Structure and element distribution in the UFe10-xAlxSi2 system

The neutron diffraction studies of powdered alloys withx = 2,2.5, and 3 gave detailed information on crystallographic data of the compounds. The structure refinement at 300 K for allx = 2,2.5, and 3 and at 450 K forx = 2 confirmed the ThMn₁₂ type of structure with uranium located in 2(a) positions, iron in 8(f), 8(i), and 8(j) positions, aluminum atoms in 8(i) and 8(j) positions, and Si atoms in 8(f) and 8(j) positions. Impurity phases, FeAl and UFe₂Si₂ have been detected in all investigated compounds.     

Crystal structure and Rietveld refinement of the new ternary compound Al14Nd5Si

The new ternary compound Al₁₄Nd₅Si has been studied by means of the X-ray powder diffraction technique and the Rietveld method. The ternary compound Al₁₄Nd₅Si has a hexagonal Ni₃Sn-type structure with space group P6₃/mmc (No.194), the lattice parameters are a = 0.64470 (2) nm and c = 0.45926 (1) nm. The Smith and Snyder figure of merit for the index, F_N, is F₃₀ = 97.8 (30). The X-ray diffraction data indicated that the crystal structure of the compound Al₁₄Nd₅Si has been successfully refined by the Rietveld method. The R-factors of Rietveld refinement are R_p = 0.088 and R_wp = 0.120, respectively. The thermal dependence of magnetization for the compound was measured by a vibrating sample magnetometer. The experimentally determined magnetic effective paramagnetic moment is μ_eff = 3.60 μ_B per Nd atom. The paramagnetic Currie temperature θ_p = −33.7 K was also obtained from the Currie-Weiss law.     

The crystal structure of R10Co9In20 (R=Er, Tm, Lu) compounds

New ternary indides R₁₀Co₉In₂₀ (R=Er, Tm, Lu) have been found in the systems {Er, Tm, Lu}–Co–In at 870 K. The crystal structure of Tm₁₀Co₉In₂₀ has been refined using single-crystal X-ray data: Ho₁₀Ni₉In₂₀ structure type, P4/nmm space group, Z=2, a=13.166(5) Å, c=9.097(4) Å, V=1577(2) ų, R=0.0315 for 335 unique reflections hkl (DARTCH-1 diffractometer, MoK_α radiation). The coordination polyhedra of the Tm atoms have 16 and 17 vertices, those of the In atoms 12 and 13 vertices and those of the Co atoms 8 and 10 vertices. The structure can be described as a stacking of polyhedra formed by In atoms.     

Isothermal section at 1673 K of the Mo–Ru–Si diagram and crystallographic structures of ternary phases

Efforts to improve the high temperature behavior of MoSi₂ in oxidizing environments led to the investigation of the Mo–Ru–Si phase diagram. The isothermal section at 1673 K was determined by X-ray diffraction, optical and scanning electron microscopies and EPMA. Five new silicides were identified and their crystallographic structure was characterized using conventional and synchrotron X-ray as well as neutron powder diffraction. Mo₁₅Ru₃₅Si₅₀, denoted α-phase, is of FeSi-type structure, space group P2₁3, a=4.7535 (5) Å, D_x=7.90 g. cm⁻³, Bragg R=7.13. Mo₆₀Ru₃₀Si₁₀ is the ordered extension of the Mo₇₀Ru₃₀ σ-phase with space group P4₂/mnm, a=9.45940(8) Å, c=4.94273(5) Å, D_x=6.14 g. cm⁻³, Bragg R=5.75.     

Polymorphism of PrIr2Si2 – In situ XRPD experiments and theoretical calculations

We have confirmed polymorphism of PrIr₂Si₂ and performed the in situ high-temperature X-ray powder diffraction (XRPD) experiments focused on the dynamics of the crystallographic phase transition from the high-temperature CaBe₂Ge₂-type phase to the low-temperature ThCr₂Si₂ phase above 250 °C. A double-exponential time evolution of this phase transformation has been observed at 325 °C. We have also performed density functional calculations to analyze why the low-temperature crystallographic modification becomes the ground state crystal structure. The important role of the c/a ratio in this process can be argued from results of calculations.     

