Characterization of Laves phase in Crofer 22 H stainless steel

This study investigated the effect of annealing temperature on the precipitation behavior of Crofer^® 22 H at 600 °C, 700 °C, and 800 °C. The grain size distribution, precipitate phase identification, and microstructure were analyzed using electron backscatter diffraction (EBSD) and energy dispersive X-ray spectroscopy (EDS). The morphology of Laves phase (Fe,Cr,Si)₂(Nb,W) precipitates having the Cr₂Nb structure changed from strip-like to needle-shaped as the annealing temperature was increased. The precipitates of the Laves phase also shifted from the grain boundaries to the grain interiors when the temperature was increased. However, the average grain size (150 μm) of the ferritic matrix did not significantly change at 600 °C, 700 °C, and 800 °C for 10 h.     

Structure imaging and vanadium substitution in cubic TiCr2 Laves phase

Properties of Laves phase compounds can be tailored by alloying and microstructural engineering. V-substituted cubic TiCr₂ Laves phase has been studied to understand the location of V atoms in the lattice, by structural imaging and first-principle computations. Even though Ti, V and Cr appear next to each other in the periodic table, V preferentially replaces the Ti lattice producing anti-site defects. The defect formation energy for V substitution in Ti and in Cr lattice is 0.29 and 0.40 eV, respectively. V replacement in the Ti lattice generates atomic scale strain. Atomic numbers of V, Ti and Cr being very close, this phase is not quite suitable for incoherent imaging for understanding the structure and the chemistry. Instead, difference in channelling behaviour of electron waves along the Ti columns and along the Cr columns could be exploited to preferentially image the individual atom columns. Nature of the exit phase wave, phase and amplitude has been used to understand the contrast qualitatively. The intensity distribution of any particular atom column that is disturbed by the presence of foreign atom has been used to detect the position of V atoms. This method could be extended to study other Laves phases and complex intermetallic structures to understand their structure, defects and interfaces.     

Mg-Al-Ca合金中三元Laves相的稳定性和电子结构

采用基于密度泛函理论的第一性原理方法,应用VASP (Vienna Ab-initio Simulation Package) 计算软件,研究了Mg-Al-Ca合金中三元Laves相,即Ca(Mg1-x,Alx)2和Al2(Ca1-x,Mgx) (x=0, 0.25, 0.50, 0.75, 1)在不同形态结构(C14, C15和C36)下的相稳定性及电子结构。计算所得的晶格常数和实验值吻合很好,形成能和相关能的计算用来研究三元Laves相的合金化能力和稳定性,结果表明：C14-Ca(Mg0.25,Al0.75)2具有很好的合金化能力,而C15- CaAl2具有很好的结构稳定性。态密度和电荷密度的计算用来研究Mg-Al-Ca合金中三元Laves相稳定性的内在微观机制。     

Superconducting properties of the C15-type Laves phase ZrIr2 with an Ir-based kagome lattice

We report systematic studies on superconducting properties of the Laves phase superconductor ZrIr$_2$. It crystallizes in a C15-type (cubic MgCu$_2$-type, space group $Fd\overline{3}m$) structure in which the Ir atoms form a kagome lattice, with cell parameters $a=b=c=7.3596(1)$ Å. Resistivity and magnetic susceptibility measurements indicate that ZrIr$_2$ is a type-II superconductor with a transition temperature of 4.0 K. The estimated lower and upper critical fields are 12.8 mT and 4.78 T, respectively. Heat capacity measurements confirm the bulk superconductivity in ZrIr$_2$. ZrIr$_2$ is found to possibly host strong-coupled s-wave superconductivity with the normalized specific heat change $\Delta C_{\rm e}/\gamma T_{\rm c} \sim 1.86$ and the coupling strength $\Delta_0/k_{\rm B}T_{\rm c} \sim 1.92$. First-principles calculations suggest that ZrIr$_2$ has three-dimensional Fermi surfaces with simple topologies, and the states at Fermi level mainly originate from the Ir-5d and Zr-4d orbitals. Similar to SrIr$_2$ and ThIr$_2$, spin--orbit coupling has dramatic influences on the band structure in ZrIr$_2$.     

