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

Our earlier study Yartys et al. (2015) showed that at high hydrogen pressures, hexagonal MgNi 2 undergoes a hydrogen assisted phase transition into the orthorhombic MoSi 2 -type structure. Here we report on a combined high resolution synchrotron and neutron diffraction investigation of the crystal structure of MgNi 2 D 3 , and ab initio calculation of its electronic structure that revealed the nature of the metal–hydrogen bonding. The diffraction data (293 and 1.8 K) are well described with a Cmca unit cell with H atoms filling the deformed octahedra Mg 4 Ni 2 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 MgNi 2 D 3 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 MgNi 2 H 3 show positive frequencies, indicating that the structure is mechanically stable. The calculated gross heat of formation for the Cmca phase of MgNi 2 H 3 is −37.3 kJ/mol-H 2 , which makes it more stable by 3 kJ/mol-H 2 than the prototype structures tested in Yartys et al. (2015). The stability of the Cmca crystal structure of MgNi 2 H 3 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. [ABSTRACT FROM AUTHOR]