Order-parameter behaviour below the 118.5K transition in PrAlO₃

Using the electron paramagnetic resonance (EPR) of Gd3+ ions substituted at Pr3+ lattice sites as symmetry probes, the order parameter of PrAlO3 has been investigated below 120K. It is found that the rotation axis of the AlO6 octahedra tilts out of the (010) symmetry planes below Tc=118.5K. Below approximately 100K, the tilt angle decreases but the crystal does not asymptotically reach the tetragonal symmetry in the quantum region.