Structure and chemical bonding in MgNi2H3 from combined high resolution synchrotron and neutron diffraction studies and ab initio electronic structure calculations (original) (raw)
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MgNi 2 intermetallic was synthesized by powder metallurgy and crystallizes with a Laves-type C36 structure (space group P6 3 /mmc (No. 194); a = 4.826; c = 15.832 Å). At 300°C during interaction with hydrogen (deuterium) gas compressed to 2.8-7.4 GPa, a trihydride MgNi 2 H(D) 3.2 was synthesized. The trihydride remained metastable at ambient conditions allowing its structure, stability and magnetic properties to be studied. The formation of MgNi 2 H 3.2 is associated with a complete rebuilding of the initial hexagonal structure into the orthorhombic distorted MoSi 2-type sublattice (space group Fmmm (No. 69); a = 4.55; b = 4.69; c = 8.80 Å). Neutron diffraction of the MgNi 2 D 3.2 demonstrated that D atoms fill sites having octahedral Mg 4 Ni 2 (D1/4b) and planar Ni2 (D2/8f) coordination. Within the framework of the density functional theory, density of states (DOS) calculations showed the formation of a structure around À10 to À6 eV caused by the chemical bonds of hydrogen and its 1s states mainly via interaction with the 3d states of Ni. Analysis of the electronic structure revealed a charge transfer from Mg to Ni, and to the H atoms. The calculated enthalpy of formation of MgNi 2 H 3 is about À30 kJ/mol-H 2 , which is consistent with the stability of the hydride at normal conditions. The initial sample contained a small amount of a secondary MgNi 3 intermetallic, which has been formed during the equilibrium interaction of magnesium and nickel at 800°C. Thus this compound should be included in the phase diagram of the Mg-Ni system. MgNi 3 decomposes under high-temperature/ high-pressure hydrogenation conditions and forms nickel monohydride.
The substitutional doping of Mn in Mg2Ni phase and the electronic structure of Mg3MnNi2 phase have been investigated by first principles density functional theory calculations. The calculation of enthalpy of formation shows that among the four different lattice sites of Mg(6f), Mg(6i), Ni(3b) and Ni(3d) in Mg2Ni unit cell, the most preferable site of substitution of Mn in Mg2Ni lattice has been confirmed to be Mg(6i) lattice site. The constructed Mg9Mn3Mg(6i)Ni6 structure by replacing 3 Mg atoms at Mg(6i) lattice sites with 3 Mn atoms in the Mg2Ni unit cell is less stable. In contrast, the cubic Mg3MnNi2 phase that has the same composition as that of Mg9Mn3Mg(6i)Ni6 structure possesses good stability. Analysis of density of states (DOS) indicates that there is a strong hybridization between Mg s, Mg p and Ni d electrons, which is dominant in controlling the structural stability of pure and Mn-doped Mg2Ni phases. The Mnsubstitution in Mg2Ni unit cell weakens the interaction between Mg s, Mg p and Ni d electrons, especially for Mg9Mn3Mg(6i)Ni6 phase. The cubic Mg3MnNi2 phase possesses a strong hybridization between Mn and Mg, Ni atomic orbits under simultaneously retaining the strong bonding among Mg s, Mg p and Ni d electrons. Based on the calculated results, the stability of phases gradually decreases along the sequence pure Mg2Ni phase >Mg3MnNi2 phase > Mn-substitution doped Mg2Ni phase.
Synthesis and properties of the Mg2Ni0.5Co0.5H4.4 hydride
Journal of Alloys and Compounds, 2015
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Thermodynamics and crystal chemistry of the RE2MgNi9H12-13 (RE= La and Nd) hydrides
Ternary RE-Mg-Ni intermetallics are promising negative electrode materials for high-energy/high-power Nickel-Metal Hydride (Ni-MH) batteries. These compounds belong to a family of hybrid layered structures (AB 3 , A 2 B 7 and A 5 B 19 ; A = RE, Mg; B = Ni), composed of stacked Laves-type layers, RE 2-x Mg x Ni 4 , and Haucke-type RENi 5 layers. In the present study structural and hydrogen storage properties of a new compound, Nd 2 MgNi 9 (PuNi 3 type; a = 4.9783(1), c = 24.1865(6) Å), are reported and compared with those of the isostructural La 2 MgNi 9 intermetallic. RE 2 MgNi 9 (RE = La and Nd) were found to easily form hydrides containing 13 (La) or 12 (Nd) H/f.u. As for La 2 MgNi 9 H 13 , formation of the Nd 2 MgNi 9 H 12 hydride proceeds via isotropic expansion of the unit cell (a = 5.3234(2), c = 26.506(2) Å; ∆V/V = 25.3 %). In situ neutron diffraction studies of the saturated deuterides La 2 MgNi 9 D 13 and Nd 2 MgNi 9 D 12 , performed at SINQ, PSI, Switzerland, revealed: (a) nearly equal distribution of H atoms within the REMgNi 4 and RENi 5 layers; (b) preferred filling of the Mg-and Ni-surrounded sites within the REMgNi 4 layers; (c) local hydrogen ordering with the H-sublattice built from stacking of MgH 6 octahedra and NiH 4 tetrahedra, indicating directional metalhydrogen bonding. In spite of the similarity of the crystal structures and hydrogenation capacities, Nd 2 MgNi 9 H 12 shows a significantly lower thermodynamic stability (∆ ∆ ∆ ∆H des = 29 kJ/mol H 2) than La 2 MgNi 9 H 13 (∆ ∆ ∆ ∆H des = 36 kJ/mol H 2).
