Order-disorder in the Jahn-Teller transition of LaMnO3: A Raman scatteringstudy

The high-T "orthorhombic-cubic" Jahn-Teller (JT) transition of a polycrysta lline sample of LaMnO3 is studied by Raman scattering. A CaMnO3 ceramic sam ple is used as a reference compound. Dramatic braodening of the LaMnO3 Rama n modes are observed as T approaches the JT transition temperature (T-JT) f rom below. The strong T dependence of the phonon linewidth suggests that co nsiderable lattice disorder is already present at temperatures well below T -JT. Our results indicate that the JT transition in LaMnO3 is of order-diso rder type and is driven by thermally activated e(g) orbital disorder of the Mn3+ ion in the MnO6 octahedra.