Toward Elusive Iodoplumbic Acid HPbI3: High-Energy Isochoric Synthesis of Hydronium Forms of the PbI3- Anion at Elevated Temperature and Pressure

High-pressure and -temperature crystallization and X-ray diffraction crystallography have revealed hydronium forms of the proposed but never demonstrated iodoplumbic acid HPbI₃. Depending on the pressure range, the reaction of PbI2 and aqueous concentrated hydriodic acid under isochoric conditions in a diamond anvil cell (DAC) held between 0.11 and 1.20 GPa produces two hydrated hydronium salts with compositions [H₃O][PbI₃].nH₂O (n=3,4). Comprised of polymeric one-dimensional PbI₃^- anions, these hydronium salts represent the so far best match for the elusive HPbI₃ progenitor of hybrid lead perovskites. We also reveal a new three-dimensional polymorph of lead iodide (PbI₂), so far known only as a layered structure.