Evidence for the existence of Hf5Si3

This article describes new evidence for the existence of Hf₅Si₃ with the Mn₅Si₃ structure type (hP16-P6₃/mmc). Binary Hf-Si alloys with Si concentrations from 11.5 to 35.0 at.% were investigated. Hf₂Si was also observed in the binary Hf-Si alloys that were investigated; this is consistent with the assessed phase diagram. The Mn₅Si₃-type structure has been observed in both binary Hf₅Si₃ and higher alloyed forms, (Hf, X)₅Si₃, where Nb and Ti were substituted for Hf. Phase identification was performed using scanning electron microscopy, electron microprobe analysis, x-ray diffraction, and automated electron backscattering pattern analysis in the scanning electron microscope. The present data indicate that high levels of O, N, or C are not required to stabilize Hf₅Si₃, and that it can exist alone as a binary compound. This result is contrary to previous reports regarding Hf₅Si₃, which claim that it only exists when stabilized by interstitials.     

Solidification morphology and phase selection in drop-tube processed Ni–Fe–Si intermetallics

Drop-tube processing was used to rapidly solidify droplets of Ni_64.7Fe₁₀Si_25.3 and Ni_59.7Fe₁₅Si_25.3 alloys. In the larger droplets, and therefore at low cooling rates, only two phases, γ-Ni₃₁Si₁₂ and β₁-Ni₃Si were observed. Conversely, in the smaller droplets, and therefore at higher cooling rates, the metastable phase Ni₂₅Si₉ was also observed. The critical cooling rate for the formation of Ni₂₅Si₉ was estimated as 5 × 10³ K s⁻¹. SEM and TEM analysis reveals three typical microstructures: (I) a regular structure, comprising single-phase γ-Ni₃₁Si₁₂ and a eutectic structure between γ-Ni₃₁Si₁₂ and β₁-Ni₃Si; (II) a refined lamellar structure with a lamellar spacing <50 nm comprising γ-Ni₃₁Si₁₂ and β₁-Ni₃Si; (III) an anomalous structure with a matrix of Ni₂₅Si₉ and only a very small proportion of a second, and as yet unidentified, phase. These results indicate that there is an extended stability field for Ni₂₅Si₉ in the Ni-rich part of the Ni–Fe–Si ternary system in comparison to the Ni–Si binary system. With an increase of cooling rate, an increasing fraction of small droplets experience high undercoolings and, therefore, can be undercooled into the Ni₂₅Si₉ stability field forming droplets consisting of only the anomalous structure (III). The Fe atoms are found to occupy different substitutional sites in different phase, i.e. Fe substitutes for Ni in the γ phase and Si in the L1₂ (β₁) phase respectively.     

Characterization of silicide phases formed during pack siliconizing coating on the Nb-1Zr-0.1C alloy

The present paper describes the morphology, chemistry and crystallography of the phases observed in the silicide coatings produced by pack cementation technique on Nb based alloys. Cross-sectional microstructures examined by transmission electron microscopy and scanning electron microscopy techniques have shown that the coating has two silicide layers: NbSi₂ and Nb₅Si₃. NbSi₂ formed at the surface of the sample and Nb₅Si₃ formed in between the substrate (Nb alloy) and NbSi₂ coating layer. Electron diffraction analyses revealed that NbSi₂ has hexagonal crystal structure with lattice parameters as a = 0.48 nm and c = 0.66 nm and Nb₅Si₃ has tetragonal crystal structure with lattice parameters as a = 0.65 nm and c = 1.19 nm. Nb₅Si₃ showed fine equiaxed grains, whereas, NbSi₂ exhibited duplex morphology having columnar grain morphology near to the Nb₅Si₃ layer and large equiaxed grains at the surface of the coating sample. The presence of duplex morphology was explained by estimating diffusion of various species and it was shown that columnar morphology of grains could be attributed to outward diffusion of Nb and equiaxed grains to inward diffusion of Si. In the case of Nb₅Si₃, growth takes place due to single element Si diffusion, leading to development of single equiaxed grain morphology of the Nb₅Si₃ phase.     

