A powder diffraction study of the phase transition in LaAlO3

The structure of LaAlO3 has been investigated around the phase transition a t T-c approximate to 800 K by neutron powder diffraction in vacuum and by X -ray powder diffraction under nitrogen atmosphere as well as by a very high resolution synchrotron experiment in air. The results were analysed in fra me of the Landau theory using the fluctuation-dissipation theorem to relate the susceptibility to the atomic displacement parameters. The room tempera ture structure is a rhombohedrally distorted perovskite structure, space gr oup R (3) over bar c, which undergoes a transition to the ideal perovskite structure, space group Pm (3) over bar m, at high temperatures. The order p arameter is a rotation of the O-6-octahedron described by one x(O)-paramete r. This parameter and the spontaneous strain (c/a - root 6), as well as the relevant atomic displacement parameter U-11(op)(O) in the order parameter system, show a critical behaviour in agreement with a second order phase tr ansition. Although the critical exponents of the order parameter and strain show the expected coupling behaviour, there is a striking difference of th e transition temperature: the metric becomes cubic roughly 30 K below the p roper T-c. This is related to spontaneous formation of domains imposing the average cubic symmetry via internal stresses.