Pressure-induced valence transitions in metal oxides: squeezing the electrons out of lone pairs

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Australian Institute of Nuclear Science and Engineering
While bonds in solid-state compounds always have some degree of covalent character, the ionic approximation is usually sufficient to understand their “crystal chemistry” using concepts like the effective ionic radius (IR). IR predicts that an atom will shrink as its oxidation state increases. This occurs gradually as electrons are removed within a shell (e.g., IR(Ir"3"+) = 0.68, IR(Ir"4"+) = 0.625, IR(Ir"5"+) = 0.57 Å in 6-fold coordination), but removing the last electron of a shell produces a much more pronounced change (e.g., IR(Bi"3"+) = 1.03, IR(Bi"5"+) = 0.76 Å). For a compound with a suitable combination of cations, it should therefore be possible to effect a net reduction in volume by transferring an electron from one to the other. Temperature and/or pressure could drive such a valence state transition; but in practice, this is extremely rare, with only three cases reported until recently. We tested this idea systematically in a series of high-pressure X-ray and neutron diffraction and spectroscopy experiments on six candidate materials containing Bi"3"+ with 4d or 5d metal cations. We observed a valence state transition in every case, suggesting that they are far more common than previously thought. This talk will present both published and unpublished experimental results, as well as ab initio calculations that shed light on the finely balanced electronic states of these compounds. The potential for tuning these transitions closer to ambient pressures, and of inverting the effect to give a volume change with an electronic stimulus, will be discussed.
Atomic ions, X-ray diffraction, Bismuth compounds, Valence, Crystal structure, Crystallography, Electrons, Neutron diffraction
Ling, C. D., Kennedy, B. J., & Avdeev, M. (2016). Pressure-induced valence transitions in metal oxides: squeezing the electrons out of lone pairs. Paper presented at 13th AINSE-ANBUG Neutron Scattering Symposium (AANSS 2016), Sydney, NSW, Australia; 29-30 November 2016.