Internal-Displacement Order Parameter below the 151-K Phase Transition in PrAlO3

EPR measurements of ${\mathrm{Gd}}^{3+}$ impurities in PrAl${\mathrm{O}}_3$ show that the direction of maximum electric field gradient at the ${\mathrm{Pr}}^{3+}$ site rotates continuously from the [101] to [001] perovskite axis below 151 K. This angle gives a direct measure of the displacement associated with the optical phonon mode participating in the Jahn-Teller transition; the angle is found to vary as ${(T_c-T)}^{\frac{1}{2}}$ near $T_c$.