Site occupancy behaviours of ternary elements (Zr,Mo, Cr) in the Laves phase of C15 NbCo2: a first-principles study

Abstract

First-principles method based upon the density functional theory has been carried out to investigate the site occupancy behaviours of typical ternary elements Zr, Cr and Mo in the C15 NbCo₂ Laves phase. The calculated equilibrium lattice constant of the Nb₈Co₁₆ unit cell is in good agreement with the previous experimental results. The calculated results indicate that all the considered ternary elements show the preference to occupy the Co sites, and the site preference of these elements in the Laves phase is in the order of Cr, Mo and Zr. Mo and Cr atoms tend to participate in the formation of the Laves phase, generating Nb₇MoCo₁₆, Nb₈Co₁₅Mo and Nb₈Co₁₅Cr, respectively. Zr atoms are not prone to concentrate in the Laves phase. The partial density of states was also calculated to investigate how atoms interact, and the charge density difference is analysed to reflect the bonding characteristics.     

Influence of hydrogen on structural and magnetic properties of the hexagonal Laves phase HoMn2

The HoMn₂ compound crystallizes in the cubic C15 or hexagonal C14 Laves phases depending on preparation. The effect of hydrogen absorption on structural and magnetic properties of HoMn₂H_x hydrides for the C14 phase has been investigated by XRD and AC/DC magnetometry in the temperature ranges of 75-380 K and 4-390 K, respectively. In addition to general features revealed by RMn₂H_x compounds (R=rare earth or Yttrium), unusual behavior of these hydrides was found. In particular, a transformation from the hexagonal to the monoclinic structure was detected, the same as that observed for cubic HoMn₂H_x compounds. The structural transformations are correlated to the magnetic behavior. The presented results are compared mainly with the properties of the cubic HoMn₂H_x hydrides as well as with those of other RMn₂H_x hydrides. Tentative magnetic and structural phase diagrams are proposed.     

Glassy phase and thermodynamics for random field Ising model on spherical lattice in magnetic field

Phase diagram and thermodynamic parameters of the random field Ising model (RFIM) on spherical lattice are studied by using mean field theory. This lattice is placed in an external magnetic field (B). The random field (hi) is assumed to be Gaussian distributed with zero mean and a variance     

Spin glass and glass-like lattice behaviour in HoB66 at low temperatures

The low-temperature specific heat, magnetic susceptibility, and thermal expansion are studied experimentally in the fcc boride HoB₆₆ and compared to similar quantities in non-magnetic LuB₆₆. The anomalous behaviour observed in HoB₆₆ is explained within a glass-like picture together with magnetic subsystem ordering processes. The character of the observed anomalies suggests the existence of a spin-glass phase transition in HoB₆₆ below a characteristic temperature of T _s ?≈?0.98?K.     

Phase transition and critical behavior of spin–orbital coupled spinel ZnV_2O_4

We present the temperature-dependent susceptibility and specific heat measurement of spinel ZnV_2O_4.The structural transition with orbital ordering and the antiferromagnetic transition with spin ordering were observed at 50 K and 37 K,respectively.By analysis of the hysteresis behavior between the specific heat curves obtained in warming and cooling processes,the structural transition was confirmed to be the first-order transition,while the antiferromagnetic transition was found to be of the second-order type.At the structural transition,the latent heat and entropy change were calculated from the excess specific heat,and the derivative of pressure with respect to temperature was obtained using the Clausius-Clapayron equation.At the magnetic transition,the width of the critical fluctuation region was obtained to be about 0.5 K by comparing with Gaussian fluctuations.In the critical region,the critical behavior was analyzed by using renormalization-group theory.The critical amplitude ratio A~+/A~- = 1.46,which deviates from the 3D Heisenburg model;while the critical exponent α is-0.011,which is close to the 3D XY model.We proposed that these abnormal critical behaviors can be attributed to strong spin-orbital coupling accompanied with the antiferromagnetic transition.Moreover,in the low temperature range(2-5 K),the Fermi energy,the density of states near the Fermi surface,and the low limit of Debye temperature were estimated to be2.42 eV,2.48 eV~(-1),and 240 K,respectively.     

