Local Moment Instability of Os in Honeycomb Li2.15Os0.85O3

Abstract

Compounds with honeycomb structures occupied by strong spin orbit coupled (SOC) moments are considered to be candidate Kitaev quantum spin liquids. Here we present the first example of Os on a honeycomb structure, Li2.15(3)Os0.85(3)O3 (C2/c, a = 5.09 Å, b = 8.81 Å, c = 9.83 Å, β = 99.3°). Neutron diffraction shows large site disorder in the honeycomb layer and X-ray absorption spectroscopy indicates a valence state of Os (4.7 ± 0.2), consistent with the nominal concentration. We observe a transport band gap of Δ = 243 ± 23 meV, a large van Vleck susceptibility, and an effective moment of 0.85 μB, much lower than expected from 70% Os(+5). No evidence of long range order is found above 0.10 K but a spin glass-like peak in ac-susceptibility is observed at 0.5 K. The specific heat displays an impurity spin contribution in addition to a power law ∝T(0.63±0.06). Applied density functional theory (DFT) leads to a reduced moment, suggesting incipient itineracy of the valence electrons, and finding evidence that Li over stoichiometry leads to Os(4+)−Os(5+) mixed valence. This local picture is discussed in light of the site disorder and a possible underlying quantum spin liquid state.

Quantum materials with strong spin–orbit coupling: challenges and opportunities for materials chemists

“…67,68 Analogous α-OsCl 3 is not known, though substoichiometric Os x Cl 3 (in which honeycomb nanodomains may be realised through local ordering of the Os-site vacancies) and partially-lithiated Li 2.15 Os 0.85 O 3 have the same structural basis. 69,70 The layered motif of these materials facilitates topochemical removal/exchange of the interlayer species. In this way, A 3 LiIr 2 O 6 (A = H, Cu, Ag) have been prepared.…”

“…For example, Li 2.15 OsO 3 is the first reported honeycomb osmate, but in contrast to the single-valence iridate analogue samples of this material were found to be approximately 70% Os 5+ and 30% Os 4+ . 69 This has a significant effect on the magnetism as the mixture of nominally J e f f = 3/2 and J e f f = 0 ions creates disorder on the honeycomb spin network. Another intriguing example is the Os 5+ /Os 6+ mixed oxide Sr 5 Os 3 O 13 , which shows complex temperature-dependent behaviour.…”

“…Precisely determining both the oxidation states present and whether they are ordered, clustered or disordered is key to understanding the structures and magnetism of these materials but is non-trivial, often requiring facility-based techniques such as X-ray absorption spectroscopy (XAS). 69 However, recent progress in oxidation state control by a variety of synthetic routes in showing that the effort can certainly be worthwhile. 114,115…”

Os4+ Instability in the Pyrochlore Structure: Tl2–xBixOs2O7–y

“…Osmium in the system studied prefers to exist in a mixed valence state, following precedent. 5,8,9,18,19 A high level of mixed valence would be difficult to stabilize in a pyrochlore containing only Bi 3+ at the A site, causing the instability of the single-phase bismuth osmate pyrochlore. Rather than adopting the pyrochlore structure, bismuth osmate will instead preferentially form the KSbO 3 structure type (Bi 3 Os 3 O 11 ), which allows for Os to remain mixed valent.…”

“…3 Osmate compounds in general have also been shown to exhibit interesting properties such as a metal−insulator transition 4 and a potential quantum spin liquid state. 5 However, few osmate oxides are known due to the difficulty in stabilizing these compounds. In fact, the rare-earth osmate pyrochlores reported to exist 6 have not to date been reproduced or characterized.…”