Crystal structures of R2Pd2Pb (R = Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, and Lu) compounds

The crystal structures of the R₂Pd₂Pb (R=Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, and Lu) compounds were determined using X-ray powder diffraction. The investigated compounds crystallize with Mo₂FeB₂ structure type (space group P4/mbm, Pearson code tP10). The importance of stabilization by polar intermetallic R–Pd bonding is underscored by a bonding analysis derived from electronic band structure calculations.     

Formation of zirconium diboride (ZrB2) by room temperature mechanochemical reaction between ZrO2, B2O3 and Mg

A mixture of magnesium, boric oxide and zirconium dioxide were mechanically milled under argon for up to 15 h in a laboratory scale ball mill. X-ray diffraction showed that there was an increasing conversion of ZrO₂ to ZrB₂ with milling time with >98% reaction after 15 h. Differential thermal analysis revealed there were multiple, overlapping reactions all of which seemed to be formation of ZrB₂. The energy evolved decreased with milling time and the sample after 15 h milling showed no thermal reaction. After milling, separation of the ZrB₂ from the coproduct MgO was easily achieved by a mild acid leaching leaving essentially pure ZrB₂ with a crystallite size of 75 nm.     

Effects of particle size of 8 mol% Y2O3 stabilized ZrO2 (YSZ) and additive Ta2O5 on the phase composition and the microstructure of sintered YSZ electrolyte

The chemical stability and sinterability of nano- and micro-sized yttria stabilized zirconia (YSZ) powders were investigated, and the effect of Ta₂O₅ additive on the sinterability of YSZ electrolyte was also studied. Addition of Ta₂O₅ easily causes phase transformation of YSZ from cubic to monoclinic, and thus unfavorable for the improvement of ionic conductivity of YSZ electrolyte. Nano-sized YSZ easily causes abnormal grain growth and hence readily results in inhomogeneous microstructure of YSZ matrix, while micro-sized YSZ displays relatively low sinterability. Comparatively, mixture with 50 wt.% nano-sized and 50 wt.% micro-sized YSZ powders exhibits high sinterability and could be sintered to homogeneous microstructure, which can be used as starting materials to prepare dense YSZ electrolyte at low temperatures.     

Achieving tensile superplasticity in 8 mol% Y2O3 cubic stabilized ZrO2 through the addition of intergranular silica

The addition of 5 wt.% SiO₂, a viscous second phase, to 8 mol% Y₂O₃ cubic stabilized ZrO₂ (8Y-CSZ) made superplastic 8Y-CSZ. This material had a fine grain size of 0.4 μm and exhibited deformations in tension as large as 520% at 1430 °C with a strain rate of 1.0 × 10⁻⁴ s⁻¹.     

Determination of the structure of UFe2Al10 compound

The atomic structure of a new ternary phase UFe₂Al₁₀ appearing in the U–Fe–Al system was determined using direct methods applied to X-ray powder diffraction data. High resolution electron microscopy combined with the methods of crystallographic image processing was used for the verification of the structural model. The UFe₂Al₁₀ phase is orthorhombic and belongs to Cmcm space group, its unit cell contains 40 Al, eight Fe, and four U atoms. The lattice parameters obtained after Rietveld refinement are: a=8.919 Å, b=10.208 Å, and c=9.018 Å. The reliability factors characterizing the Rietveld refinement procedure are: R_p=5.9%, R_wp=8.1%, and R_b=2.9%.     

Crystal structure and magnetic properties of ZnV2O4

We present structural and magnetic data on ZnV₂O₄ single crystals. Single crystal X-ray diffraction shows the measured crystals to be of very high quality, especially with respect to atomic order. The measured magnetic susceptibility resembles to that of a spin glass system, surprising for a translational invariant structure. The results are discussed in the framework of disorder in a magnetically frustrated lattice.     

