First-principles investigation of Mg2Ni phase and high/low temperature Mg2NiH4 complex hydrides

Using a density functional approach calculation, the structural, energetic and electronic properties of Mg₂Ni phase as well as its high/low temperature (HT/LT)-Mg₂NiH₄ complex hydrides are systematically investigated. The optimized structural parameters including lattice constants and atomic positions are very close to the experimental data determined from X-ray and neutron powder diffraction. A detailed study of the electronic structures including the energy band, density of states (DOS) and charge density distribution reveals the orbital hybridization and characteristics of bonding orbits within Mg₂Ni and its hydrides. Based on the calculated results of the reaction heat of hydrogenation, enthalpy of formation and energy cost to remove H atoms, it is found that the formation ability of LT-Mg₂NiH₄ is higher than that of the HT phase during the hydrogenation of Mg₂Ni alloy; moreover, LT-Mg₂NiH₄ has a relatively higher structural stability than HT phase, which is also well explained through the DOS and the charge distributions of HT/LT-Mg₂NiH₄ phases.     

Electronic structural, elastic properties and thermodynamics of Mg17Al12, Mg2Si and Al2Y phases from first-principles calculations

Electronic structures, elastic properties and thermal stabilities of Mg₁₇Al₁₂, Mg₂Si and Al₂Y have been determined from first-principle calculations. The calculated heats of formation and cohesive energies show that Al₂Y has the strongest alloying ability and structural stability. The brittle behavior and structural stability mechanism is also explained through the electronic structures of these intermetallic compounds. The elastic constants are calculated, the bulk moduli, shear moduli, Young's moduli and Poisson ratio value are derived, the brittleness and plasticity of these phases are discussed. Gibbs free energy, Debye temperature and heat capacity are calculated and discussed.     

Magnetic properties of a Tb3Ni single crystal

Magnetization measurements, performed on a single crystal of the orthorhombic compound Tb₃Ni, are presented. In this compound, nickel is not magnetic. Below the ordering temperature (62K) the magnetization vs. applied field variations along the three symmetry axes exhibit one or two transitions. These transitions correspond to metamagnetic processes associated with a complex antiferromagnetic structure which is confirmed from neutron diffraction experiments. The analysis of the bulk magnetization results leads to moment directions of terbium atoms which differ from one site to the other. These directions are due to the effect of the strong crystalline electric field which has a very low symmetry. The huge resulting magnetocrystalline anisotropy is also responsible for the large hysteresis loops associated with the metamagnetic transitions at low temperature.     

Magnetic properties of NdAu2Ge2

Physical properties of NdAu₂Ge₂, crystallising with the tetragonal ThCr₂Si₂-type crystal structure, were investigated by means of magnetic, calorimetric and electrical transport measurements as well as by neutron diffraction. The compound exhibits antiferromagnetic ordering below T_N=4.5 K with a collinear magnetic structure of the AFI-type. The neodymium magnetic moments are parallel to the c-axis and amount to 1.04(4) μ_B at 1.5 K. The observed magnetic behaviour is strongly influenced by crystalline electric field effect.     

Magnetic properties of NdCo2Ge2, ErCo2Ge2 and PrFe2Ge2 compounds

Magnetometric and neutron diffraction studies of polycrystalline NdCo₂GE₂, ErCo₂Ge₂ and PrFe₂Ge₂ compounds were carried out in the temperature range between 4.2 and 300 K. All samples are antiferromagnetic with Néel temperature 26.5, ～ 4.2 and 13 K, respectively. The RECo₂Ge₂ compounds have collinear antiferromagnetic order of +?+? type. For PrFe₂Ge₂ a sinusoidal magnetic structure is observed. Magnetic moment is localized on RE atoms only and is equal to that of RE³⁺ free ion value. In ErCo₂Ge₂ the magnetic moment of Er atoms is perpendicular to the c-axis, whereas for remaining compounds it is parallel to the c-axis.     

Magnetic, thermal and transport properties of NdMn2

Susceptibility, magnetisation, specific-heat, thermal expansion and electrical resistivity measurements have been performed on NdMn₂ between 1.5 K and room temperature. Discontinuities are observed in the susceptibility, magnetisation and resistivity and large anomalies in the specific heat and thermal expansion at T_N(=104 K). Additional anomalies are found at 7 K and around 20 K in the specific heat and thermal expansion. The results are shown to be compatible with the picture for this compound proposed by Ballou et al.     

Piezo-optic behavior of forsterite,Mg2SiO4

The piezo-optic behavior of forsterite, crystallizing in the orthorhombic system, has been investigated up to a maximum pressure of 7 kbars. It is found that the variation of the refractive indices with pressure, dn/dP, are 0·03₅×10⁻³/kbar, 0·04₆×10⁻³/kbar, and 0·06₃×10⁻³/kbar for the n_α, n_β, and n_γ respectively. These values are the lowest on record. The corresponding values for the variation of the refractive indices with volume strain are 0·04₄, 0·05₉ and 0·08₀ respectively. These results are interpreted in terms of the bonding and coordination number of the oxygen ions.     

