure generated largely by transition metal 3d states. In particular, a 4f-conduction electron hybridization or strong correlations can be ruled out as origin for the pseudogap.     

Crystal field effects in rare-earth ferroborates RFe3(BO3)4 with R = Nd, Tb, Dy, or Er

Crystal field effects in rare-earth ferroborates RFe₃(BO₃)₄ with various rare-earth elements are studied theoretically. The rare-earth magnetoelastic interaction Hamiltonian is written in a multipole approximation. The field and temperature dependences of the multipole moments and strain susceptibilities of rare-earth ions in the ferroborate structure are calculated. A comparative analysis of anomalies in the thermal expansion and elastic constants of rare-earth ferroborates with Tb and Dy ions having identical magnetic structure but differing in the degree of anisotropy of the rare-earth ion is performed.     

Magnetic susceptibility of the suicides and germanides Me5Si3 and Me5Ge3 (Me=Sc,Ti, V)

Russian Physics Journal -     

An ab initio study of filled skutterudites ROs4P12 (R = Sm,Eu and Gd)

The elastic, electronic, magnetic and optical properties of filled skutterudite ROs₄P₁₂ (R = Sm, Eu and Gd) have been studied by first principles calculation. The full-potential linearized augmented plane wave method based on density functional theory was employed. For the exchange-correlation potential, local spin density approximation + Coulomb repulsion (LSDA + U) is used to treat the f-electrons more effectively. The numerical values of the elastic parameters are estimated in the framework of the Voigt–Reuss–Hill approximation. OsP-based filled skutterudite with localized 4f and 5d-electrons shows dense energy bands near Fermi energy originating from rare earth and Os atoms. The dense density of states near E_F reveals that these compounds are suitable for thermoelectric application. Optical constants including dielectric function, optical reflectivity and refractive index are calculated for photon energy radiation up to 12 eV. The exchange-splitting of R-4f states were analyzed to explain the ferromagnetic behavior of ROs₄P₁₂.     

Superconductivity in the ‘triple-fluorite-layer’ copper oxides (Cu,M)-1232 (M=Mo, W, Re, Pb)

Here two new high-T_c superconductors are reported of the (Cu,M)-1232 structure containing a fluorite-structured (Ce,Y)-O₂-(Ce,Y)-O₂-(Ce,Y) triple-layer block between two adjacent superconductive CuO₂ planes. The new superconductors were found through substitution studies at the Mo site in the recently synthesized superconductor, (Cu_0.75Mo_0.25)Sr₂(Ce_0.67Y_0.33)₃Cu₂O_11+δ or (Cu_0.75Mo_0.25)-1232 [Y. Morita, T. Nagai, Y. Matsui, H. Yamauchi, M. Karppinen, Phys. Rev. B 70 (2004) 174515. [10a]]. Essentially single-phase samples of the (Cu_0.75M_0.25)-1232 phase with M=W, Re and Pb were obtained by an ambient-pressure solid-state synthesis method. Through a subsequent high-pressure oxygenation (HPO) treatment carried out at 5 GPa and 500 °C in the presence of Ag₂O₂ as an oxygen source the M=W and Re samples were successfully ‘superconductorized’. Introduction of Pb was found to markedly enhance the speed of the formation of the 1232 phase, but the (Cu,Pb)-1232 samples did not exhibit superconductivity. Among the superconductive (Cu,M)-1232 samples, the highest T_c value of 56 K was achieved for (Cu_0.75Re_0.25)-1232. It is likely that the higher the average valence of cations in the (Cu,M)O_1+δ charge-reservoir is, the more can the phase accommodate excess oxygen upon HPO treatment and the higher is the resulting T_c value.     

Structural phase stability,electronic structure and mechanical properties of alkali metal hydrides AMH4 (A=Li,Na; M=B,AL)

The structural stability of Alkali metal hydrides AMH₄ (A=Li, Na; M=B, Al) is analyzed among the various crystal structures, namely hexagonal (P6₃mc), tetragonal (P4₂/nmc), tetragonal (P-42₁c), tetragonal (I4₁/a), orthorhombic (Pnma) and monoclinic (P2₁/c). It is observed that, orthorhombic (Pnma) phase is the most stable structure for LiBH₄, monoclinic (P2₁/c) for LiAlH₄, tetragonal (P4₂/nmc) for NaBH₄ and tetragonal (I4₁/a) for NaAlH₄ at normal pressure. Pressure induced structural phase transitions are observed in LiBH₄, LiAlH₄, NaBH₄ and NaAlH₄ at the pressures of 4 GPa, 36.1 GPa, 26.5 GPa and 46 GPa respectively. The electronic structure reveals that these metal hydrides are wide band gap insulators. The calculated elastic constants indicate that these metal hydrides are mechanically stable at normal pressure.     

Optical properties of the alkaline-earth fluorohalides matlockite-type structure SrFX (X=Cl, Br, I) compounds

The optical properties of the SrFX (X=Cl, Br, I) compound have been reported using the full potential linearized augmented plane wave (FP-LAPW) method as implemented in the WIEN2K code. We employed the generalized gradient approximation (GGA), which is based on exchange-correlation energy optimization to calculate the total energy. Also we have used the Engel-Vosko GGA formalism, which optimizes the corresponding potential for band structure calculations. Our calculations show that the valence band maximum (VBM) and conduction band minimum (CBM) are located at Γ resulting in a direct energy gap. We present calculations of the frequency-dependent complex dielectric function ε(ω) and its zero-frequency limit ε₁(0). We find that the value of ε₁(0) increases on decreasing the energy gap. The reflectivity spectra and absorption coefficient have been calculated and compared with the available experimental data.     

