Structural Stabilities and Electronic Properties of Cobalt Hydrides

Cobalt forms ferromagnetic hydrides CoH$_x$ at high pressures of hydrogen [1]. As the hydrogen pressure increases at temperatures 250-350${}^{\circ}$C, the concentration of hydrogen in the hcp phase monotonically increases, and reaches $x\sim 0.6$ at 7 GPa. At higher pressures, an fcc-based hydride with $x\sim 1.0$ is formed. At ambient pressure and 120 K, hydrogen atoms in the solution with $x\le 0.26$ are randomly distributed over octahedral interstitial sites [2]. In the solution with $x=0.34$ ($x\ge 0.38$), hydrogen atoms occupy every third (second) layer. The magnetic moments of the hcp-based hydrides are oriented to the $c$-axis, and are decreased with increasing hydrogen concentration at a rate of about 0.36 $\mu_{\mathrm{B}} $ per hydrogen atom. In this study, we optimize the structural parameters for several structures, and investigate the structural stabilities and related electronic properties by using first-principles calculations. The full-potential linearized augmented plane wave method with the generalized gradient approximation is adopted.\\[4pt] [1] V. E. Antonov, J. Alloys Compd. \textbf{330}-\textbf{332}, 110 (2002).\\[0pt] [2] V. K. Fedotov, V. E. Antonov, T. E. Antonova, E. L. Bokhenkov, B. Dorner, G. Grosse, and F. E. Wagner, J. Alloys Compd. \textbf{291}, 1 (1999).