Super-high Curie temperature ferroelectrics and superior TC tunability: lattice instability and ferroelectric phase transitions.

Ferroelectric materials with high Curie temperatures, T_C, and tuning of the T_C over a large temperature range are very important for the basic sciences and engineering. This paper reviews the various types of high-temperature ferroelectrics with perovskite, bismuth layered, tungsten bronze, and perovskite slab structures, especially Sr₂Nb₂O₇ with T_C = 1342 °C and its family. The lattice instability of the high-temperature ferroelectric phase transitions is also discussed. The superior tunability of the T_C is achieved by the substitution of cations at the A- and B-sites in the perovskite-like structures. The variation of the soft optic and relaxation modes and elastic anomalies related to structural phase transitions were discussed in relation to the A- and B-site substitutions of oxygen octahedral ferroelectrics, especially Ba₂NaNb_5(1-x)Ta_5xO₁₅ with a tungsten bronze structure and (Ba_xSr_1-x)₂Nb₂O₇, Sr₂(Nb_1-xTa_x)₂O₇ with the perovskite slab structure. [ABSTRACT FROM AUTHOR]