Thermoelectric properties of n-type Sr x M y Co4Sb12 (M=Yb, Ba) double-filled skutterudites

Alkaline-earth (AE) and rare-earth (RE) atoms are usually used as void fillers in the caged compound CoSb₃ to improve the thermoelectric performance of the filled system. Polycrystalline single-filled Sr_0.21Co₄Sb₁₂, double-filled Sr _x Yb _y Co₄Sb₁₂, and Sr _x Ba _y Co₄Sb₁₂ skutterudites have been synthesized. Rietveld structure refinement confirms that both Sr and Yb occupy the Sb-icosaedron voids in skutterudite frame work. In this paper, we report the high-temperature thermoelectric properties including electrical conductivity, Seebeck coefficient, and thermal conductivity. Double filling of the Sr–Yb combinations shows a stronger suppression on lattice thermal conductivity than that of Sr–Ba combination. Furthermore, the double-filled Sr _x Yb _y Co₄Sb₁₂ skutterudites exhibit a much higher power factor than the Sr-filled system. The maximum power factor for Sr_0.22Yb_0.03Co₄Sb_12.12 reaches 41 μW cm⁻¹ K⁻² at room temperature and 57.5 μW cm⁻¹ K⁻² at 850 K, respectively. The enhanced thermoelectric figures of merit are 1.32 for Sr _x Yb _y Co₄Sb₁₂ and 1.22 for Sr _x Ba _y Co₄Sb₁₂ at 850 K, respectively.     

NMR in Li2 M 3Al(MoO4)4 triple molybdates (M = Rb,Cs)

Nuclear magnetic resonance (magic angle spinning) spectra of ⁷Li, ²⁷Al, ⁸⁷Rb, and ¹³³Cs nuclei are measured in Li₂ M ₃Al(MoO₄)₄ triple molybdates (M = Rb, Cs) for the first time. Analysis of the nuclear magnetic resonance spectra reveal considerable asymmetry in the distribution of the electric charge throughout the crystal lattices of the compounds.     

碱土金属氧化物阳离子2MO+(M=Ca, Sr, Ba) 介入N2O与CO反应的理论研究

本文采用UB3LYP/6-311G(2d)+SDD//CCSD(T)/6-311+G(2d)+SDD方法, 计算研究了气相中碱土金属氧化物阳离子2MO+(M=Ca, Sr, Ba)参与N2O (X1∑+) + CO (X1∑+) → N2 (X1∑+g) + CO2 (X1∑+g) 的反应机理. 通过计算亲氧性得到在三种氧化物阳离子中只有2CaO+从N2O得到O原子并传递给CO的过程是热力学允许的. 碱土金属氧化物阳离子2MO+参与主题反应的机理通过以下两种方式进行, 其一为2MO+从N2O获取O原子生成2MO2+, 进而向CO提供O原子得到2MO+和CO2, 该过程为催化反应机理; 其二为2MO+先与N2O复合生成中间体IM1, 之后IM1继续与CO复合生成中间体IM2', 经过一系列反应过程最终生成2MO+, N2和CO2. 通过对两种反应过程的热力学性质和动力学因素分析得到, 2MO+(M=Ca, Sr, Ba)参与反应N2O (X1∑+) + CO (X1∑+) → N2 (X1∑+g) + CO2 (X1∑+g)的机理为后一路径, 所得结果与实验观测相符.     

碱土金属氧化物阳离子2MO+(M=Ca,Sr,Ba)参与N2O与CO反应理论研究

本文采用UB3LYP/6-311G(2d) +SDD//CCSD(T)/6-311+ G(2d) +SDD方法,计算研究了气相中碱土金属氧化物阳离子2MO+ (M=Ca,Sr,Ba)参与N2O (X1∑+)+CO(X1∑+)→N2(X1∑g+)+CO2 (X1∑g+)的反应机理.通过计算亲氧性得到在三种氧化物阳离子中只有2CaO+从N2O得到O原子并传递给CO的过程是热力学允许的.碱土金属氧化物阳离子2MO+参与主题反应的机理通过以下两种方式进行,其一为2 MO+从N2O获取O原子生成2MO+,进而向CO提供O原子得到2MO+和CO2,该过程为催化反应机理;其二为2MO+先与N2O复合生成中间体IM1,之后IM1继续与CO复合生成中间体IM2',经过一系列反应过程最终生成2MO+,N2和CO2.通过对两种反应过程的热力学性质和动力学因素分析得到,2 MO+ (M=Ca,Sr,Ba)参与反应N2O(X1∑+)+CO(X1∑+)→N2(X1∑g+)+CO2(X1∑g+)的机理为后一路径,所得结果与实验观测相符.     

Theoretical study of magnetism in RM12B6 compounds (R=Y,La or Ce; M=Fe,Co)

Electronic structure study for RM₁₂B₆ intermetallics (R=Y, La or Ce; M=Fe, Co) was performed. Fixed spin moment calculations for different volumes of unit cell were used to find low and high moment states in LaFe₁₂B₆. Obtained results are in agreement with previously obtained experimental and theoretical results. Total magnetic moments obtained for YCo₁₂B₆ and LaCo₁₂B₆ are in fair agreement with experimental values. In CeCo₁₂B₆ discrepancy between theory and experiment seems to be more pronounced. It seems that the calculated Co magnetic moments could be overestimated in the studied RCo₁₂B₆ compounds. Present calculations indicate that Fe (Co) atoms prefer 18(h) (18(g)) atomic positions what is in agreement with experiment.