Crystal chemistry and thermodynamic properties of anisotropic Ce2Ni7H4.7 hydride

A new intermetallic deuteride Ce2Ni7D4.7 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 CeNi2 slabs only. All D atoms are located inside these slabs and on the border between CeNi2 and CeNi5. Ordering of D atoms in the bulk of CeNi2 is accompanied by substantial deformation of these slabs thus lowering the hexagonal symmetry to orthorhombic [space group Pmcn (No. 62); a=4.9251(3) Å, b=8.4933(4) Å, c=29.773(1) Å]. Inside the CeNi2 layer the hydrogen sublattice is completely ordered; all D–D distances exceed 2.0 Å. Local coordination of Ni by D inside the CeNi2 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: ΔH=−22.4 kJ/molH, ΔS=−59.9 J/(K molH).