Calculation of Thermodynamic, Electronic, and Optical Properties of Monoclinic Mg₂NiH₄

Ab initio total-energy density functional theory is used to investigate the low temperature (LT) monoclinic form of Mg₂NiH₄. The calculated minimum energy geometry of LT Mg₂NiH₄ is close to that determined from neutron diffraction data, and the NiH₄ complex is close to a regular tetrahedron. The enthalpies of the phase change to high temperature (HT) pseudo-cubic Mg₂NiH₄ and of hydrogen absorption by Mg₂Ni are calculated and compared with experimental values. LT Mg₂NiH₄ is found to be a semiconductor with an indirect band gap of 1.4 eV. The optical dielectric function of LT Mg₂NiH₄ differs somewhat from that of the HT phase. A calculated thin film transmittance spectrum is consistent with an experimental spectrum.