Atomic ordering in the hcp cobalt hydrides and deuterides

Cobalt hydrides and deuterides with the hcp metal lattice and H(D)-to-metal atomic ratios 0.18 less than or equal to x less than or equal to 0.5 were prepared under high pressures of hydrogen or deuterium, respectively, and s tudied by neutron diffraction in a metastable state at 120 K and ambient pr essure. A profile analysis of the spectra showed that in all samples the hy drogen and deuterium atoms occupy octahedral interstitial sites. In the sam ples with x less than or equal to 0.26, the hydrogen and deuterium atoms ar e randomly distributed over these sites. In the samples with x greater than or equal to 0.34, they form layered superstructures, occupying every third octahedral base layer at x = 0.34 and every second layer at x = 0.38 and 0 .5. (C) 1999 Elsevier Science S.A. All rights reserved.