Order-disorder in the Jahn-Teller transition ofLaMnO3:A Raman scattering study

The high-$T$ “orthorhombic-cubic” Jahn-Teller (JT) transition of a polycrystalline sample of ${\mathrm{LaMnO}}_3$ is studied by Raman scattering. A ${\mathrm{CaMnO}}_3$ ceramic sample is used as a reference compound. Dramatic broadening of the ${\mathrm{LaMnO}}_3$ Raman modes are observed as T approaches the JT transition temperature ${(T}_{\mathrm{JT}})$ from below. The strong T dependence of the phonon linewidth suggests that considerable lattice disorder is already present at temperatures well below $T_{\mathrm{JT}}.$ Our results indicate that the JT transition in ${\mathrm{LaMnO}}_3$ is of order-disorder type and is driven by thermally activated $e_g$ orbital disorder of the ${\mathrm{Mn}}^{3+}$ ion in the ${\mathrm{MnO}}_6$ octahedra.