Thermally-induced first-order phase transition in the (FC6H4C2H4NH3)(2)[PbI4] photoluminescent organic-inorganic material.

The thermal properties of the perovskite slab alkylammonium lead iodide (FC6H4C2H4NH3)(2)[PbI4] are investigated using spectroscopic ellipsometry, differential scanning calorimetry, photoluminescence, and Raman spectroscopy. The spectroscopic ellipsometry, performed in the heating mode, clearly evidenced the presence of a singularity at 375 K. This is corroborated by the temperature dependence of the photoluminescence, which pointed out a first-order order-disorder phase transition at similar to 375 K, with a hysteresis loop of 40 K width. Raman spectroscopy data suggest that this transition arises from a dynamic rotational disordering of the ammonium headgroups of the alkylammonium chain. In contrast, differential scanning calorimetry measurements on a pellet sample led to an entropy change value Delta S approximate to 0.39 J/K/mol at the transition, suggesting the existence of a residual short-range order of the NH3+ on cooling from the high temperature phase. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3690920]