Structures of a Complex Hydrazinium Lead Iodide, (N₂ H₅ )₁₅ Pb₃ I₂₁ , Possessing [Pb₂ I₉ ]^5- , [PbI₆ ]^4- , and I^-...

Photovoltaic perovskites, most notably methylammonium lead triiodide, (NH₃ Me)PbI₃ , have recently attracted considerable attention, and based upon the modified "Goldschmidt" as well as a "revised" tolerance factors, hydrazinium should be able to occupy the same cation site as methylammonium, and form a cubic unit cell. The reaction of N₂ H₅⁺ I^- with PbI₂ in dimethylformamide results in three types of yellow crystals; hexagonal, needle-like, and rod-like, the structures of which were determined at 100 K. The hexagonal (P6₃ /m: a=10.8906(10) Å; b=37.845(5) Å) crystals possess isolated face-sharing octahedral [Pb₂ I₉ ]^5- , [PbI₆ ]^4- , and I^- ions. IR spectroscopy indicates the presence of hydrogen-bonded N₂ H₅⁺ and the composition was determined by single-crystal X-ray diffraction, density measurements, combustion elemental analysis, and thermogravimetric analysis to be (N₂ H₅ )₁₅ Pb₃ I₂₁ , which is photoluminescent at 50 K, but not at room temperature. The needle and rod crystals have an orthorhombic (Pnma: a=11.1385(7) Å; b=4.4806(3) Å; c=17.6241(11) Å) and hexagonal (P6₃ /mmc: a=8.7386(9) Å; b=8.2006(9) Å) unit cells, respectively, possessing the perovskite ABX₃ composition of (N₂ H₅ )PbI₃ , but neither exhibits the cubic Perovskite structure type. The structures of α- and β-(N₂ H₅ )PbI₃ possess parallel ribbons of Pb₂ I₄ and chains of PbI₂ , respectively. Strong inter-hydrazinium hydrogen bonding due to it possessing both hydrogen bonding donor and acceptor sites (unlike NH₃ Me⁺ ) appear to stabilize the observed extended ribbon motif for (N₂ H₅ )₁₅ Pb₃ I₂₁ and α-(N₂ H₅ )PbI₃ . (N₂ H₅ )₁₅ Pb₃ I₂₁ has a band-gap-like absorption of 2.34 eV, and both α- and β-(N₂ H₅ )PbI₃ have a 2.70 eV band-gap-like absorptions.