Thermal cycle behavior of plasma sprayed La2O3, Y2O3 stabilized ZrO2 coatings

The effects of La₂O₃ addition on thermal conductivity, phase stability and thermal cycle life of Y₂O₃ stabilized ZrO₂ plasma sprayed coatings were investigated. Although low thermal conductivity as well as high resistance to sintering was achieved by La₂O₃ addition, it tended to also result in lower phase stability and thermal cycle life of the coatings. Optimization of the composition and structure of the coatings improved these properties, and the optimized coatings showed prolonged thermal cycle life.     

Structure and unusual magnetic properties in the itinerant electron system Hf0.8Ta0.2(Fe1−xCox)2

The crystal structure and magnetization of Hf_0.8Ta_0.2(Fe_1−xCo_x)₂ are investigated by X-ray powder diffraction and magnetization measurements. The compounds exhibit the Laves C14 structure for x=0.0–0.2 and the C15 structure for x≥0.3. The structural transition from C14 to C15 leads to an anomaly of the unit cell volume between x=0.2 and 0.3. When x=0.0, the compound undergoes a magnetic phase transition from ferromagnetic to paramagnetic state via the antiferromagnetic state, in which a field-induced metamagnetic transition is observed. When x=0.1 and 0.2, the compounds exhibit unusually small saturation moments, which are considered as antiferromagnetism (with weak ferromagnetic impurities) and weak ferromagnetism or ferrimagnetism, respectively. The formation of the AFM state is associated with a small bond length of Fe atom in the 6h site. When x≥0.3, the compounds exhibit a ferromagnetic to paramagnetic transition, which can be explained by itinerant electron metamagnetism.     

Ce(Au,Sb)2 with UHg2 structure type, a new member of the AlB2 family

A new ternary compound Ce(Au,Sb)₂, with a homogeneity range has been observed from X-ray powder diffraction of as cast alloys, a = 4.743–4.712 Å, c = 3.567–3.768 Å. Its crystal structure was investigated by X-ray diffraction from Ce(Au_1−xSb_x)₂ (x = 0.266) single crystal: CAD-4 automatic diffractometer, Mo K radiation, a = 4.7256(6) Å, c = 3.6711(6) Å, P6/mmm space group, V = 70.997(17) Å³, Z = 1, ρ = 10.732 Mg/m³, μ = 76.369 mm⁻¹, R₁ = 0.0415, wR₂ = 0.0793 for 99 reflections with I > 2σ(I₀). The coordination polyhedron of X (X = 0.734Au + 0.266Sb) atom is a full-capped trigonal prism [XCe₆X₃X₂]. Ce atom is coordinated by 14 atoms: [CeX₁₂Ce₂]. The compound is isotypic with UHg₂ structure, a deformation derivative of AlB₂ structure type. It forms isostructural compounds with La and Pr.     

Crystal structures and structural relationships of KFe2(SeO2OH)(SeO3)3 and SrCo2(SeO2OH)2(SeO3)2

The new phases KFe₂(SeO₂OH)(SeO₃)₃ and SrCo₂(SeO₂OH)₂(SeO₃)₂ have been synthesized under low-hydrothermal conditions and their structures were determined by single-crystal X-ray methods. Both compounds are monoclinic; KFe₂(SeO₂OH)(SeO₃)₃: space group P2, A = 9.983(4), B = 5.270(1), C = 10.614(4) Å, β = 97.42(2)°, V = 553.7 ų, Z = 2; SrCo₂(SeO₂OH)₂(SeO₃)₂: space group P2ln, A = 14.984(2), B = 5.286(1), C = 13.790(2) Å, β = 94.72(1)°, V = 1088.5 ų , Z = 4. The refinements converged to R-values of 2.9 and 3.6% respectively.

