EXAFS Study on the Phase Transition (Phase α’-δ) in CH ₃ NH ₃ I

The local structure of CH 3 NH 3 I around the I - ion in Phase α' and δ was studied by iodine K-edge EXAFS. The crystal structure in Phase α' was redetermined by single crystal X-ray diffraction. The Debye-Waller factor σ (2) derived from EXAFS spectra shows an anomalous behavior around the order-disorder transition temperature from Phase δ to α'. The phase transition has been interpreted as the disordering process which involves not only the H atoms but also the C atoms in the cation.