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

Abstract

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 approximately 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 approximately 4 displays various orthorhombic lattice distortions, delta = (b/ radical3 - a)/a. Hydrogen pressures of approximately 30 bar stabilize a phase having a negative distortion (delta < 0). Upon a decrease in the pressure, this phase transforms via a two-phase region into another phase having a positive distortion (delta > 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 delta is considered to be an order parameter. Neither magnetic nor structural transitions have been detected down to 1.5 K.

Mg substitution effect on the hydrogenation behaviour, thermodynamic and structural properties of the La2Ni7–H(D)2 system

“…The structures of the hydrides of the hybrid intermetallic compounds have been studied in detail for representatives belonging to different structure types, including PuNi 3 ( [3] and references therein), CeNi 3 [1], and Ce 2 Ni 7 [4][5][6]. The formation of the hydrides is accompanied by either isotropic or anisotropic expansion of the unit cells.…”

In-situ powder neutron diffraction study on the formation process of LaMg 2 NiH 7

“…Thus, the NiH1.9 and NiH3.3 units could be the basis for the formation of the complex anion [NiH4] 4-in LaMg2NiH7. Such units have also been reported in hydrides of rare earth and transition metals (e.g., Ce2Ni7H4.7), MgNi2H3 and Pd-based complex hydrides (e.g., Li2PdH2) [13][14][15][16][17][18][19]. Since the NiH1.9 and NiH3.3 units were reminiscent of [NiH4] 4-and similar to their related hydrides, they were suggested to be precursors for formation of the complex anion…”

Deuterium site occupancies in Ce2Ni7D∼4 and comparison with CeNi3D2.8

“…In a previous paper we have reported structure data for the cerium-nickel deuteride Ce 2 Ni 7 D ∼4 [2]. Deuterium was found to occupy interstices in the CeNi 2 slabs only, while those in the bulk of the CeNi 5 slabs remain empty.…”

“…Both compounds react easily with hydrogen and form so-called "interstitial" hydrides. Those which have been structurally fully characterized are deuterides of composition Ce 2 Ni 7 D 4.1 [2] and CeNi 3 D 2.8 [6]. A striking aspect of the corresponding hydrides is their hydrogen equilibrium pressures.…”

“…Furthermore, in contrast to the lanthanum system La 2 Ni 7 -H which appears to remain hexagonal (La 2 Ni 7 D 6.5 : space group P6 3 /mmc [8]), the cerium system undergoes various degrees of orthorhombic lattice distortions, δ = (b/ √ 3 − a)/a. Hydrogen pressures of ∼30 bar, for example, stabilize a phase having a negative distortion (δ < 0), while decreasing the pressure leads to another phase having a positive distortion (δ > 0) [2]. Both phases are nearly isostructural, and have the same space group symmetry and nearly the same composition.…”