High-pressure observation of elusive iodoplumbic acid in different hydronium-hydrate solid forms

High-pressure and high-temperature isochoric crystallization combined with single-crystal X-ray diffraction revealed the proposed, but previously never demonstrated, hydronium acid hydrates of [PbI3]-, the closest forms of the elusive iodoplumbic acid. Depending on the pressure range, the reaction of PbI2 and aqueous concentrated hydriodic acid under isochoric conditions in a diamond anvil cell held between 0.11 and 1.20 GPa produces two hydrated acids with compositions [H3O][PbI3]center dot nH2O (n = 3, 4). Comprised of polymeric one-dimensional double-chain PbI3- anions, analogous to those seen in archetypal lead-halide perovskites such as RbPbI3 and CsPbI3, these hydrates offer the first observation of the iodoplumbic acid progenitor of hybrid lead perovskites. We also reveal the 'hidden' polymorph of lead(ii) iodide, adopting a three-dimensional Pb-I bonded network, contrasting with the prototypic ambient-pressure layered PbI2 structure. High-energy conditions stabilize iodoplumbic acid in the form of hydronium acid hydrates. The high-pressure reaction of PbI2 and aqueous concentrated hydriodic acid led to two types of hydrated acids and revealed a new polymorph of lead(ii) iodide.