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Crystal chemistry and thermodynamic properties of anisotropic Ce2Ni7H4.7 hydride |
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| Authors: | R.V. Denys V.A. Yartys Masashi Sato R.G. Delaplane |
| Affiliation: | a Institute for Energy Technology, P.O. Box 40, Kjeller NO-2027, Norway b Physico-Mechanical Institute of the National Academy of Science of Ukraine, 5 Naukova Street, Lviv 79601, Ukraine c Department of Applied Chemistry, School of Engineering, Tokai University, 1117 Kita-Kaname, Hiratsuka, Kanagawa 259-1292, Japan d The Studsvik Neutron Research Laboratory, Uppsala University, S-611 82 Nyköping, Sweden |
| Abstract: | 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). |
| Keywords: | Metal hydrides Crystal structure and symmetry Neutron diffraction Pressure-composition-temperature relationships |
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