Electrical properties and spin fluctuations studies of Y(Co1−xNix)2 compounds

Synthesis by arc melting, the structural and the electric properties of Y(Co_1−xNi_x)₂ alloys were studied by X-ray diffraction (XRD) and four probe dc electrical measurements. XRD analysis (300 K) shows that all samples crystallize in a cubic MgCu₂-type structure. The lattice parameters linearly decrease with Ni content. Electrical resistivity for the Y(Co_1−xNi_x)₂ intermetallic series was measured in a temperature range of 15-1100 K. The parameters involved in the dependence of resistivity on temperature were determined. Residual, phonon and spin fluctuations resistivity were separated from electrical resistivity using both the Matthiesen formula and the Bloch-Gruneisen formula. The spin fluctuations resistivity of the Y(Co_1−xNi_x)₂ series are compared to the mean square amplitudes of spin fluctuations previously calculated by the Linear Muffin Tin Orbital-Tight Binding Approach method for these series in the literature. The contribution of spin fluctuations to total resistivity ρ_sf is proportional to T² at low temperatures. The proportionality parameter strongly reduces across the Y(Co_1−xNi_x)₂ series.     

Vortex matter phase transition in the C15-Laves-phase superconductor CeRu2: comparison with Bi-2212

We present a comparative study of peak-effect in DC magnetization in single-crystal samples of CeRu₂ and Bi-2212. While the PE in Bi-2212 can be associated with the second-order phase transition from Bragg-glass to vortex-glass, the PE in CeRu₂ is anomalous and can be understood as accompanying a first-order phase transition in the vortex matter.     

Magnetic Properties of an Antiferromagnetic Spin-1/2 XYZ Model in the Presence of Different Magnetic Fields: Finite-Size Effects of Inhomogeneity Property*

Magnetic and thermodynamic properties of the anisotropic XYZ spin-1/2 finite chain under both homogeneous and inhomogeneous magnetic fields are theoretically studied at low temperature. Using exact diagonalization method (ED), we study the magnetization, magnetic susceptibility, and specific heat of the model characterized in terms of the finite correlation length in the presence of three different magnetic fields including longitudinal, transverse, and transverse staggered magnetic fields. The magnetization, susceptibility, and the specific heat of the model are investigated under two conditions separately: (i) When the model is putted in the presence of homogeneous magnetic fields. (ii) When finite inhomogeneities are considered for all applied magnetic fields in the Hamiltonian. We show that for the finite-size XYZ chains at low temperature, the evident magnetization plateaus gradually convert to their counterpart quasi-plateaus when the transverse magnetic field increases. Moreover, the influence of the transverse and staggered transverse magnetic fields, and their corresponding inhomogeneities on the magnetization process, magnetic susceptibility, and specific heat are reported in detail. Our exact results illustrate that by altering the inhomogeneity parameters, magnetization plateaus gradually convert to their counterpart quasi-plateaus. The specific heat manifests Schottky-type maximum, double-peak, and triple-peak, as well as, transformation between them by varying considered inhomogeneity parameters in the Hamiltonian.     

Origin of excess low-energy vibrations in densified B2O3 glasses

Low-temperature experiments of Raman scattering and heat capacity have been performed in a B₂O₃ glass, pressure quenched from 1200 °C in order to obtain the density as largest as possible (ρ = 2373 kg/m³). When compared to those of compacted B₂O₃ glasses having smaller density, the Raman spectrum of this glass exhibits a strong decrease of the intensities of the Boson peak and the band at 808 cm^?1, both the features being determined by the decrease of the boroxol ring population. Moreover, the Boson peak exhibits a large shift to 68 cm^?1 (from 26 cm^?1 observed in normal vitreous B₂O₃). The high atomic packing of the glassy network also leads to a marked decrease of the excess heat capacity over the Debye T³-behaviour characterizing the crystal. The density g(ν) of low-frequency vibrational states has been assessed by using the low-frequency Raman intensity to determine the temperature dependence of the low-temperature heat capacity. The observations performed over a wide range of glass densities are compared to the predictions of theoretical models and computer simulations explaining the nature of the Boson peak. Consistency with the results of a simulation study concerning the vibrations of jammed particles leads to evaluate a nanometre length scale which suggests the existence of poorly packed domains formed from several connected boroxols. These soft regions are believed to be the main source of low-frequency optic-like vibrations giving rise to the Boson peak.     

