Orbital molecules in oxides

Orbital molecules are weakly bonded clusters of transition metal ions within an orbitally ordered solid.[i] The importance of these states has become apparent in recent years following the discovery of ‘trimeron’ orbital molecules in the ground state of magnetite (Fe₃O₄).[ii] Determination of the full superstructure below the famous Verwey transition at 125 K showed that Fe²⁺/Fe³⁺ charge ordering occurs with a pronounced orbital ordering of Fe²⁺ states that leads to localization of electrons in the linear, three-Fe trimerons. The same complex electronic order is observed in a natural magnetite sample.[iii] Site-selective doping of ordered charge states in magnetite has recently been observed.[iv] Electronic phase separation driven by trimeron formation has recently been reported in CaFe₃O₅.[v]
X-ray pair distribution function (XPDF) analysis reveals that disordered orbital molecules persist to high temperatures in the vanadium spinels AlV₂O₄[vi] and the new analog GaV₂O₄.[vii,viii]. Orbital molecules may also be formed in LiV₂O₄ at pressure [ix]. XPDF also demonstrates that local structural fluctuations in magnetite emerge below the Curie transition at 850 K, indicating that fluctuations in Fe-Fe bonding arising from magnetic order are the primary electronic instability and hence the origin of the Verwey transition.[x]

[i] J.P. Attfield, APL Mats. 2015, 3, 041510.
[ii] M.S. Senn, J.P. Wright, J.P. Attfield, Nature 2012, 481, 173.
[iii] G. Perversi, et al, Chem. Comm. 2016, 52, 4864.
[iv] E. Pachoud, J. Cumby, G. Perversi, J.P. Wright, J.P. Attfield, Nature Comm. 2020, 11,1671.
[v] K.H. Hong, et al Nature Comm. 2018, 9, 2975.
[vi] A.J. Browne; S.A.J. Kimber; J.P. Attfield. Phys. Rev. Mat. 2017, 1, 052003.
[vii] A.J. Browne; et al Inorg. Chem. 2018, 57, 2815.
[viii] A.J. Browne; J. P. Attfield. Phys. Rev. B 2020, 101, 024112.
[ix] A.J. Browne; et al. Phys. Rev. Mat. 2020, 4, 015002.
[x] G. Perversi, et al, Nature Comm. 2019, 10, 2857.