Unconventional electron-deficient multicenter bonds in AIO3 perovskites

ABX3 and BX3 perovskites and their distorted variants are solid-state systems with exceptional properties, which enable them for a plethora of potential technological applications. This notwithstanding, the nature of the chemical B‒X bonding, which forms the framework where the A atoms can be inserted, is still under debate. Through a joint experimental and theoretical study of AIO3 (A= K, Rb, Cs, Tl, NH4) compounds and in particular in cesium iodate (CsIO3) under compression, we show how the IO3‒ polyanions, present in these compounds at room pressure, undergo a gradual pressure-induced polymerization (PIP) process in three dimensions (3D). This results in a pressure-induced symmetrization of the crystalline structure that leads to a tetragonal perovskite structure, with IO5+1 units, in CsIO3 and eventually to a cubic perovskite, with IO6 units, in other AIO3 compounds. We demonstrate that the PIP process induces a change in the chemical bonding from the resonant delocalized I‒O bonds in IO3‒ polyanions towards the unconventional I‒O electron-deficient multicenter bonds (EDMBs) in AIO3 cubic perovskites. The process of EDMB formation in the cubic perovskites agrees with the recently proposed unified theory of multicenter bonding and contradicts previous assumptions that considered these bonds to be impossible in valence electron-rich elements, such as chalcogens and halogens. Interestingly, our results suggest that: i) the formation of the cubic and slightly distorted ABX3 and BX3 perovskites with A, B, and X being main-group elements at high pressure is driven by the formation of 3D EDMBs due to the PIP of BX3 units leading to the cubic skeleton of regular BX6 units; and ii) unconventional EDMBs could be already present at room conditions in the cubic or slightly distorted ABX3 and BX3 perovskites with A, B, and X being main-group elements perovskites. The presence of unconventional EDMBs could explain the extraordinary properties of these perovskites.