Crystal chemistry and thermodynamic properties of anisotropic Ce{sub 2}Ni{sub 7}H{sub 4.7} hydride

• Published in: Journal of solid state chemistry Vol. 180; no. 9
• Main Authors: Denys, R.V., Physico-Mechanical Institute of the National Academy of Science of Ukraine, 5 Naukova Street, Lviv 79601, Yartys, V.A., Sato, Masashi, Riabov, A.B., Delaplane, R.G., Studsvik Neutron Research Laboratory, Uppsala University, S-611 82 Nykoeping
• Format: Journal Article
• Language: English
• Published: United States 15.09.2007
• Subjects: ANISOTROPY; CERIUM HYDRIDES; CRYSTALLOGRAPHY; DEUTERIDES; HYDROGEN; HYDROGENATION; INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; INTERMETALLIC COMPOUNDS; LAYERS; NEUTRON DIFFRACTION; NICKEL HYDRIDES; ORTHORHOMBIC LATTICES; SPACE GROUPS; SYMMETRY; TEMPERATURE MEASUREMENT; THERMODYNAMIC PROPERTIES
• ISSN: 0022-4596; 1095-726X
• DOI: 10.1016/j.jssc.2007.07.002

A new intermetallic deuteride Ce{sub 2}Ni{sub 7}D{sub 4.7} with an anomalous volume expansion has been studied. Its structure was solved on the basis of in situ neutron diffraction data. Expansion proceeds along the c-axis and within the CeNi{sub 2} slabs only. All D atoms are located inside these slabs and on the border between CeNi{sub 2} and CeNi{sub 5}. Ordering of D atoms in the bulk of CeNi{sub 2} is accompanied by substantial deformation of these slabs thus lowering the hexagonal symmetry to orthorhombic [space group Pmcn (No. 62); a=4.9251(3) A, b=8.4933(4) A, c=29.773(1) A]. Inside the CeNi{sub 2} layer the hydrogen sublattice is completely ordered; all D-D distances exceed 2.0 A. Local coordination of Ni by D inside the CeNi{sub 2} blocks is of 'open', saddle-like type. Hydrogen ordering is mainly determined by Ce-H and H-H interactions. The pressure-composition-temperature measurements yielded the following thermodynamic parameters of the formation of the hydride: {delta}H=-22.4 kJ/mol{sub H}, {delta}S=-59.9 J/(K mol{sub H}). - Graphical abstract: An extremely large volume expansion takes place during the hydrogenation of the hexagonal Ce{sub 2}Ni{sub 7} intermetallic compound (left). The expansion proceeds exclusively along one crystallographic direction ([001]) and dramatically modifies the metal sublattice. Hydrogen atoms in Ce{sub 2}Ni{sub 7}D{sub 4.7} deuteride form an ordered sublattice and preferably fill sites centred by Ni Ce{sub 6} octahedra (right)