EXAFS study of the Jahn-Teller distortion in layered nickel oxyhydroxide

Electronic Structure of the (Undoped and Fe-Doped) NiOOH O2 Evolution Electrocatalyst

“…Examining the coordination environment of the Ni 3+ species, in all cases, one finds that two Ni−O bonds in the trans position are longer than the other four by 0.1−0.15 Å (see Figure 1 for an example). This result, which agrees with some reported EXAFS data, 50 probably reflects the existence of Jahn−Teller distortion, which can be expected in these systems because the Ni 3+ ion (which, as explained below, has a low-spin electronic configuration) has an odd number of electrons and the e g -type manifold that presumably contains the unpaired electron would be degenerate in a trigonally distorted octahedral coordination. The unpaired electron of Ni 3+ thus seems to be present in an orbital of d z 2 type, as can be ascertained from the shape of its spin density distribution shown in Figure 1 Considering now the energies obtained, it is observed that a range of values appear, spanning an interval of ca.…”

Phase Stability of Nickel Hydroxides and Oxyhydroxides

“…One mechanism by which such structural phase transformations can occur is by passage of dislocations through the intercalation layers. 20 The work by Delmas et al 19 suggests, however, that this may not be the dominant mechanism to kinetically facilitate the ␤͑II͒ → ␤͑III͒ transformation. They showed that oxidation of Ni͑OH͒ 2 leads to a NiOOH compound having an amorphous-like X-ray diffraction pattern ͑XRD͒, which nevertheless, reverts back to a crystalline form of Ni͑OH͒ 2 upon reduction.…”

“…While passage of dislocations to facilitate the ␤͑II͒ → ␤͑III͒ transformation will introduce structural defects which could conceivably lead to an amorphization of the host structure, the damage resulting from dislocation passage is irreversible and is unlikely to be removed upon reduction of NiOOH to Ni͑OH͒ 2 . Instead, the mechanism by which the ␤͑II͒ → ␤͑III͒ transformation actually proceeds may be linked to the occurrence of noncooperative Jahn-Teller distortions of the oxygen octahedra around the Ni ions, observed by Delmas et al 19 in NiOOH. Such noncooperative Jahn-Teller distortions in NiOOH can locally accommodate the strain induced by the stacking sequence change during the ␤͑II͒ → ␤͑III͒ transformation, and because the strain accommodation is local, the stacking sequence shifts need not be cooperative over long distance, leading to a structure that may appear amorphous with XRD measurements.…”

“…2 The general chemical formula of the ␥ form can be written as H x A y ͑H 2 O͒ z NiO 2 in which A is typically Na or K and y and z are often claimed to be 0.33 and 0.66, respectively. 2,19 A common form 2,5 of ␥ is H x K 0.33 ͑H 2 O͒ 0.66 NiO 2 , which can be obtained by overcharging ␤-NiOOH. The potassium and water come from the electrolyte according to a reaction such as…”

Nickel-oxido structure of a water-oxidizing catalyst film

“…In both catalysts, there is no indication for mono-m-O(H) bridging (corner-sharing octahedra, typical M-M distance of 3.0-3.5 A ˚). A Ni-Ni distance of 2.82 A ˚has been reported for Ni III OOH, 24,25 a layered Ni oxide with di-m 3 -O(H) bridges between Ni ions. We find significantly more M-M interactions (N in Table 1) in the B i -NiCat as compared to the CoCat (see EXAFS fit results in Table S2z and magnitude of FT peaks in Fig.…”