High pressure electrical transport behavior in SrF_2 nanoplates

The charge transport behavior of strontium fluoride nanocrystals has been investigated by in situ impedance measurement up to 35 GPa.It was found that the parameters changed discontinuously at each phase transition.The charge carriers in SrF_2 nanocrystals include both F~-ions and electrons.In the Fm3 m phase,pressure makes the electronic transport easier,while makes it more difficult in the Pnma phase.The defects at grain boundaries dominate the electronic transport process.Pressure could make the charge-discharge processes in the Fm3 m phase much easier,but make it more difficult in the Pnma phase.     

Electron mass enhancement and magnetic phase separation near the Mott transition in double-layer ruthenates

We present a detailed investigation of the specific heat of Ca₃(Ru_1-xM_x)₂O₇ (M = Ti, Fe, Mn) single crystals. Depending on the dopant and doping level, three distinct regions are present: a quasitwo-dimensional metallic state with antiferromagnetic (AFM) order formed by ferromagnetic bilayers (AFM-b), a Mott insulating state with G-type AFM order (G-AFM), and a localized state with a mixed AFM-b and G-AFM phase. Our specific heat data provide deep insights into the Mott transitions induced by Ti and Mn doping. We observed not only an anomalous large mass enhancement, but also an additional term in the specific heat, i.e., C ∝ T², in the localized region. The C ∝ T² term is most likely due to long-wavelength excitations with both FM and AFM components. A decrease in the Debye temperature is observed in the G-type AFM region, indicating lattice softening associated with the Mott transition.     

Dynamic phase transitions in the kinetic mixed spin-1/2 and spin-5/2 Ising model under a time-dependent oscillating magnetic field

We calculate the dynamic phase transition (DPT) temperatures and present the dynamic phase diagrams in the kinetic mixed spin-1/2 and spin-5/2 Ising model under the presence of a time-dependent oscillating external magnetic field. We employ the Glauber transition rates to construct the set of mean-field dynamic equations. The time variation of the average magnetizations and the thermal behavior of the dynamic magnetizations are investigated, extensively. The nature (continuous or discontinuous) of the transitions is characterized by studying the thermal behaviors of the dynamic magnetizations. The DPT points are obtained and the phase diagrams are presented in two different planes. Phase diagrams contain four fundamental phases and three coexistence or mixed phases, which strongly depend on interaction parameters. The phase diagrams are discussed and a comparison is made with the results of the other mixed spin Ising systems.     

2-(2-喹啉偶氮)-5-二甲氨基苯酚固相萃取光度法测定铀的研究

合成了新试剂2-(2-喹啉偶氮)-5-二甲氨基苯酚(QADMAP),用红外光谱、核磁共振氢谱、质谱、可见吸收光谱和元素分析鉴定了其结构。研究了QADMAP与铀的显色反应,在pH 7.8的三乙醇胺-盐酸缓冲介质中,Triton X-100和氟离子存在下,QADMAP与铀(Ⅵ)和氟离子反应生成1∶1∶1紫色三元稳定络合物,络合物的λ_max＝590 nm,ε＝1.05×105 L·mol-1·cm-1,铀含量在0～20 μg·10 mL-1内符合比尔定律。环境水样中的铀用TBP萃淋树脂固相萃取柱分离和富集后用该方法测定,方法相对标准偏差在2.2%～3.6%之间,标准回收率在94%～105%。     

Magnetocaloric and barocaloric effects in a Gd5Si2Ge2 compound

The first-order phase transition in Gd5Si2Ge2 is sensitive to both magnetic field and pressure.It may indicate that the influences of the magnetic field and the pressure on the phase transition are virtually equivalent.Moreover,theoretical analyses reveal that the total entropy change is almost definite at a certain Curie temperature no matter whether the applied external field is a magnetic field or a pressure.The entropy change curve can be broadened dramatically under pressure,and the refrigerant capacity is improved from 284.7 J/kg to 447.0 J/kg.     

Orbital fluctuations and orbital order in FeCr2S4

Heat-capacity investigations on the ferrimagnetic spinel FeCr₂S₄ poly- and single crystals provide experimental evidence of orbital liquid and orbital glass states. The low-temperature transition in the polycrystals at 10 K arises from orbital order and is very sensitive to the sample stoichiometry. In the single crystals the orbital order is fully suppressed resulting in an orbital glass state with the heat capacity following a strict T² dependence towards zero temperature. At elevated temperatures, FeCr₂S₄ exhibits an unexpected large linear term of about 100 mJ mol⁻¹ K⁻² as the fingerprint of the orbital liquid.