Complexity in the Phase Diagram of Perovskite Ferroelectrics

The temperature/doping dependent phase diagram of ferroelectric perovskite oxides is shown to be characterized by six different temperature scales: (1) At high temperatures a crossover from paraelectric to paraelectric/ferroelastic behavior sets in followed by (2) the onset of fluctuating ferroelastic clusters with doping- and temperature-dependent size. (3) The cluster size anti-correlates with their total system volume which defines another temperature scale. (4) The onset of ferroelectricity is not sharp since para- and ferroelectric solutions coexist in a small temperature regime. (5) The ferroelectric phase shows novel effects since beyond a certain transition temperature this state is incomplete, i.e., ferroelastic clusters fluctuate in a polar matrix. Finally (6) for low enough temperatures, ferroelectricity is completely suppressed by quantum fluctuations.