The ferroelastic phase transition of ${\mathrm{}\left(\mathrm{KBr}\right)\mathrm{}}_{0.27}$${\mathrm{}\left(\mathrm{KCN}\right)\mathrm{}}_{0.73}$ has been studied by x-ray diffraction, ultrasonics, and inelastic neutron scattering. It is the first example of a cubic crystal where the elastic shear constant $C_{44}$ softens completely corresponding to the $m=2\mathrm{}$ universality class. $C_{44}$ and the Bragg intensities show a nonclassical critical behavior.

• Received 24 July 1985

DOI:https://doi.org/10.1103/PhysRevLett.55.2445