The phase diagram of electrically poled, tetragonal PLZT materials has been investigated. In the higher La concentration range, a tetragonal, so-called β-phase occurs, which has no clear FE properties. The transition to the FEt phase has first order character and takes place only after a strong electric field has been applied to the material. The energetics of the βt ah FEt transition were investigated in several ways. The heat-effects associated with this transition have a small value of 6–20cal/mol, dependent on composition. The transition βt ah FEt is accompanied by a relatively large activation energy ΔG2. The ΔG2(T) curve shows a minimum for materials, which do not show a spontaneous transition to the FEt phase. This minimum value of ΔG2 (1–2.5 cal/mol) is large in comparison to the average thermal energy kT and prevents the spontaneous transition from the βt phase to the FEt phase.