Magnetic study of Mg0.95Mn0.05Fe2O4 ferrite nanoparticles

The magnetic properties of Mg_0.95Mn_0.05Fe₂O₄ ferrite samples with an average particle size of ∼6.0±0.6 nm have been studied using X-ray diffraction, Mössbauer spectroscopy, dc magnetization and frequency dependent real χ^′(T) and imaginary χ^″(T) parts of ac susceptibility measurements. A magnetic transition to an ordered state is observed at about 195 K from Mössbauer measurements. The zero-field-cooled (ZFC) and field-cooled (FC) magnetization have been recorded at low field and show the typical behavior of a small particle system. The ZFC curve displays a broad maximum at , a temperature which depends upon the distribution of particle volumes in the sample. The FC curve was nearly flat below , as compared with monotonically increasing characteristics of non-interacting superparamagnetic systems indicating the existence of strong interactions among the nanoparticles. A frequency-dependent peak observed in χ^′(T) is well described by Vogel-Fulcher law, yielding a relaxation time and an interaction parameter . Such values show the strong interactions and rule out the possibility of spin-glass (SG) features among the nanoparticle system. On the other hand fitting with the Néel-Brown model and the power law yields an unphysical large value of τ₀ (∼6×10⁻⁶⁹ and 1.2×10⁻²² s respectively).     

Magnetic properties of n-type CuInS2

The magnetic susceptibility, χ, of a polycrystalline n-type CuInS₂ sample has been measured in the temperature range 4.2–300 K. A shallow donor level at 0.017 eV has been identified and an antiferromagnetic exchange coupling between donors is observed. These results are inferred from the excellent fit of the data to the equation $\text{χ(10}{}^{\mathrm{?6}}\text{cm}{}^3\text{mole}{}^{\mathrm{?1}}\text{) = -83.7 + 185}\text{[1?exp (}\text{?100}\text{T}\text{)]}\text{(T + 3)}$ in the whole temperature range.     

Magnetic properties of large-area one-dimensional WO2 and MoO2 nanorods

Large-area one-dimensional (1D) monoclinic WO₂ and MoO₂ nanorods in the space group P2₁/c were synthesized by reactive thermal evaporation. The as-synthesized 1D WO₂ and MoO₂ nanorods become soft magnetic materials at 10 K, implying that structural or magnetic transitions occur. There are large differences in saturation magnetization, the coercive field, and remanence between the 1D WO₂ and MoO₂ nanorods, although both 1D nanorods have a similar shape.     

Magnetic properties of antiferromagnetic YMn2 and magnetic structure of GdMn2

Electronic structure of d-electrons in antiferromagnetic YMn₂ is calculated in the tight-binding approximation. By making use of the calculated density-of-state curve, various magnetic properties of this compound are discussed and a good agreement between the calculated and observed results is obtained. A new model of the magnetic structure of GdMn₂ is proposed.     

Magnetic properties of Mg1-pMnpTe2 near p = 0.5

Precise magnetization measurements and neutron diffraction measurements are carried out for Mg_1-pMn_pTe₂ near the critical concentration p = 0.5 for the antiferromagnetic ordering. A magnetization property which is similar to micto-magnetism is observed for p = 0.35 and p = 0.45 samples. The susceptibility shows an anomaly at T_f and field-cooling effect is notable below T_f. Neutron diffraction experiments show that most of the manganese moments are ordered antiferromagnetically for p = 0.53 while small fractions of Mn²⁺ are ordered for p = 0.45 and p = 0.35.     

Structural transformation in Mg2NiH4

X-Ray diffraction measurements show that on heating Mg₂NiH₄ in a 1 atm pressure H₂ atmosphere, above ～250°C it transforms into a cubic structure, metal atoms in CaF₂ arrangement, a = 6.525 Å. It is concluded that the H atoms are in tetrahedral clusters, and that the structure is only weakly ionic. This conclusion is also supported by NMR measurements. The 20°C structure of Mg₂NiH₄ is shown to be describable primarily as a slight monoclinic distortion of the cubic unit cell; a = 6.594 Å, b = 6.412 Å, c = 6.490 Å and β = 93.1°. However, weak small angle lines show that a longer range order exists and that the true unit cell, which we have not determined, must be very large. To what extent the cubic phase should be considered a high temperature and/or low concentration (Mg₂NiH_4??) phase is not resolved.     

