Neutron-diffraction study of the Jahn-Teller transition in stoichiometricLaMnO3

The parent compound of the giant magnetoresistance Mn-perovskite, ${\mathrm{LaMnO}}_3$, has been studied by thermal analysis and high-resolution neutron-powder diffraction. The orthorhombic $\mathrm{Pbnm}$ structure at room temperature is characterized by an antiferrodistorsive orbital ordering due to the Jahn-Teller effect. This ordering is evidenced by the spatial distribution of the observed Mn-O bond lengths. ${\mathrm{LaMnO}}_3$ undergoes a structural phase transition at $T_{\mathrm{JT}}\approx 750$ K, above which the orbital ordering disappears. There is no change in symmetry although the lattice becomes metrically cubic on the high-temperature side. The ${\mathrm{MnO}}_6$ octahedra become nearly regular above $T_{\mathrm{JT}}$ and the thermal parameter of oxygen atoms increases significantly. The observed average cubic lattice is probably the result of dynamic spatial fluctuations of the underlying orthorhombic distortion.