1H NMR study of the α phase of Mg2NiHx system

Hydrogen behavior in the α phase of Mg₂NiH_x system was studied by ¹H NMR. ¹H NMR spectra and spin-lattice relaxation times, T₁ and T_1ρ, of Mg₂NiH_0.22 were measured in the temperature range between 100 and 480 K. The drastic change in the linewidth is observed between 170 and 340 K, and ¹H rigid lattice is observed below 170 K, from which it is deduced that the hydrogen atoms are randomly distributed in α-Mg₂NiH_x. The relaxation mechanism for t₁ is the paramagnetic one, while the T_1ρ value is determined partially by hydrogen diffusion. The hydrogen diffusion rate has been determined from the linewidth and the T_1ρ value. The paramagnetic relaxations observed in T₁ and T_1ρ have been discussed relating to the hydrogen diffusion.     

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.     

Étude par rmn de la localisation et des mouvements des protons dans les hydrures MgH2 et Mg2NiH4

Wide line and pulsed NMR studies are reported for MgH₂; and Mg₂NiH₄ hydrides at 79 and 30 MHz in the temperature range between 100 and 500 K. Line shape data confirm a rutile type structure for MgH₂, and lead to the evaluation that 97% of the hydrogen is present in this phase, the remainder being in solid solution MgH_x, (x<0.04).Mg₂NiH₄ shows a Gaussian line, whose peak-to-peak width decreases from 6.16 to 4.28 G in the range 320–365 K. From 365 to 480 K the spectrum shows a second, narrower, line (0.85 G), implying that approximately 7% of the protons have migrated from their initial position to energetically less stable sites. The thermal behaviour of the T₁ and T₂ relaxation times shows a dramatic variation in the 320–370 K temperature range in connection with the change of the proton localization. Relaxation mechanisms can be attributed mainly to conduction electron-nucleus interactions. With rising temperature, diffusion mechanisms are also involved. A diffusion activation energy of about 0.35 eV has been determined, with a diffusion coefficient 3.45 × 10^?8 < D < 4.6 × 10^?8 cm²/s.     

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.     

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     

Properties of Mg-based materials for hydrogen storage

In the presented paper, microstructures of as-cast MgNi_14.0 and MgNi_27.8 alloys are described. The alloys are composed of α(Mg) phase and Mg₂Ni intermetallic (space group P6₂22) of primary and/or eutectic nature. The α(Mg)+Mg₂Ni eutectic is characterized by an asymmetric zone of coupled growth. Primary Mg₂Ni phase shows a branched dendritic morphology and eutectic Mg₂Ni phase forms narrow and interconnected lamellae. The MgNi_27.8 alloy was electrochemically hydrided in 6 M KOH. XRD proved that the hydriding led to a transformation of the Mg₂Ni phase into Mg₂NiH_x (x=0–0.3) solid solution. It was shown that the hydriding rate increased with bath temperature and that the optimum voltage between cathode and anode was 1.5 V. Higher voltages resulted in H₂ evolution which reduced hydriding efficiency. An 1 h hydriding at 90 °C and 1.5 V was able to produce almost 20 μm layer of the Mg₂NiH_0.3 phase. Further hydriding probably led to a formation of Mg₂NiH₄ hydride which retarded the inward diffusion of H.     

Magnetic properties of MgCo2, MgNi2 and Mg2Ni

The magnetic properties of MgCo₂, MgNi₂ and Mg₂Ni were investigated in the range 4–400 K. The compound MgCo₂ is ferromagnetic below T_c = 321 but gives rise to a first order ferromagnetic-antiferromagnetic transition near 45 K. The compounds MgNi₂ and Mg₂Ni are both Pauli paramagnetic; they have an almost temperature-independent magnetic susceptibility. For all compounds the lattice constants were determined.     

Matrix isolation of Mg2 and Mgn molecules in neon,argon, and nitrogen hosts

The Mg₂ and Mg_n molecules have been formed and studied in neon, argon, and nitrogen matrices at approximately 4° and 12 K. Bands observed between 2000 and 3500 Å have been investigated as a function of gradual changes in trapping conditions and assigned to various stages of magnesium atom polymerization. The A¹Σ_u⁺ ← X¹Σ_g⁺ system of Mg₂ relative to the gas exhibits a pronounced red shift which increases with the polarizability of the matrix host. A band at 2653 Å in argon was observed which can be tentatively assigned to another Mg₂ system.     

X-ray photoelectron spectroscopic,electrical and magnetic studies on Mg2NiH4

X-ray photoelectron and X-ray induced Auger spectroscopic (XPS and XAES) investigations show that, in Mg₂NiH₄, both Mg and Ni are in the metallic state. Support for this observation has been obtained from thermopower and magnetic measurements.     

Structure and magnetic properties of Ca1−x−yMgxCu2+yO3 influenced by isoelectronic substitution x

Phases of the composition Ca_1−x−yMg_xCu_2+yO₃ have been prepared for the first time. The compounds are isostructural with the known end-members CaCu₂O₃ and MgCu₂O₃ showing a two-leg spin-ladder-like connection of copper and oxygen atoms within the Cu₂O₃-layer. Opposite the spin ladders this layer is folded, which results in a long-range antiferromagnetic ordering of these phases. The Néel temperature can be adapted by variation of x in Ca_1−x−yMg_xCu_2+yO₃ between 24 and 80 K. Several structural features, which influence the magnetic ordering, are discussed.     

