Crystal structure and local structure of Mg_2-xPr _xNi₄ (x = 0.6 and 1.0) deuteride using in situ neutron total scattering

We studied crystal structure and local structure of Mg_2-xPr _xNi₄ (x = 0.6 and 1.0) and their deuterides using in situ neutron total scattering and first-principles calculations. The total scattering data were analyzed using Rietveld refinement and pair distribution function analysis (PDF). The crystal structure of Mg_2-xPr_xNi ₄ before deuterium absorption was C15b in space group F4̄3m. No difference between the crystal and local (PDF) structures was observed. The crystal structure of Mg_1.0Pr_1.0Ni₄D _∼4 was found to be orthorhombic in space group Pmn2₁, with three deuterium occupation sites: PrNi₃ and two types of bipyramidal Pr₂MgNi₂ that have a plane of symmetry composed of MgNi₂. There is no significant difference between the crystal structure and the local structure of Mg_1.0Pr _1.0Ni₄D_∼4. On the other hand, the average crystal structure of the Mg-rich Mg_1.4Pr_0.6Ni ₄D_∼3.6 was C15b with two deuterium occupation sites: PrNi₃ and MgPrNi₂ suggesting that the deuterium occupation shifts away from the Pr₂MgNi₂ bipyramid. First-principles relaxed structures also showed the shift of the hydrogen occupation site toward the Pr atom of the bipyramid, when induced by Mg substitution for the opposing Pr, resulting in hydrogen occupation in the MgPrNi₂ tetrahedral site. The PDF pattern of Mg_1.4Pr_0.6Ni₄D _∼3.6 cannot be refined below 7.2 Å in atomic distances using the C15b structure which was obtained from Rietveld refinement but can be done using an orthorhombic structure. It suggests that Mg_1.4Pr _0.6Ni₄D_∼3.6 was locally distorted to the orthorhombic.