Magnetic properties of PrCo2 and its ternary hydride PrCo2H4

Magnetization and susceptibility data on PrCo₂ and PrCo₂H₄ are presented. The ac susceptibility of PrCo₂ measured in zero dc field displays a sharp and high peak at T_c = (39.9 ± 0.2) K. The magnetization versus temperature curves show ferromagnetic behaviour for B >1 T, but display a maximum at lower values of the applied field. These results, together with the behaviour of the hysteresis loops at different temperatures below T_c, indicate that PrCo₂ orders ferromagnetically, the magnetic hardness increasing strongly for T → 0. The saturation moment at 4.2 K equals 3.9 μ_B per formula unit, as found from the magnetization curve measured in a pulsed-field magnet up to B = 30 T.Similar experiments on PrCo₂H₄ provide evidence that the introduction of hydrogen in PrCo₂ not only destroys the long-range atomic order, but also considerably reduces the ferromagnetic interactions. Such an effect of the hydrogen is commonly observed in cobalt intermetallics. Part of the PrCo₂H₄ is found to have decomposed into PrH₂ and free Co. The clusters of free Co atoms give rise to a maximum in the zero-field ac susceptibility versus temperature curves, similar as observed in spin glasses or magnetic glasses. By increasing the ac frequency, the maximum shifts to higher temperatures. The behavior can be explained in terms of the Néel model for superparamagnetic particles with randomly oriented local anisotropy axes.     

Magnetic properties of MnCr2O4 nanoparticle

The exchange interactions and the magnetic exchange energies are calculated by using the mean field theory and the probability law of Zn_1−xMn_xCr₂O₄ nanoparticles. The high-temperature series expansions have been applied in the spinels Zn_1−xMn_xCr₂O₄ systems, combined with the Padé approximants method, to determine the magnetic phase diagram, i.e. T_C versus dilution x. The critical exponent associated with the magnetic susceptibility (γ) is deduced. The obtained value of γ is insensitive to the dilution ratio x and may be compared with other theoretical results based on the 3D Heisenberg model.     

第一性原理计算Mg2Ni氢化物的电子结构及其稳定性分析

对Mg₂Ni及其氢化物的能量和电子结构进行了第一性原理计算,并对Mg₂Ni低温氢化物和高温氢化物的稳定性进行了分析.结果发现:在Mg₂Ni中,Mg与Ni存在较强的相互作用.H原子的加入使得Mg的价电子向Ni的3d轨道转移,并在Ni-3d轨道和Ni-4p轨道间形成了带隙.Ni-4s和H-1s电子之间的成键作用,使得Mg和Ni间相互作用减弱了;从LT-Mg₂NiH₄中去掉一个H原子比HT-Mg_{关键词： 第一性原理 2}Ni')" href="#">Mg₂Ni 2NiH₄')" href="#">Mg₂NiH₄ 结构稳定性     

第一性原理研究Mg2 Si同质异相体的结构、电子结构和弹性性质

在第一性原理框架下,采用赝势平面波方法研究了三种Mg₂Si同质异相体的晶胞结构、电子结构和弹性性质随压强的变化关系.研究发现,反萤石结构Mg₂Si、反氯铅矿结构Mg₂Si和Ni₂In型Mg₂Si分别在压强为0-7 GPa,7.5-20.2 GPa和21.9-40 GPa范围内能够保持结构稳定.计算获得了不同压强下Mg₂Si的弹性常数、体模量、剪切模量、杨氏模量、泊松比和各向异 关键词： 态密度 电子结构 弹性常数 第一性原理     

Magnetic ordering and low-temperature thermodynamic properties of ErFe2Ge2

Polycrystalline sample of ErFe₂Ge₂ was investigated by means of magnetic susceptibility, heat capacity and electrical resistivity measurements, as well as by powder neutron diffraction. All these experiments yielded an evidence of magnetic ordering setting at about 3 K. The low-temperature neutron data revealed the formation of a sine-modulated commensurate antiferromagnetic structure characterized by the propagation vector k=(0, 0, ). The erbium magnetic moment is aligned parallel to the crystallographic a-axis. At T=1.55 K it is equal to 7.06(5) μ_B.     

Magnetic and transport properties of Mn-doped Bi2Se3 and Sb2Se3

We report on the single crystal growth and thermoelectric and magnetic properties of Mn-doped Bi₂Se₃ and Sb₂Se₃ single crystals prepared by the temperature gradient solidification method. The composition and crystal structure were determined using electron probe microanalysis and θ–2θ powder X-ray diffraction studies, respectively. The lattice constants of several percent Mn-doped Bi₂Se₃ and Sb₂Se₃ were slightly smaller than those of the undoped sample due to the smaller Mn atomic radius (1.40 Å) than those of Bi (1.60 Å) and Sb (1.45 Å). Mn-doped Bi₂Se₃ and Sb₂Se₃ showed spin-glass and paramagnetic properties, respectively.