THE FCC-BCC STRUCTURAL TRANSITION .2. A MEAN-FIELD MODEL FOR FINITE-TEMPERATURE EFFECTS

We present a model for temperature induced martensitic phase transitio ns where the excitations leading to the phase instability are local fl uctuations of the respective order parameter which describes the displ acement of a single atom or an atomic plane when transforming structur e A into structure B. The fluctuations of the order parameter are heat ed in the framework of a Landau-Ginzburg model and allow us to describ e the temperature induced first-order phase transition as being driven by strain fluctuations (caused by anharmonicities) rather than by pho non softening. In some materials this phenomenological mean field trea tment represents a possible model for martensitic phase transitions.