Abstract
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.
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