Evidence and detailed study of a second-order phase transition in the (C6H11NH3)2[PbI4] organic-inorganic hybrid material

The thermal properties of the organic-inorganic hybrid material (C6H11NH3)2[PbI4] are investigated using diffuse reflectivity, spectroscopic ellipsometry, differential scanning calorimetry, Raman spectroscopy, and X-ray diffraction. The diffuse reflectivity, performed in heating mode, clearly evidences the presence of a singularity at 336 K. This is confirmed by the temperature dependence of the spectroscopic ellipsometry spectra, which points out a second-order phase transition at 336 K with a critical exponent ∼0.5. Differential scanning calorimetry measurements on a polycrystalline powder of (C6H11NH3)2[PbI4] show a reversible phase transition detected at TC = 336 K without hysteresis. Raman spectroscopy data suggest that this transition arises from a change in the interactions between inorganic sheets ({[PbI4]2−}∞) and organic protonated molecules ([C6H11NH3]+). The structural analysis from power X-ray diffraction reveals an incomplete order-disorder transition of the cyclohexylammonium cation, causing a subtle contraction of the inter-plane distance. The transition results from repulsive close contacts between the organic molecules in the interlayer spacing.