The tetragonal rotation of Ti${\mathrm{O}}_6$ octahedra in SrTi${\mathrm{O}}_3$ and the trigonal rotation of Al${\mathrm{O}}_6$ octahedra in LaAl${\mathrm{O}}_3$ were measured by paramagnetic resonance below their respective phase transitions. The normalized rotation angles $\alpha$ vary quantitatively in the same way as a function of reduced temperature. Thus $\alpha$ is the order parameter in such transitions, which are characteristic for perovskite compounds.

• Received 1 July 1968

DOI:https://doi.org/10.1103/PhysRevLett.21.814