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: D174h ↔ D182h ⇄ D164h. 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 D182h ⇄ D164h phase transition: inclusion of the two order parameters circumvents the need for postulating a D102h phase between the D182h and D164h phases and for higher order temperature dependent terms in the free energy expansion. The D174h ↔ D182h phase transition is characterized by a strong Landau–Khalatnikov contribution to c66 and a large dynamical critical behavior.