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Structure and chemical bonding in MgNi2H3 from combined high resolution synchrotron and neutron diffraction studies and ab initio electronic structure calculations.
- Source :
-
Acta Materialia . Oct2015, Vol. 98, p416-422. 7p. - Publication Year :
- 2015
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Abstract
- 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]
Details
- Language :
- English
- ISSN :
- 13596454
- Volume :
- 98
- Database :
- Academic Search Index
- Journal :
- Acta Materialia
- Publication Type :
- Academic Journal
- Accession number :
- 109045580
- Full Text :
- https://doi.org/10.1016/j.actamat.2015.07.053