Synthesis and crystal structure of Ba3Si4C2

SiC powder prepared by the Na flux method at 1023 K for 24 h and Ba were used as starting materials for synthesis of tribarium tetrasilicide acetylenide, Ba₃Si₄C₂. Single crystals of the compound were obtained by heating the starting materials with Na at 1123 K for 1 h and by cooling to 573 K at a cooling rate of −5.5 K/h. The single crystal X-ray diffraction peaks were indexed with tetragonal cell dimensions of a = 8.7693(4) and c = 12.3885(6) Å, space group I4/mcm (No.140). Ba₃Si₄C₂ has the Ba₃Ge₄C₂ type structure which can be described as a cluster-replacement derivative of perovskite (CaTiO₃), and contains isolated anion groups of slightly compressed [Si₄]⁴⁻ tetrahedra and [C₂]²⁻ dumbbells. The electrical conductivity measured for a not well-sintered polycrystalline sample was 2.6 × 10⁻²–7 × 10⁻³ S cm⁻¹ in the temperature range of 370–600 K and slightly increased with increasing temperature. The Seebeck coefficient showed negative values of around −200 to −300 μV K⁻¹.     

The influence of gallium on the magnetocaloric properties of Gd5Si2Ge2

Gd₅Si₂Ge₂ was alloyed with varying amounts of Ga to study its influence on the giant magnetocaloric effect. Investigations on Gd₅(Si_2−xGe_2−x)Ga_2x with 2x = 0.03, 0.05 and 0.13 were carried out using X-ray powder diffraction, temperature and magnetic field dependent magnetization measurements, and differential scanning calorimetry. We observe that as the Ga content increases, the temperature stability range of the monoclinic phase narrows, and the orthorhombic structure gains stability. This is expected to be related to the decrease in the (Si/Ge)(Si/Ge) bond distance in the monoclinic phase. The maximum entropy change for the parent compound at 270 K was found to be 9.8 J kg⁻¹ K⁻¹ in an applied field of 5 T. For 2x = 0.03, this value reduces to 8.5 J kg⁻¹ K⁻¹, and the temperature corresponding to the maximum entropy change shifts marginally to 278 K. For other 2x values, the maximum entropy change further decreases.     

Nanocrystallization of Pd74Si18Au8 metallic glass

The crystallization process of Pd₇₄Si₁₈Au₈ amorphous alloy has been investigated by transmission electron microscopy, small angle X-ray scattering and three-dimensional atom probe techniques. Although literature suggests that the alloy decomposes into two glassy phases prior to the crystallization, we found that the crystallization occurs directly from a single amorphous phase by the primary crystallization of fcc Pd–Au solid solution, followed by the polymorphous crystallization of the remaining amorphous phase to a Pd₃Si phase.     

Thermochemistry of ytterbium silicides

The results of the investigation of the high temperature decomposition reactions in vacuum under equilibrium conditions of ytterbium silicides in the whole composition range are reported. By means of the Knudsen Effusion–Mass Spectrometry (KE–MS) and the Knudsen Effusion–Weight Loss (KE–WL) techniques, the Yb(g) vapour pressures in equilibrium over the various high temperature and low temperature biphasic regions were measured in the temperature range 781–1395 K and the reaction enthalpies for the respective decompositions were derived. From this set of experimental data we derived for the first time the heats of formation of all the six known Si–Yb intermediate phases. The following values Δ_fH°₂₉₈ are recommended: Si₃Yb₅=−48.3±3.6, Si₄Yb₅=−53.2±4.6, SiYb=−51.1±5.1, Si₄Yb₃=−48.0±3.1, Si₅Yb₃=−41.3±2.6, Si_1.74Yb=−37.4±0.9, all in kJ/mol atoms.     

Mixed cerium valence and unusual Ce–Ru bonding in Ce23Ru7Cd4

The rare earth-rich compounds Ce₂₃Ru₇Cd₄ and Pr₂₃Ru₇Cd₄ were synthesized from the elements in sealed tantalum ampoules in an induction furnace. Both structures were refined on the basis of diffractometer data: P6₃mc, Z = 2, a = 988.7(3), c = 2241.6(5) pm, wR2 = 0.0439, 1976 F² values for Ce₂₃Ru₇Cd₄ and a = 992.7(2), c = 2236.4(7) pm, wR2 = 0.0466, 2528 F² values for Pr₂₃Ru₇Cd₄ with 74 variables per refinement. Striking structural motifs are ruthenium centered trigonal prisms RuCe₆ and RuPr₆ which are condensed via common edges and corners, building rigid three-dimensional networks. Larger voids within these networks are filled by slightly elongated Cd₄ tetrahedra. Five of the nine crystallographically independent cerium sites in Ce₂₃Ru₇Cd₄ show Ce–Ru distances which are shorter than the Pr–Ru distances in Pr₂₃Ru₇Cd₄. This strong hint for mixed cerium valence is supported by the magnetic behavior. Pr₂₃Ru₇Cd₄ shows Curie–Weiss behavior above 50 K with an experimental magnetic moment of 3.62 μ_B/Pr atom, indicating stable trivalent praseodymium. Complex magnetic ordering sets in at 13 K. Ce₂₃Ru₇Cd₄ shows a reduced magnetic moment of 2.05 μ_B/Ce atom. The trivalent cerium atoms show ferro- or ferrimagnetic ordering below T_C = 3.6 K.     

