Soft acoustic mode at the cooperative Jahn-Teller phase transition in Dy VO4

The temperature dependence of the elastic constant 12(c11-c12) shows that the B1g acoustic phonon branch is the soft mode at the cooperative Jahn-Teller phase transition in DyVO4. For T>2T D (T D is the transition temperature) the data can be described by mean field theory and show that the dominant driving mechanism for the transition is the coupling of the Jahn-Teller ions to the static elastic strain. However, for T D<T<2T D we find significant deviations from mean field theory due to critical fluctuations. © 1972 The American Physical Society.