Neutron-diffraction study of the Jahn-Teller transition in stoichiometric<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">LaMnO</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>

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 ${T}_{\mathrm{JT}}\ensuremath{\approx}750$ 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.