Structure and element distribution in the UFe10-x Al x Si2 system

The neutron diffraction studies of powdered alloys withx = 2,2.5, and 3 gave detailed information on crystallographic data of the compounds. The structure refinement at 300 K for allx = 2,2.5, and 3 and at 450 K forx = 2 confirmed the ThMn₁₂ type of structure with uranium located in 2(a) positions, iron in 8(f), 8(i), and 8(j) positions, aluminum atoms in 8(i) and 8(j) positions, and Si atoms in 8(f) and 8(j) positions. Impurity phases, FeAl and UFe₂Si₂ have been detected in all investigated compounds.     

The corrosion behavior of amorphous and nanocrystalline Fe73.5Cu1Nb3Si15.5B7 alloy

The potentiodynamic polarization curves in 0.5 M NaCl solution before and after crystallization of Fe_73.5Cu₁Nb₃Si_15.5B₇ alloy have been studied in relation to the microstructure and alloy composition. It was shown that the corrosion resistance of the alloy strongly depending on these two factors. The observed decrease in corrosion resistance of the alloy after the heat treatment up to 480 °C in comparison to the corrosion resistance of the alloy in the as prepared state is attributed to the increased inhomogeneity of the alloy that coincides with the first appearance of Fe₃Si phase. Further heating (up to 600 °C) resulted in an increase in the number of Fe₃Si nanocrystallites and the appearance of a FeCu₄ phase. After annealing at 600 °C the lowest corrosion rate, 0.004 mm a⁻¹, was observed. Annealing of the samples at higher temperatures (>600 °C) induced formation of six crystalline phases which proved detrimental to the corrosion resistance of the Fe_73.5Cu₁Nb₃Si_15.5B₇ alloy. Solid corrosion products were identified on the surface of the samples after anodic polarization.     

Coexistence of localized magnetism and itinerant ferromagnetism in a Gd5Y2Pd3 single crystal

Experimental results of the single-crystal X-ray diffraction, XPS, ac-magnetic susceptibility (ac-χ), dc-magnetization M(T), and electrical resistivity (ρ) measurements for the hexagonal Th₇Fe₃-type Gd₅Y₂Pd₃ single crystal are presented. Anomalies in (ac-χ), (T) and M(T)-curves have allowed to establish that Gd₅Y₂Pd₃ undergoes a long-range ferromagnetic-type ordering at T_C = 263 K, followed by a spin-reorientation below 190 K. The magnetization data indicate that there is an excess of the magnetic moment for the Gd³⁺ ions. The observed XPS, magnetic and electrical resistivity behaviour points to the coexistence of localized magnetism from the magnetic Gd³⁺ ions and itinerant ferromagnetism from 4d- and 5d-electron bands. We discuss the magnetic behaviour of the Gd_7−xY_xPd₃ solid solutions in terms of three competitive mechanisms: RKKY-interaction, magnetic frustration and spin-fluctuation.     

Si合金化对C15 NbCr2 Laves相稳定性和断裂韧性影响的第一性原理研究

基于第一性原理计算方法,通过对形成焓、结合能、原子自由体积和电子结构的计算,研究了Si合金化对C15 NbCr₂Laves相稳定性和断裂韧性的影响。位点占据能表示Si原子倾向于占据Cr位点。形成焓和结合能计算表明,随着Si含量的增加,Nb₈Cr_16-xSi_x(X= 0~ 5)相的形成能力和稳定性均得到提升且与Si含量保持线性相关性。原子自由体积计算表明,Nb₈Cr_16-xSi_x相的原子自由体积较NbCr₂基体相均得到增加,其中在Si含量为8.33 at%(Nb₈Cr₁₄Si₂)时,原子自由体积取得最大值,断裂韧性达到最优。电子结构计算表明,Si合金化使得DOS曲线右移,费米能级向赝能隙峰谷靠近,稳定了NbCr₂基体相,同时所有的成键峰变得下降和展宽,削弱了Nb-Cr原子的键合强度,使得剪切变形易于进行,从而提高韧性。     

