Crystal and Local Structures of Pr2Co7D x during Its Deuterium Absorption Process Determined by Neutron Diffraction

We investigated crystal and local structures of Pr2Co7, Pr2Co7D2.7 and Pr2Co7D7.2 by neutron diffraction. The determined structural model of Pr2Co7D2.7 was the Ce2Ni7-type structure by Rietveld refinement, where the deuterium atoms occupied both the CaCu5-type (I) and the MgZn2-type cells. The deuterium contents in these cells were 0.10 and 0.80 D/M, respectively. The expansions of the lattice parameters of Pr2Co7D2.7 from the original intermetallics were Delta a = 0.3% and Delta c = 7.3%. The crystal structure transformed in the order Ce2Ni7-type Pr2Co7 -> Ce2Ni7-type Pr2Co7D2.7 -> orthorhombic Pr2Co7D7.2. The expansions of the lattice parameters of Pr2Co7D7.2 from the original intermetallics were Delta a = 5.9%, Delta b = 6.2%, and Delta c = 6.4%; nearly isotropic expansion was observed. The deuterium contents in the MgZn2-type, CaCu5-type (I), and CaCu5-type (II) cells were 1.11, 0.44, and 0.83 D/M, respectively. The refined structural parameters by a pair distribution function (PDF) analysis on Pr2Co7 (0.18 < r < 5.0 nm) and Pr2Co7D2.7 (0.13 < r < 5.0 nm) obtained were consistent with those obtained by Rietveld refinement. The Rietveld refinement results of the refined lattice parameters of Pr2Co7D7.2 corresponded with the PDF analysis results in the region of 0.13 < r < 5.0 nm, whereas the refined atomic coordinates of the atoms D1, D2, D4, D6, and D9 obtained by the two methods differed. The refined structural parameters of the short-range structure (0.13 < r < 1.0 nm) in Pr2Co7D7.2 did not correspond to those of the long-range structure (0.13 < r < 5.0 nm). The local distortion within the domain of approximately 1.0 nm in size exists in Pr2Co7D7.2.