Raman scattering in Mg2Sn and Mg2Si

The Raman spectra of single crystal Mg₂Sn and Mg₂Si have been studied at room temperature. In Mg₂Sn we observe a single relatively strong line produced by the degenerate Raman active F_2g modes, seen previously by neutron inelastic scattering. Two weaker peaks are seen, one occuring at the F_2g overtone; the assignment for the other is uncertain. In Mg₂Si we find a strong line having a shift comparable to the value calculated by previous workers.     

Structural, electronic, elastic and thermal properties of Mg2Si

First-principles calculations of the lattice parameter, electron density maps, density of states and elastic constants of Mg₂Si are reported. The lattice parameter is found to differ by less than 0.8% from the experimental data. Calculations of density of states and electron density maps are also performed to describe the orbital mixing and the nature of chemical bonding. Our results indicate that the bonding interactions in the Mg₂Si crystal are more covalent than ionic. The quasi-harmonic Debye model, by means of total energy versus volume calculations obtained with the plane-wave pseudopotential method, is applied to study the elastic, thermal and vibrational effects. The variations of bulk modulus, Grüneisen parameter, Debye temperature, heat capacity C_v, C_p and entropy with pressure P up to 7 GPa in the temperature interval 0-1300 K have been systemically investigated. Significant differences in properties are observed at high pressure and high temperature. When T<1300 K, the calculated entropy and heat capacity agree reasonably with available experimental data. Therefore, the present results indicate that the combination of first-principles and quasi-harmonic Debye model is an efficient approach to simulate the behavior of Mg₂Si.     

Search for superconductivity in hydrogen-implanted platinum

A recent theoretical calculation by Papaconstantopoulos predicted superconductivity in platinum hydride, possibly with a rather high critical temperature (about 10 K as in PdH). We have produced PtH_x (x ? 4.00) by ion-implantation, and find no evidence for superconductivity above 1.7 K. Residual resistivity measurements indicate that (in contrast to previous results on NiH_x and PdH_x) the Pt monohydride is not formed. We have considered the possibility that the dihydride might be formed.     

High-temperature electrical properties of magnesiowustite Mg1 − xFexO and spinel Fe3 − x − yMgxCryO4 ceramics

Phase relationships, thermal expansion and electrical properties of Mg_1 − xFe_xO (x = 0.1-0.45) cubic solid solutions and Fe_{3 − x − y}Mg_xCr_yO_4 ± δ (x = 0.7-0.95; y = 0 or 0.5) spinels were studied at 300-1770 K in the oxygen partial pressure range from 10 Pa to 21 kPa. Increasing iron content enlarges the spinel phase stability domain at reduced oxygen pressures and elevated temperatures. The total conductivity of the spinel ceramics is predominantly n-type electronic and is essentially p(O₂)-independent within the stability domain. The computer simulations using molecular dynamics technique confirmed that overall level of ion diffusion remains low even at high temperatures close to the melting point. Temperature dependencies of the total conductivity in air exhibit a complex behavior associated with changing the dominant defect-chemistry mechanism from prevailing formation of the interstitial cations above 1370-1470 K to the generation of cation vacancies at lower temperatures, and with kinetically frozen cation redistribution in spinel lattice below 700-800 K. The average thermal expansion coefficients of the spinel ceramics calculated from dilatometric data in air vary in the range (9.6-10.0) × 10^− 6 K^− 1 at 300-500 K and (13.2-16.1) × 10^− 6 K^− 1 at 1050-1370 K. Mg_1 − xFe_xO solid solutions undergo partial decomposition on heating under oxidizing and mildly reducing conditions, resulting in the segregation of spinel phase and conductivity decrease.     

Electrochemical luminescence of Mg1−xCaxIn2O4:Er electrodes

Improvement for electrochemical luminescence (ECL) property of MgIn₂O₄ is attempted by partial exchange of Mg²⁺ ion in MgIn₂O₄ to Ca²⁺ ion. Mg_1−xCa_xIn₂O₄ solid solution was obtained in the region 0<x<0.4. Efficiency for ECL per unit current for Mg_1−xCa_xIn₂O₄:Er³⁺ increased with the increase in the ratio of Ca²⁺ ion, and showed a peak at x=0.25 and then decreased steeply. ECL efficiency for other rare-earth ion-(RE:Sm, Eu, Ho) doped Mg_1−xCa_xIn₂O₄ also increased comparing with those for MgIn₂O₄:RE. This Ca²⁺ addition effect on the ECL efficiency seems to be caused by the improvement of the efficiency for the impact activation.     