Cu content and annealing temperature dependence of martensitic transformation of Ti36Ni49−xHf15Cux melt spun ribbons

The martensitic transformation in Ti₃₆Ni_49−xHf₁₅Cu_x (x = 1, 3, 5, 8) ribbons has been investigated. Only B2 to B19′ transformation was detected in all the present ribbons. The martensitic transformation temperatures do not change obviously with increase in the Cu content except that they decrease when the Cu content is 3 at.%. The lattice parameters of B19′ martensite, a and c increase, b almost remains constant, while the monoclinic angle β decreases with increase in the Cu content. For the ribbons with Cu content of 1 and 3 at.%, the martensitic transformation temperatures change slightly when the annealing temperature increases. For the ribbons with Cu content of 5 and 8 at.%, with increase in the annealing temperature, the martensitic transformation temperatures almost do not change and then decrease rapidly when the annealing temperature is higher than 873 K. TEM observation shows that the microstructure of the ribbons with Cu content of 1 and 3 at.% contains the martensite matrix and the (Ti,Hf)₂Ni particles with the size of about 150 nm, which does not change obviously when the annealing temperature increases. This results in that the martensitic transformation temperatures are not sensitive to the annealing temperature in the ribbons with 1 and 3 at.% Cu content. However, nano-scale (Ti,Hf)₂Ni particles precipitate in the ribbons with Cu content of 5 and 8 at.% when the annealing temperature is 773 and 873 K, and then the (Ti,Hf)₂Ni particles grow and coarsen rapidly with further increase in the annealing temperature. The coarsening of the (Ti,Hf)₂Ni particles should be responsible for the dramatic decrease of the martensitic transformation when the annealing temperature is higher than 873 K. For all the present ribbons, the substructure of B19′ martensite is (001) compound twins, and the inter-variant relationship is mainly (011) type I twinning.     

High-temperature SO2-gas corrosion of TiN–Ti5Si3 composites prepared by polymer pyrolysis

By pyrolyzing a mixture of Si-containing pre-ceramic polymers and TiH₂ powders in a N₂ atmosphere, a TiN–Ti₅Si₃ composite was synthesized. The composite was then corroded between 700 °C and 1000 °C for 20 h in an Ar–0.2% SO₂ atmosphere. TiN was mainly oxidized to rutile TiO₂. Ti₅Si₃ was oxidized to TiO₂ supersaturated with Si ions, and sulfidized to Ti₂S supersaturated with Si ions. At initial stage of corrosion, oxidation dominated sulfidation. As corrosion proceeded, sulfidation progressively occurred underneath the oxide scale based on the decreased oxygen potential and increased sulfur potential near the scale/matrix interface.     

A new semiconductor Al2Fe3Si3 with complex crystal structure

Al₂Fe₃Si₃, a new semiconductor with complex triclinic structure was synthesized by arc melting and spark plasma sintering, followed by heat treatment. The nominal compositions of samples have been changed to compensate Al evaporation during synthesis process, and single Al₂Fe₃Si₃ phase has been obtained with the nominal composition of Al: Fe: Si = 26: 37: 37 (6 at.% Al excess against stoichiometry). In this study, we measured the sound velocity, thermal expansion coefficient, Vickers hardness, fracture toughness, electrical conductivity, Seebeck coefficient, and thermal conductivity of the new semiconductor Al₂Fe₃Si₃. The Al₂Fe₃Si₃ sample displayed positive Seebeck coefficient from 300 to 850 K, with a maximum Seebeck coefficient of 110 μV/K at 430 K. The Debye temperature of Al₂Fe₃Si₃ was 640 K, which was similar to or higher than those of other Al, Fe, Si based thermoelectric materials, but the lattice thermal conductivity was lower, 4–5 W/mK, due to the complex crystal structure of Al₂Fe₃Si₃. The maximum ZT value was 0.06 at 580 K.