On the Demystification of “HPbI 3 ” and the Peculiarities of the Non‐innocent Solvents H 2 O and DMF

Detailed investigations by XRD reveal that the precursor “HPbI 3 ” that was obtained by reaction of aq. conc. hydroiodic acid HI and PbI 2 in DMF is (CH 3 ) 2 NH 2 PbI 3 . (CH 3 ) 2 NH 2 + is formed by solvent reaction as already described in the literature but not properly assigned. Attempts to synthesize HPbI 3 by gas phase reaction of PbI 2 with aq. conc. HI yielded light‐yellow crystals of the oxonium salt H 18 O 8 Pb 3 I 8 ( Pbam, Z = 2, a = 10.075, b = 30.162, c = 4.5664 Å). The crystal structure of H 18 O 8 Pb 3 I 8 consists of trimeric ribbons of edge‐sharing PbI 6 octahedra. These ribbons [Pb 3 I 8 ] 2– are separated by protonated fragments of crystalline ice [H 18 O 8 ] 2+ or (H 2 O) 6 (H 3 O + ) 2 . H 18 O 8 Pb 3 I 8 can also be precipitated from PbI 2 and aq. conc. HI. H 18 O 8 Pb 3 I 8 is not stable at room temperature but transforms within a few days to light‐yellow (H 3 O) 2 x (H 2 O) 2–2 x Pb 1– x I 2 with x ≈ 0.23 ( R 3 m, Z = 3, a = 4.5554, c = 29.524 Å). The crystal structure represents a CdCl 2 ‐type layer structure with H 2 O/H 3 O + in between. Charge compensation is achieved by Pb 2+ vacancies. Via topotactic reaction (H 3 O) 2 x (H 2 O) 2–2 x Pb 1– x I 2 releases H 2 O/HI and forms crystals of the pristine PbI 2 . All steps were characterized by P‐XRD, IR/Raman spectra, and UV/Vis spectra. H 18 O 8 Pb 3 I 8 acts as a precursor for the synthesis of MAPbI 3 because the reaction with gaseous CH 3 NH 2 yields MAPbI 3 , so it can mimic a composition “HPbI 3 ”.