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]·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.