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

We investigated crystal and local structures of Pr₂Co₇, Pr₂Co₇D_2.7 and Pr₂Co₇D_7.2 by neutron diffraction. The determined structural model of Pr₂Co₇D_2.7 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 Pr₂Co₇D_2.7 from the original intermetallics were Δa = 0.3% and Δc = 7.3%. The crystal structure transformed in the order Ce₂Ni₇-type Pr₂Co₇ → Ce₂Ni₇-type Pr₂Co₇D_2.7 → orthorhombic Pr₂Co₇D_7.2. The expansions of the lattice parameters of Pr₂Co₇D_7.2 from the original intermetallics were Δa = 5.9%, Δb = 6.2%, and Δc = 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 Pr₂Co₇D_2.7 (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 Pr₂Co₇D_7.2 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 Pr₂Co₇D_7.2 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 Pr₂Co₇D_7.2.