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Fedotov et al. performed the neutron diffraction investigation of cobalt hydrides CoHx to determine the atomic ordering [1]. The hydrides show ferromagnetism at room temperature. As the pressure of hydrogen increases, the solubility of hydrogen monotonically increases. At around temperatures of 250-350ºC, the concentration of hydrogen in the hcp phase becomes x~0.6 at 7 GPa. At higher pressures, an fcc-based hydride with x~1.0 is formed. At ambient pressure and 120 K, hydrogen atoms in the solution with x≤0.26 are randomly distributed over octahedral interstitial sites. In the solution with x=0.34 (x≥0.38), hydrogen atoms occupy every third (second) layer. In the ordered structures, the Co layers separated by hydrogen move apart while those with no hydrogen in between get closer with each other. 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 µB per hydrogen atom. In this study, we optimize the structural parameters, namely the lattice constants and internal coordinates for several structures, and investigate the structural stability and related electronic properties by using first-principles calculations. The full-potential linearized augmented plane wave method with the generalized gradient approximation is adopted. [1] V. K. Fedotov, V. E. Antonov, T. E. Antonova, E. L. Bokhenkov, B. Dorner, G. Grosse, and F. E. Wagner: J. Alloys Compd. 291 (1999) 1. |
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