Tetrahedral D Atom Coordination of Nickel and Evidence for Anti-isostructural Phase Transition in Orthorhombic Ce₂Ni₇D_∼₄

Hydrogenation of hexagonal Ce2Ni7 was investigated by synchrotron X-ray and neutron powder diffraction. In contrast to the recently investigated lanthanum analogue, which remains hexagonal (La2Ni7D6.5: space group P63/mmc), the cerium compound becomes orthorhombic (Ce2Ni7D∼4: space group Pmcn). As in the structurally related CeNi3D2.8, deuterium occupies CeNi2 slabs only, while the bulk of the CeNi5 slabs remains empty. A significant amount of deuterium is bonded in tetrahedral NiD4 units similar to those in nickel-based complex metal hydrides. These findings provide further evidence for directional bonding effects in hydrides that are traditionally considered as “interstitial”. Ce2Ni7D∼4 displays various orthorhombic lattice distortions, δ = (b/√3 − a)/a. Hydrogen pressures of ∼30 bar stabilize a phase having a negative distortion (δ < 0). Upon a decrease in the pressure, this phase transforms via a two-phase region into another phase having a positive distortion (δ > 0). Both phases are nearly isostructural and have the same space group symmetry and nearly the same composition. This situation is typical for a so-called anti-isostructural phase transition in which δ is considered to be an order parameter. Neither magnetic nor structural transitions have been detected down to 1.5 K.