Crystal and\nLocal Structures of Pr<sub>2</sub>Co<sub>7</sub>D<i><sub>x</sub></i> during Its Deuterium Absorption\nProcess Determined by Neutron Diffraction

We investigated crystal and local structures of Pr<sub>2</sub>Co<sub>7</sub>, Pr<sub>2</sub>Co<sub>7</sub>D<sub>2.7</sub> and Pr<sub>2</sub>Co<sub>7</sub>D<sub>7.2</sub> by neutron diffraction.\nThe determined\nstructural model of Pr<sub>2</sub>Co<sub>7</sub>D<sub>2.7</sub> was\nthe Ce<sub>2</sub>Ni<sub>7</sub>-type structure by Rietveld refinement,\nwhere the deuterium atoms occupied both the CaCu<sub>5</sub>-type\n(I) and the MgZn<sub>2</sub>-type cells. The deuterium contents in\nthese cells were 0.10 and 0.80 D/M, respectively. The expansions of\nthe lattice parameters of Pr<sub>2</sub>Co<sub>7</sub>D<sub>2.7</sub> from the original intermetallics were Δ<i>a</i> =\n0.3% and Δ<i>c</i> = 7.3%. The crystal structure transformed\nin the order Ce<sub>2</sub>Ni<sub>7</sub>-type Pr<sub>2</sub>Co<sub>7</sub> → Ce<sub>2</sub>Ni<sub>7</sub>-type Pr<sub>2</sub>Co<sub>7</sub>D<sub>2.7</sub> → orthorhombic Pr<sub>2</sub>Co<sub>7</sub>D<sub>7.2</sub>. The expansions of the lattice parameters\nof Pr<sub>2</sub>Co<sub>7</sub>D<sub>7.2</sub> from the original intermetallics\nwere Δ<i>a</i> = 5.9%, Δ<i>b</i> =\n6.2%, and Δ<i>c</i> = 6.4%; nearly isotropic expansion\nwas observed. The deuterium contents in the MgZn<sub>2</sub>-type,\nCaCu<sub>5</sub>-type (I), and CaCu<sub>5</sub>-type (II) cells were\n1.11, 0.44, and 0.83 D/M, respectively. The refined structural parameters\nby a pair distribution function (PDF) analysis on Pr<sub>2</sub>Co<sub>7</sub> (0.18 < <i>r</i> < 5.0 nm) and Pr<sub>2</sub>Co<sub>7</sub>D<sub>2.7</sub> (0.13 < <i>r</i> <\n5.0 nm) obtained were consistent with those obtained by Rietveld refinement.\nThe Rietveld refinement results of the refined lattice parameters\nof Pr<sub>2</sub>Co<sub>7</sub>D<sub>7.2</sub> corresponded with the\nPDF analysis results in the region of 0.13 < <i>r</i> < 5.0 nm, whereas the refined atomic coordinates of the atoms\nD1, D2, D4, D6, and D9 obtained by the two methods differed. The refined\nstructural parameters of the short-range structure (0.13 < <i>r</i> < 1.0 nm) in Pr<sub>2</sub>Co<sub>7</sub>D<sub>7.2</sub> did not correspond to those of the long-range structure (0.13 < <i>r</i> < 5.0 nm). The local distortion within the domain of\napproximately 1.0 nm in size exists in Pr<sub>2</sub>Co<sub>7</sub>D<sub>7.2</sub>.