Local structure ofLiNiO2studied by neutron diffraction

Charge disproportionation and Jahn-Teller distortion in LiNiO2and NaNiO2: A density functional theory study

“…A slight monoclinic distortion was observed at low temperatures (10 K) by the neutron diffraction study of [5] and a better fit found to the C2/m space group, but detailed analysis showed that the collinear ordering of the Jahn--Teller distortions implied by this space group was not supported by a combination of the Rietveld refinement and the neutron PDF data. The electron diffraction study of [12] was analysed using the Pm space group (which is the simplest space group that can incorporate a trimer ordering model).…”

“…Reynard et al 11 suggested, on the basis of anomalies in the spin susceptibility observed at 10 K and 400 K, that there were at least two energy scales involved, corresponding to spin and orbital interactions, and that the possibility of orbital frustration should be considered. The neutron studies of [5] argue against this since this would imply that the number of "short" and "long" Ni-O bonds would be equal. The authors suggest instead that the magnetic properties should be explained by the assumption that there is local orbital ordering: the 3 !…”

“…Two different Ni--O bond lengths have been observed both in EXAFS 8,9 and neutron diffraction 5 which are attributed to a local Jahn--Teller distortion, unlike the cooperative distortion seen in other Jahn--Teller active systems such as NaNiO 2 and LiMnO 2 . The analysis of the neutron partial density function (PDF) in [5] supports the hypothesis of a Jahn--Teller distortion since their results show four bond--lengths grouped as "long bonds" (2.04 Ǻ and 2.06 Ǻ with an average length of 2.05 Ǻ) and "short bonds" (1.90 Ǻ and 1.96 Ǻ with an average length of 1.93 Ǻ) suggestive of the 2:1 ratio of short to long bonds expected for Jahn--Teller distortion. However, the long--range PDF peaks increase in height with temperature -an unusual effect that the authors attribute to domain formation.…”

“…Even the most recent experimental studies struggle to obtain truly stoichiometric material. Where quoted, typical defect concentrations are of the order of a few percent -nickel is found on the lithium site of the perfect structure 5 .…”

Surface properties of LiCoO2, LiNiO2 and LiNi1−xCoxO2

“…Following the same reasoning for LiNiO 2 , however, requires the formation of Ni 3+ , an uncommon state for solid state nickel oxides and one that is often stabilized by the formation of defects or hydroxides that produce mixed Ni 2+ /Ni 3+ oxidation states. Indeed, two general descriptions have emerged in the literature in relation to the chemical nature of nickel in LiNiO 2 , one analogous to that found for LiCoO 2 in which the oxidation state for the nickel is Ni 3+ [23], [24], [25], [26], [27] and [28] and a second one that relies on localized Ni 2+ -O − pairs in which charge is transferred from a neighboring lattice oxygen onto the nickel to preserve, or at least more closely approximate, the favored 2 + state [29], [30], [31], [32], [33] and [34]. We present here a study of surface composition of LiCoO 2 , LiNiO 2 and LiNi 0.5 Co 0.5 O 2 which suggests that, at least in the near-surface region, the nickel-containing lithium metal oxide is stabilized by dilithiation to produce a nickel cation with an average electron density closer to that of the more favored Ni 2+ state.…”