The atomic arrangement in KFe₂(SeO₂OH)(SeO₃)₃ and SrCo₂(SeO₂OH)₂(SeO₃)₂ is based on isolated MO₆ octahedra (M = Fe³⁺, Co²⁺), which are corner-linked via trigonal pyramidal selenite groups to a framework structure. Interstitials are occupied by potassium or strontium atoms in ten- or eight-coordination respectively, and by the lone-pair electrons of the Se⁴⁺ atoms. Both compounds are not isotypic but are closely related and may be interpreted as different distortions of an idealized structure type in space group P2/m, which was modelled for a theoretical compound SrFe₂(SeO₃)₄ by distance least squares refinement (program ).     

Dynamic visualization of crack propagation and bridging stress using the mechano-luminescence of SrAl2O4: (Eu,Dy,Nd)

The goal of the present investigation was to visualize the propagating crack in a mechano-luminescence (ML) material to enable the measurement of instantaneous R-curves and directly observe the bridging (shielding) stress in a fast-propagating crack system. The well-known ML compound, SrAl₂O₄: Eu²⁺, was used as a model test material. Two additional trivalent rare earth elements, Dy and Nd, were introduced as co-dopants to improve the luminescent efficiency. The initiation and growth of a crack from the mechanically machined sharp notch tip in a disc shaped compact tension (CT) specimen at a relatively high loading rate were found to be associated with the extent of light emission around the crack. An in-situ measurement of crack length and applied load for 0.3 sec yielded an instantaneous R-curve at the conventional crack propagation speed.     

New ambient pressure organic superconductor with Tc = 8.1 K based on unsymmetrical donor molecule, ethylenedithiotetrathiafulvalene: (EDT-TTF)4Hg3−δI8, δ 0.1–0.2

The new ambient pressure organic superconductor based on unsymmetrical donor, ethylenedithiotetrathiafulvalene, and the iodomercurate anion, (EDT-TTF)₄Hg_3−δI₈, δ0.1–0.2, with T_c=8.1 K has been synthesized. Transport and magnetic properties have been investigated. Upper critical fields have been studied under parallel and perpendicular magnetic fields up to 17 T in the temperature region down to 1.3 K. The data show that upper critical fields are within the paramagnetic limit in all directions. The coherence lengths indicate that (EDT-TTF)₄Hg_3−δI₈, δ0.1–0.2, is a quasi-two-dimensional organic superconductor.     

NiAl-based polyphase in situ composites in the NiAl---Ta---X (X = Cr, Mo, or V) systems

Polyphase in situ composites were generated by directional solidification of ternary eutectics. This work was performed to discover if a balance of properties could be produced by combining the NiAl-Laves phase and the NiAl-refractory metal phase eutectics. The systems investigated were the Ni---Al---Ta---X (X = Cr, Mo, or V) alloys. Ternary eutectics were found in each of these systems and the eutectic composition, temperature, and morphology were determined. The ternary eutectic systems examined were the NiAl---NiAlTa---(Mo, Ta), NiAl---(Cr, Al) NiTa---Cr, and the NiAl---NiAlTa---V systems. Each eutectic consists of NiAl, a C14 Laves phase, and a refractory metal phase. Directional solidification was performed by containerless processing techniques in a levitation zone refiner to minimize alloy contamination. Room temperature fracture toughness of these materials was determined by a four-point bend test. Preliminary creep behavior was determined by compression tests at elevated temperatures, 1100–1400 K. Of the ternary eutectics, the one in the NiAl---Ta---Cr system was found to be the most promising. The fracture toughness of the NiAl---(Cr, Al)NiTa---Cr eutectic was intermediate between the values of the NiAl---NiAlTa eutectic and the NiAl---Cr eutectic. The creep strength of this ternary eutectic was similar to or greater than that of the NiAl---Cr eutectic.     

Magnetic phase transitions in Nd1−xYxMn2Ge2 (0 ≤ x ≤ 0.6)

The structure and magnetic properties of Nd_1−xY_xMn₂Ge₂ (0.0≤x≤0.6) were studied by X-ray powder diffraction and magnetization measurements. All compounds crystallize in the ThCr₂Si₂-type structure with space group I4/mmm. Substitution of Y for Mn led to a linear decrease in the lattice constants and the unit cell volume. Increasing substitution of Y for Nd in NdMn₂Ge₂ shows a depression of ferromagnetic ordering and the gradual development of antiferromagnetic ordering.     

