Calculation of thermodynamic, electronic, and optical properties of monoclinic Mg[sub 2]NiH[sub 4].

Ab initio total-energy density functional theory is used to investigate the low temperature (LT) monoclinic form of Mg[sub 2]NiH[sub 4]. The calculated minimum energy geometry of LT Mg[sub 2]NiH[sub 4] is close to that determined from neutron diffraction data, and the NiH[sub 4] complex is close to a regular tetrahedron. The enthalpies of the phase change to high temperature (HT) pseudo-cubic Mg[sub 2]NiH[sub 4] and of hydrogen absorption by Mg[sub 2]Ni are calculated and compared with experimental values. LT Mg[sub 2]NiH[sub 4] is found to be a semiconductor with an indirect band gap of 1.4 eV. The optical dielectric function of LT Mg[sub 2]NiH[sub 4] differs somewhat from that of the HT phase. A calculated thin film transmittance spectrum is consistent with an experimental spectrum. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]