Results in Physics, 2012
The present paper reports a comprehensive study of structural, elastic, electronic and phonon properties of MgN and GdN compounds in its rocksalt phase by means of first principles density functional theory and use of pseudo-potentials within generalized-gradient approximation to the exchange correlation functional for all calculations except phonons in the case of GdN. The computed equilibrium lattice constant and mechanical properties for both compounds agree well with the available experimental and theoretical data. The electronic band structure calculation shows an ionic component along with the covalent like character and clear half-metallic nature in rocksalt GdN, while rocksalt MgN shows covalent like character. Spin-polarized calculation suggests that the MgN has a negligible magnetization while GdN shows significant magnetization at equilibrium lattice constant. The phonon dispersion curves, phonon density of states and allied properties are also calculated. Zone center phonon frequencies are in good agreement with experimental Raman spectra. The structures observed in Raman spectra can be attributed to phonons near the zone center due to the presence of defect or disorder and the combination of LO(L) and LA(L) phonons.
International Journal of Hydrogen Energy, 2018
Available online xxx Keywords: Hydrogen storage Intermetallic hydrides Kinetic Monte Carlo Ab initio calculations Thermodynamic properties Kinetic of absorption a b s t r a c t First-principle calculations and kinetic Monte-Carlo simulations were performed to study the hydrogen storage properties of the intermetallic hydrides MgNiH 3 , MgCoH 3 and thier mixture namely MgCo 0.5 Ni 0.5 H 3. Based on the heat of formation, desoprtion temperature, activation energies computed from DFT calculations and KMC simulations, we show that the MgNiH 3 involves a fast kinetic while it is thermodynamically unstable (À9.96 kJ/mol.H 2 ; 76.61 K) whereas MgCoH 3 has a high thermodynamic stability (À73.32 kJ/mol.H 2 ; 560.97 K) which prevents their application for mobile hydrogen storage. On the other hand, the electronic structures show that the Ni weakens the strong CoeH bonding of the mixture MgCo 0.5 Ni 0.5 H 3 , which enhances significantly its stability and its desorption temperature (À45.92 kJ/mol.H 2 and 351.33 K) without reducing its high volu-metric capacity 133.73 g.H 2 /l. Kinetic Monte-Carlo simulations show that MgCo 0.5 Ni 0.5 H 3 exhibits a fast charging time (only 4.6 min at 400 K and 10 bar). Thermodynamic properties including entropy S, Gibbs free energy G and thermal expansion coefficient are predicted within the quasi-harmonic approximation. It is verified that crystal structure of MgCo 0.5 Ni 0.5 H 3 is stable.
New structure results for hydrides and deuterides of the hydrogen storage material Mg2Ni
Journal of The Less Common Metals, 1980
X-ray and neutron diffraction experiments performed on Mg,Ni(H,D), (0 < x < 3.9) confirmed the existence of a structural phase transformation at about 235 "C. The high temperature phase (a = 6.49 8, space group Fm3m) has an antifluorite-type metal structure in which the deuterium atoms surround the nickel atoms octahedrally in a disordered manner (D-Ni = 1.47 8, D-Mg = 2.30 a). Refined atomic parameters of Mg,Ni as well as absorption and desorption isotherms for the deuteride and hydride phases are reported.
Hydrogen-stabilized Mg2RhH1.1 with filled Ti2Ni-type structure
Journal of Alloys and Compounds, 1992
Mg2RhHI.1 is the first example for a hydrogen stabilized binary metal compound with filled Ti2Ni-type structure. Its composition was refined from X-ray and neutron powder diffraction data on the deuteride and found to be Mg2RhDI.I. The deuterium atoms in the structure occupy octahedral holes formed by Mg atoms and tetrahedral holes formed by Mg and/or Rh atoms. Retrieval of hydrogen by desorption destabilizes the structure and leads to a hitherto unknown binary metal compound of composition Mg2Rh which crystallizes with the Ti2Pd-type structure, a branch of the MoSi2-type structure.