Structure and chemical bonding in MgNi2H3 from combined high resolution synchrotron and neutron diffraction studies and ab initio electronic structure calculations

Abstract

Our earlier study Yartys et al. (2015) showed that at high hydrogen pressures, hexagonal MgNi2 undergoes a hydrogen assisted phase transition into the orthorhombic MoSi2-type structure. Here we report on a combined high resolution synchrotron and neutron diffraction investigation of the crystal structure of MgNi2D3, and ab initio calculation of its electronic structure that revealed the nature of the metal–hydrogen bonding. The diffraction data (293 and 1.8K) are well described with a Cmca unit cell with H atoms filling the deformed octahedra Mg4Ni2 and the positions within the buckled nets –Ni–H–Ni–H– penetrating through the structure. DFT and phonon calculations showed that the Cmca structure of MgNi2D3 is the most stable, both from the electronic structure and the lattice dynamical arguments. The Bader charge analysis indicates an electronic transfer from Mg (−1.59e−) to Ni (+0.21e−), H1 (+0.55e−) and H2 (+0.31e−). The phonon dispersion curves of MgNi2H3 show positive frequencies, indicating that the structure is mechanically stable. The calculated gross heat of formation for the Cmca phase of MgNi2H3 is −37.3kJ/mol-H2, which makes it more stable by 3kJ/mol-H2 than the prototype structures tested in Yartys et al. (2015). The stability of the Cmca crystal structure of MgNi2H3 is enhanced by the formation of the directional Ni–H covalent bonds supplemented by the electron transfer from Mg to both Ni and H. The heat capacity as a function of temperature is obtained by phonon calculation in the quasi-harmonic approximation.