Structural, electronic properties and heat of formation of Mg₂FeH₆complex hydride: anab initiostudy

Abstract

In this work, the structural, energetic and electronic properties of Mg2FeH6 complex hydride have been studied using two different methods based on the density functional theory (DFT): the full potential linearized augmented plane wave plus local orbitals (FP-LAPW + lo) and the pseudo-potentials plane waves (PP-PW) methods. Based on the calculated results, the ground state of Mg2FeH6 is found to be paramagnetic (PM), which is in agreement with experimental results, and also, the optimized structural parameters including lattice constants and atomic positions are very close to the experimental and other theoretical works. For a detailed study, electronic structure calculations including those of the energy band, density of states (DOS) and charge density distribution have been performed. It is found that Mg2FeH6 hydride is a semiconductor with a direct energy gap at the X-point of about 1.87 and 2.25 eV for the FP-LAPW + lo and PW-PP methods, respectively, and the Mg atom acts as a donor to form a cation ion and the negatively charged FeH6 complex results in a mainly ionic bonding between the Mg and FeH6 complex. Other physical and energetic properties such as bulk modulus, cohesive energy and formation enthalpy of Mg2FeH6 are in good agreement with the theoretically and experimentally determined values.

Transition metal doping of Mg2FeH6– a DFT insight into synthesis and electronic structure

“…The electronic structure of Mg 2 FeH 6 is in agreement with earlier reports, 6,37 showing that this hydride is a semiconductor. The calculated width of the band gap is 1.85 eV, which is in excellent agreement with the results obtained from other calculations, 32,44 while to the best of our knowledge the experimental value of the band gap is not reported. Partial substitution of iron with other metals leads to the changes in electronic structure, as shown in Fig.…”

Insights into a Thermodynamically Optimal Synthesis of the Ternary Complex Hydride Mg2FeH6 for High-Density Thermal Energy Storage

“…Thus, the hydrogenation processes of the as-prepared 2Mg + Fe samples in two steps can be described in the following eqs and . The direct hydrogenation process in one step was usually considered for the synthesis reaction of Mg 2 FeH 6 , as expressed in eq . ,, …”

“…To date, Mg-based hydrides have been potential materials for different energy-related applications. − Among these materials, the ternary complex hydride Mg 2 FeH 6 has recently attracted research interest. Due to its appealing attributes such as high hydrogen storage capacity, 5.4 wt %, high reaction enthalpies (77–124 kJ/mol H 2 : the wide range indeed emphasizes the controversy in the thermodynamic characterization of this hydride material), and high volumetric density (150 kgH 2 /m 3 ), Mg 2 FeH 6 shows promise in solid-state hydrogen/heat storage applications − and recently into electrochemical energy storage by a combination with Li-ion batteries. , Moreover, based on a handful of theoretical studies, Mg 2 FeH 6 is viewed as semiconductors with a direct band gap ranging from 1.5 to 2.25 eV, , which opens prospects into solar cell technology.…”

“…Some temperature-related thermodynamic properties, such as free energy, internal energy, entropy, and so forth, which were based on a quasi-harmonic approximation of the calculated density of phonon states, were proposed. Zareii and Sarhaddi investigated the structural, energetic, and electronic properties of Mg 2 FeH 6 using two different methods (full potential linearized and pseudo-potentials plane waves) based on DFT. The results indicated that the ground state of Mg 2 FeH 6 is most likely paramagnetic, which showed good agreement with experiments.…”

New Insight on the Hydrogen Absorption Evolution of the Mg–Fe–H System under Equilibrium Conditions

“…The formation of Mg 2 FeH 6 from elemental Mg and Fe is the most thermodynamically favorable reaction. Ab initio calculations, based on density functional theory (DFT) about the formation mechanism of Mg 2 FeH 6 , have also reported reaction (1) as the optimum route from the thermodynamic point of view, regardless of any kinetic constraints [49]. Considering all the experimental evidence ( Figures 1-5 and Table 1) and based on the calculations of the standard free energy for the possible formation reactions of Mg 2 FeH 6 , it is possible to propose a reaction mechanism for Mg 2 FeH 6 formation under equilibrium conditions.…”