The cubic-tetragonal phase equilibria in the ZrO2-R2O3 (R = Y, Gd, Sm, Nd) systems

The cubic-tetragonal (c-t) phase equilibria in the ZrO₂-R₂O₃ (R = Nd, Sm, Gd, Y) systems are examined both experimentally and theoretically. The width of the c-t two-phase field is not simply described as a function of oxygen vacancies as proposed by Hillert and Sakuma (Ref 6) but is dependent on ionic radius of trivalent cations. The larger the ionic radius, the wider the two-phase field. The result is satisfactorily explained by the addition of the excess energy term caused by strain energy in cubic solid solutions to the original model.     

Crystal growth and structure of La3MGa5O14 (M=Ti, Zr, Hf)

La₃M⁴⁺Ga₅O₁₄ (M=Ti, Zr, Hf) compounds were crystallized using the micro-pulling down and Czochralski techniques. Both growth methods showed that these three crystals are peritectic compounds. The structure of La₃TiGa₅O₁₄ (LTiG) crystal was refined using single-crystal X-ray diffraction data. LTiG was observed to be isostructural to Ca₃Ga₂Ge₄O₁₄ (P321 (No.150), Z=1) and the lattice parameters are a=8.223(1), c=5.109(1) Å. The Ti atoms were found to occupy octahedral (1a) and tetrahedral (3f) sites coordinated by six and four oxygen atoms, respectively.     

稀土元素M(M=La,Ce,Pr)掺杂对LiNH2解氢性能的影响

采用基于密度泛函理论的第一性原理赝势平面波方法,研究了稀土元素M(M=La,Ce,Pr)对LiNH2解氢性能的影响.计算了合金形成热、H原子解离能和电子态密度,分析了结构稳定性.结果表明:稀土元素M替代LiNH2中部分Li时,费米能级附近能隙△EH-L变窄,使得N-H键间共价键特性减弱,系统结构稳定性降低,提高了LiNH2的解氢能力.     

Synthesis, crystal structure and properties of the new semiconductor (EVT)4·[Pt(CN)4]

A novel radical cation salt based on of the donor (4,5-ethylenedithio-4′,5′-vinylenedithio)tetrathiafulvalene (EVT) with the square planar anion Pt(CN)₄²⁻ has been synthesized: (EVT)₄·[Pt(CN)₄] (1). According to the X-ray analysis its crystal structure includes EVT cation layers alternating with anion layers along the a-axis of the unit cell. The radical cation layer is formed by EVT stacks with β-packing type, the donors in stacks are tetramerized. The EPR spectra of a plate-like crystal of (EVT)₄·[Pt(CN)₄] salt shows a very weak signal with typical parameters of TTF derivative. The room temperature conductivity of salt 1 is 8×10⁻² Ω⁻¹ cm⁻¹ and the temperature dependence of the conductivity exhibits semiconducting character.     

Surface spectroscopy and temperature-programmed desorption studies of the interaction of water vapor with Ni3(Al, Ti) (100) and (111) surfaces as a function of temperature

The interaction of water vapor with single crystal Ni₃(Al, Ti) has been studied with Auger electron spectroscopy, photoemission spectroscopy and thermal desorption. Water adsorbs molecularly onto the (111) and (100) surfaces at 140 K. When warmed up to room temperature, water remains as an intact molecule on (111), but dissociates to surface hydroxyl and atomic oxygen on (100). The latter process is accompanied by the evolution of hydrogen and oxidation of Al. It appears that the dissociation of water is structure-sensitive and that Al is the active species in controlling the dissociation of water on Ni₃Al surfaces.