Critical exponents for isosymmetric phase transitions in BiFeO3

Recent studies of BiFeO3 have revealed two first-order phase transitions that do not change symmetry (Pbnm-Pbnm at approx. 1200 K and Cc-Cc under uniaxial stress). Twenty years ago Ishibashi and Hidaka (1991 J. Phys. Soc. Japan 60 1634) showed that such systems generally exhibit a phase diagram with tricritical points and critical end points and gave the unusual mean-field critical exponents alpha = 2/3, beta = 1/3 and gamma = 2/3 at the end points. In the present paper I extend that to show delta = 3, nu = 1/3 and eta = 0 and suggest some experimental tests.