The isomorphic phase transition in the system ${\mathrm{Ce}}_{1-x}{\mathrm{Th}}_x$, which is characterized by a delocalization of the $4f$ electron and a concomitant volume change, has been studied through measurements of the density and resitivity. For samples near the critical composition ($x\simeq 0.264$) derivatives of these quantities diverge with mean-field exponents. Such mean-field behavior implies a long-force range, and suggests that electroelastic coupling dominates the critical behavior.

• Received 19 May 1975

DOI:https://doi.org/10.1103/PhysRevLett.35.289