Brillouin scattering studies of the successive phase transitions in (CH₃NH₃)₂FeCl₄

Brillouin scattering spectra are reported and analyzed for the layered perovskite compound (CH₃NH₃)₂FeCl₄. Particular attention has been given to successive phase transitions in this system: D¹⁷_4h ↔ D¹⁸_2h ⇄ D¹⁶_4h. It is found that the elastic constant c₆₆ is the major contributor to instabilities in this system. A two‐dimensional order parameter at the Brillouin zone boundary X point is introduced into the Landau free‐energy to account for this sequence of transitions. An additional secondary instability (one‐dimensional order parameter) at the Z point is used to generate the D¹⁸_2h ⇄ D¹⁶_4h phase transition: inclusion of the two order parameters circumvents the need for postulating a D¹⁰_2h phase between the D¹⁸_2h and D¹⁶_4h phases and for higher order temperature dependent terms in the free energy expansion. The D¹⁷_4h ↔ D¹⁸_2h phase transition is characterized by a strong Landau–Khalatnikov contribution to c₆₆ and a large dynamical critical behavior.