| Abstract
| - Upon hydrogenation of the hexagonal Ce2Ni7, 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 the structurally related CeNi3D2.8 and those in nickel-based complex metal hydrides. Ce2Ni7D∼4 displays a transition involving two phases with different degrees of the orthorhombic lattice distortion.
- Hydrogenation of hexagonal Ce2Ni7 was investigated by synchrotron X-ray and neutron powder diffraction. In contrastto 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 ofdeuterium is bonded in tetrahedral NiD4 units similar to those in nickel-based complex metal hydrides. Thesefindings 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 phasetransforms via a two-phase region into another phase having a positive distortion (δ > 0). Both phases are nearlyisostructural and have the same space group symmetry and nearly the same composition. This situation is typicalfor a so-called anti-isostructural phase transition in which δ is considered to be an order parameter. Neither magneticnor structural transitions have been detected down to 1.5 K.
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