Thermochromic Phase Transitions of Long Odd-Chained Inorganic-Organic Layered Perovskite-Type Hybrids [(C_nH_{2 n+1}NH₃)₂PbI₄], n = 11, 13, and 15

We investigate the last members of a series of inorganic-organic hybrid materials of the general formula [(CnH2n+1NH3)2PbI4] (abbreviated CnPbI). The self-assembly of the inorganic and organic components has a perovskite-like structure as the two-dimensional (2D) inorganic layers have four corners of the lead(II) iodide octahedra being shared out. The inorganic layers are separated by bilayers of alkylammonium chains, in this case with n = 11, 13, and 15. These materials exhibit complex phase behavior in the temperature range from -20 to + 81 °C. Differential scanning calorimetry and single-crystal X-ray diffraction enabled the phase transition temperatures and enthalpies to be determined and the structural changes that occur at the phase transition temperature. The number of phases is dependent on the chain length: for n = 11 and 15, there are three phases, and for n = 13, there are four phases. Regardless of the number of phases, all three compounds have identical lowest-temperature phases (all stable below room temperature), which have inorganic layers that are staggered, alkylammonium chains that are planar and nonplanar, and yellow crystals. The room-temperature phases for the three compounds differ, but all are orange. C11PbI has staggered layers, and C13PbI and C15PbI have identical room-temperature phases with eclipsed layers. C13PbI and C15PbI also show an additional phase between the lowest-temperature and room-temperature phases.