A simple approach for determination of the phase transition temperature using infrared temperature-induced isosbestic points.

The appearance and formation of an isosbestic point in the temperature-dependent IR spectra can be used to reveal a phase transition at a certain temperature. This conclusion was made by a thorough investigation of the IR (transmission and ATR) spectra of methylammonium iodide (MAI) and formamidinium iodide (FAI) recorded in a wide temperature range starting from -170 to +200 °C. By investigating the isosbestic points, it was found that MAI undergoes two phase transitions at -110 and +146 °C. The results obtained for FAI also showed two phase transitions at 73 and 115 °C. Furthermore, it was found that the shift of certain bands that are provoked by the phase change could be used to calculate the transition temperature. So far, according to the literature data, no attempts have been made to reveal the exact temperature of phase transitions using IR spectroscopic techniques.