Pressure-induced inter-site valence transitions involving geometric frustration in hexagonal perovskites

Solid-state compounds are generally thought of as consisting of ions with well-defined oxidation states. While ionic bonds always have some degree of covalent character, the ionic approximation is usually sufficient to understand their "crystal chemistry" in conjunction with concepts such as bond valence sum (BVS) and 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(Ir³⁺) = 0.68, IR(Ir⁴⁺) = 0.625, IR(Ir⁵⁺) = 0.57 Å in 6-fold coordination), but removing the last electron of a shell produces a more pronounced change (e.g., IR(Bi³⁺) = 1.03, IR(Bi⁵⁺) = 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. A change in temperature and/or pressure could make such a valence state transition favourable; but in practice, such transitions are extremely rare. We have observed such a pressure-induced charge transfer, from Bi to Ir (or Ru) in the hexagonal perovskites Ba_3+nBiM_2+nO_9+3n (n = 0,1; M = Ir, Ru) using high-pressure synchrotron x-ray and neutron powder diffraction. They all show ~1% first-order volume contractions at room temperature above 5 GPa, due to the large reduction in the IR of Bi when the 6s shell is emptied on oxidation. These are the first such transitions involving 4d and 5d compounds, and double the total number of cases ever observed. Ab initio calculations suggest that magnetic interactions through very short (~2.6 Å) M–M bonds contribute to the finely balanced nature of their electronic states.