RESUMO
A detailed study of lead halide-layered perovskites with general formula A2PbX4 (where A is cyclohexylammonium (CHA) or cyclopentylammonium (CPA) cation and X is Cl- or Br- anion) is presented. Using variable temperature synchrotron X-ray powder diffraction, we observe that these compounds exhibit diverse crystal structures above room temperature. Very interestingly, we report some unconventional thermomechanical responses such as uniaxial negative thermal expansion and colossal positive thermal expansion in a perpendicular direction. For the polymorphs of (CHA)2PbBr4, the volumetric thermal expansion coefficient is among the highest reported for any extended inorganic crystalline solid, reaching 480 MK-1. The phase transitions are confirmed by calorimetry and dielectric measurements, where the dielectric versus temperature curves show anomalies related with the order-disorder phase transitions. In addition, these compounds exhibit a broad photoluminescence (PL) emission with a large Stokes shift, which is related with an exciton PL emission.