The temperature dependence of the rotational displacement parameters below the second-order phase transitions in SrTi${\mathrm{O}}_3$ and LaAl${\mathrm{O}}_3$ at $T_a=105.5\mathrm{} \mathrm{and} 797$°K is described by an exponent $\beta =0.33\mathrm{}\pm 0.02$ down to $t=\frac{T}{T_a}=0.95\mathrm{}$. For smaller $t$'s there occurs a change to Landau behavior approximately followed between $t=0.9\mathrm{} \mathrm{and} 0.7$. The observation of static critical exponents near displacive phase transitions confirms now the notion of universality in this field.

• Received 27 October 1970

DOI:https://doi.org/10.1103/PhysRevLett.26.13