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

Vibrational spectra of methylammonium iodide and formamidinium iodide in a wide temperature range

“…According to the presented spectra in Figure 9, one can conclude that there is not only a band shift, but there is also a change in the intensity of the bands. It is interesting to mention that the band from the rocking NH3 + vibrations has a change in the intensity right at the phase transition temperature [17,23], as can be observed in the spectra in Figure 9b. It is important to study the changes that occur in narrower temperature intervals in order not to be misled.…”

“…The formation of the later band is shown in Figure 11b. It is interesting that both bands (680 and 754 cm -1 ) disappear above 75 ºC, and then reappear at 115 ºC when the phase transition takes place [17].…”

“…The synthesis of methylammonium iodide is rather straightforward [16]. Minor modifications suggested by the authors contributed to more stable product and have been previously described [17]. The MAI was synthesized using methylamine water solution and concentrated HI.…”

“…The synthesis of the formamidinium iodide and its purification have been previously reported [17,18]. As described, the synthesis was performed at 0 C in ethanol solution of formamidinium acetate while adding the concentrated HI drop wise.…”

“…T increases The important band assembly is the one formed by the asymmetric bending CH3 vibrations. Its importance is in the fact that these bands are part of an isosbestic point formation that can reveal the exact temperature at which the phase transition occurs [17]. Additionally, they show the appear-ance/disappearance of the bands so their evolution and assignation can be followed only by investigating the temperature dependent spectra.…”