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

We investigated crystal and local structures of PrCo, PrCoD and PrCoD by neutron diffraction. The determined structural model of PrCoD was the CeNi-type structure by Rietveld refinement, where the deuterium atoms occupied both the CaCu-type (I) and the MgZn-type cells. The deuterium contents in these cells were 0.10 and 0.80 D/M, respectively. The expansions of the lattice parameters of PrCoD from the original intermetallics were Δ = 0.3% and Δ = 7.3%. The crystal structure transformed in the order CeNi-type PrCo → CeNi-type PrCoD → orthorhombic PrCoD. The expansions of the lattice parameters of PrCoD from the original intermetallics were Δ = 5.9%, Δ = 6.2%, and Δ = 6.4%; nearly isotropic expansion was observed. The deuterium contents in the MgZn-type, CaCu-type (I), and CaCu-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 PrCo (0.18 < < 5.0 nm) and PrCoD (0.13 < < 5.0 nm) obtained were consistent with those obtained by Rietveld refinement. The Rietveld refinement results of the refined lattice parameters of PrCoD corresponded with the PDF analysis results in the region of 0.13 < < 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 < < 1.0 nm) in PrCoD did not correspond to those of the long-range structure (0.13 < < 5.0 nm). The local distortion within the domain of approximately 1.0 nm in size exists in PrCoD.