Unconventional Electron-Deficient Multicenter Bonds in AIO₃ Perovskites

ABX ₃ and BX ₃ perovskites and their distorted variants are solidstate systems with exceptional properties, which allow them to be used in 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 toward 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 ABX ₃ and BX ₃ 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 process of the BX ₃ units (monomers) leading to the formation of regular BX ₆ units; and (ii) unconventional EDMBs could be already present at room conditions in the cubic or slightly distorted ABX ₃ and BX ₃ perovskites, with A, B, and X being main-group elements. The presence of unconventional EDMBs could explain the extraordinary properties of these perovskites.