The electronic structure of the MgY2H6+δ high-pressure hydride has been investigated by means of the full potential linearized augmented plane wave method, and the stability of the octahedral and tetrahedral sites for H atoms has been discussed. The calculation models used were MgY3H8-12, corresponding to H/M = 2-3. From the calculation of the density of states for MgY3H11, it was found that the states originating from H-s and Y-d mainly formed valence and conduction bands, respectively. In the valence band, bonding states originating from H-s of tetrahedral sites, Y-d, Y-p, and Mg-p can be observed in a wide energy range, especially between -3 ∼ -5 eV below the Fermi level. In addition, the contour plots of the valence electron charge density revealed that the bonding between Y and H atoms on the tetrahedral sites was predominantly covalent, while the H atom on the octahedral site showed a weak interaction with Y and H atoms on the tetrahedral sites. The cell volume optimization indicated that the calculated equilibrium volume linearly increased with increasing the number of vacancies on the octahedral sites, while it decreased in the case of introducing the vacancy on the tetrahedral site. From these observations, it can be concluded that the H atoms on the tetrahedral sites seem to be responsible for holding its crystal structure, while ones on the octahedral sites had a certain level of contribution to enhancing the bond strength. These observations support the experimental results that the crystal structure of MgY2H6-δ is stable even after the partial desorption of hydrogen, which presumably occupy the octahedral sites, at around 600 K.
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