Synthesis, Crystal Structure, and Optical and Thermal Properties of (C4H9NH3)2MI4 (M = Ge, Sn, Pb)

Single crystals of the organic-inorganic layered perovskites (C4H9NH3)2MI4 (M = Ge, Sn, Pb) have been grown from aqueous hydriodic acid solutions. X-ray diffraction, thermal analysis, and photoluminescence spectroscopy were used to compare crystal structure, metal atom lone-pair stereoactivity, and physical properties as a function of group IVB element. The orthorhombic (C4H9NH3)2GeI4 structure, refined in the space group Pcmn, consists of single-layer-thick perovskite sheets of distorted corner-sharing GeI6 octahedra separated by n-butylammonium cation bilayers. (C4H9NH3)2SnI4 and (C4H9NH3)2PbI4 are structurally very similar but adopt the space group Pbca, with a more ideal octahedral iodine coordination around the divalent group IVB atoms. Within the more general tin(II)-based family, (C4H9NH3)2(CH3NH3)n-1SnnI3n+1, a structural comparison between the title semiconducting n = 1 compound and the previously reported semimetallic n = 3 and metallic n → ∞ members demonstrates a correlation between perovskite sheet thickness, Sn(II) lone-pair stereochemical activity, average Sn−I bond length, and electrical conductivity within this series. While (C4H9NH3)2GeI4 melts at 222(2) °C, significantly below its bulk decomposition temperature, (C4H9NH3)2SnI4 (Tm = 256(2) °C) and (C4H9NH3)2PbI4 (Tm = 285(4) °C) melt/decompose at progressively higher temperatures and are less stable as a melt. Room-temperature photoluminescence from the title compounds exhibits a pronounced spectral peak in the visible range, with the peak wavelength varying between 690(5), 625(1), and 525(1) nm for the M = Ge, Sn, and Pb compounds, respectively.