Ordering phenomena in intermetallic CeMX (M = Ni, Cu and X = Si, Ge, Sn) upon hydrogenation

CeMX ternary compounds (M = Ni, Cu and X = Si, Ge, Sn) crystallize in derivative structures of the hexagonal AlB2-type which can be either the hexagonal ZrBeSi- or LiGaGe- type structure, either the orthorhombic TiNiSi-type structure or the tetragonal LaPtSi-type structure depending on M and X. Under hydrogenation, these ternary compounds form stable hydrides. Their crystallographic structure has been investigated by X-ray diffraction and electron transmission microscopy. The structural type of CeNiSiH0.8 remains that of the corresponding initial compound (i.e. the tetragonal LaPtSi-type structure). The hydrogenation only induces an anisotropic variation of the unit cell parameters (a decreases and c increases) without significant increase of the unit cell volume (+0.1%). The two other compounds based on nickel, CeNiGe and CeNiSn, undergo a structural transition during the absorption of hydrogen. The corresponding hydrides, CeNiGeH1.6 and CeNiSnH1.8 crystallize in the hexagonal ZrBeSi-type structure. For the compounds based on copper, no structural transitions are observed. Instead electron diffraction experiments point out the presence of superstructures forCeCuSiH1.3 andCeCuGeH(1.0) hydrides. These superstructures reveal the presence of an order between hydrogen and vacancies in the partially occupied (1/3 2/3 z approximate to 0.436 in the ZrBeSi-type structure) site. The volume of the cell corresponding to these superstructures is directly related to the amount of hydrogen atoms in the structure. The c parameter has to be multiplied by 5 for CeCuSiH1.35 compared with the ternary compound while it has only to be multiplied by 3 for the CeCuGeH1.0. The crystallographic evolution resulting from the hydrogenation of CeCuSn is more complex since incommensurate modulation was observed by electron diffraction and confirmed by single crystal X-ray diffraction. A short range ordering, leading to a displacement of heavy atoms takes place in this compound. (c) 2006 Elsevier Ltd. All rights reserved.