Order parameter saturation in LaAlO3

High-resolution x-ray rocking diffraction has been used to measure the spontaneous strain associated with the cubic-rhombohedral phase transition in LaAlO3 in the range 10 K ≤ T ≤ 750 K. The data are consistent with a second-order Landau-like model at high temperatures, with TC = 834(2) K. At lower temperatures, the strain data display order parameter saturation, related to the quantum saturation of the phonon modes. Comparison of the saturation temperature for the spontaneous strain (θS = 95 K) with the saturation temperatures for independent measurements of the rotation (θS = 260 K) and distortion (θS = 150 K) of the AlO6 octahedra indicates that the phase transition consists of two coupled processes, and that the coupling does not have the same effect in the classical and quantum saturation limits