Crystal structure of the high-temperature polymorph of C(NH2)3PbI3 and its thermal decomposition

Abstract

The synthesis of guanidinium lead iodide, C(NH2)3PbI3 (GUAPbI3), was conducted by slow evaporation of the mixture obtained by dissolving PbI2 and C(NH2)3I in acetonitrile. When the evaporation is done at 40 ºC, a yellow needle-like crystals are being formed. The sample was characterized by elemental analysis, density measurements, scanning electron microscopy, thermal analyses, high-temperature X-ray powder diffraction and infrared spectroscopy measurements. The elemental analysis of the obtained crystals confirmed the proposed stoichiometry. The performed thermal analyses showed an endothermic peak associated with structural transition around 160 ºC. On the other hand, the endothermic temperature effects above 300 ºC are accompanied with mass loss and were interpreted as compound degradation. The crystal structure of high temperature polymorph between 160 ºC and 300 ºC was determined using high-temperature powder diffraction data measurements at 280 ºC using simulated annealing technique in order to obtain initial structural model. The structure was refined using the Rietveld method. At temperatures higher than 160 ºC, C(NH2)3PbI3 crystallizes in hexagonal space group P63mc with unit cell parameter a increasing from 9.269 Å to 9.337 Å between 160 ºC and 300 ºC and c parameter increasing from 15.211 Å to 15.287 Å in the same temperature range. The structure consists of PbI6 octahedra couples sharing a common face, linked with corners. Guanidinium cations are situated in the channels between Pb2I9 couples in a manner that the plane of the molecule is perpendicular to the c-axis.