THE JAHN-TELLER STRUCTURAL TRANSITION IN STOICHIOMETRIC LAMNO3

The parent compound of the giant magnetoresistance perovskite Mn-oxide s, LaMnO3, has an orthorhombic Pbnm crystal structure at room temperat ure characterized by the presence of an anti-ferrodistorsive e(g)(1):e (g)(2) orbital ordering due to a cooperative Jahn-Teller effect. The c ompound undergoes a structural phase transition at T(JT)approximate to 790 K, above which the orbital ordering disappears. There is no chang e in symmetry, although the lattice becomes metrically cubic on the hi gh-temperature side. The refinement of the crystal structure shows tha t MnO6 octahedra become much more regular above T-JT and the thermal p arameter of oxygen atoms increases significantly. The observed average cubic lattice is probably the result of dynamical spatial fluctuation s of the underlying small orthorhombic distortion.