Stability of the long-period stacking order in Mg3ln AND Mg3(In1−yCdy) alloys

The stability of the stacking ordered structures of close-packed layers in Mg-In alloys near Mg₃In and pseudo-binary alloys Mg₃(In_1?yCd_y) for 0 ? y ? 1 is studied from a standpoint based on the pseudopotential theory. An expression of the structure-dependent energy for an arbitrary type of layer stacking is given by the method described in the previous papers [1,2]. The numerical results explain well the observed trends in the composition and pressure dependences of stacking sequence; the orders of appearance of stacking variants are 3R → 12R → 2H with an increasing Mg-content for Mg-In alloys near Mg₃In, 12R → 18R → 2H with an increasing Cd-content for Mg₃(In_1?yCd_y) and 12R → 18R → 24R with an increasing pressure for Mg₃ln. Speaking in detail, however, the present calculation fails to reproduce a 12R-structure (β₁-phase) at 25% In in the Mg-In system, an 18R-structure of Mg₃(In_0.65Cd_0.35) and an 18R-structure of Mg₃In found under pressure of 20–55 kb, although the energy differences are small in all the cases. From the analysis of components of the structure-dependent energy, it is concluded that the favorable type of layer stacking is determined predominantly by a contribution from the band-structure energy term of a state with the disordered arrangement of constituent atoms and also that an energy required to the alternation of stacking of layers is apparently smaller than the ordering energy. If a specific layer sequence is characterized by a hexagonality, the alloys vary their sequences in such a way that the hexagonality increases from zero (3R) to unity (2H) as the electron-to-atom ratio of the alloys decreases from 2·3 to 2·0.The Ewald and Madelung constants are calculated for various types of layer structure. Both constants change linearly with hexagonality, without regard to period, symmetry and layer sequence.     

Phase separation in hydrides of Zr2Ni

Room temperature X-ray and Mössbauer measurements show three single phase fields for hydrogen concentrations up to Zr₂NiH_5.1. Narrow two phase regions exist around Zr₂NiH_4.1 and between Zr₂NiH_4.9 and Zr₂NiH_5.1. Hydrogen effusion curves at constant pressure indicate two different desorption stages with activation energies of 50 kJ (mol H₂)^?1 and 110 kJ(mol H₂)^?1 which are in general agreement with theoretical values.     

Effects of Gd and Lu co-doping on the long afterglow properties of yellowish-orange phosphor Y2O2S:Ti, Mg

A new long afterglow phosphor Y₂O₂S:Ti⁴⁺, Mg²⁺ co-doped with Gd³⁺ and Lu³⁺ was synthesized by solid-state reaction in inert gas ambient. Its properties were systematically analyzed by X-ray diffraction (XRD), luminescence spectra, afterglow decay curves and thermoluminescence (TL) spectra. It was found that the long afterglow performance of Y₂O₂S:Ti⁴⁺, Mg²⁺ such as brightness and persistent time was largely improved when co-doped with Gd³⁺ and Lu³⁺. By analyzing the TL curve the activation energy E were calculated to be 0.64 eV for 388 K peak and 0.98 eV for 508 K peak, and the trap intensity related to 388 K peak is much stronger than that related to 508 K peak. The mechanism of the long afterglow was also discussed in this paper.     

Cu(In,Ga)Se2 superstrate-type solar cells with Zn1−xMgxO buffer layers

Superstrate-type Cu(In,Ga)Se₂ (CIGS) thin film solar cells were fabricated using Zn_1−xMg_xO buffer layers. Due to the diffusion of Cd into CIGS during the growth of the CIGS layer, the conventional buffer material of CdS is not suitable. ZnO is a good candidate because of higher thermal tolerance but the conduction band offset (CBO) of ZnO/CIGS is not appropriate. In this study, the Zn_1−xMg_xO buffer layers were used to fulfill both the requirements. The superstrate-type solar cells with a soda-lime glass/In₂O₃:Sn/Zn_1−xMg_xO/CIGS/Au structure were fabricated with different band gap energies of the Zn_1−xMg_xO layer. The CIGS layers [Ga/(In + Ga)∼0.25] were deposited by co-evaporation method. The substrate temperature during the CIGS deposition of 450 °C did not cause the intermixing of the Zn_1−xMg_xO and CIGS layers. The conversion efficiency of the cell with Zn_1−xMg_xO was higher than that with ZnO due to the improvement of open-circuit voltage and shunt resistance. The results well corresponded to the behavior of the adjustment of CBO, demonstrating that the usefulness of the Zn_1−xMg_xO layer for the CBO control in the superstrate-type CIGS solar cells.     

Synthesis and magnetic properties of ferrites spinels MgxCu1−xFe2O4

Polycrystalline Mg_0.6Cu_0.4Fe₂O₄ ferrites have been prepared using solid-state reaction technique. Their structural and magnetic properties have been studied, using X-ray diffraction and magnetic measurements.Using mean field theory and high-temperature series expansions (HTSE), extrapolated with the padé approximants method, the magnetic properties of Mg_1−xCu_xFe₂O₄ have been studied. The nearest neighbor super-exchange interactions for intra-site and inter-site of the Mg_1−xCu_xFe₂O₄ ferrites spinels, in the range 0≤x≤1, have been computed using the probability approach, based on Mössbauer data. The Curie temperature T_c is calculated as a function of Mg concentration. The obtained theoretical results are in good agreement with experimental ones obtained by magnetic measurements.