Ferroic phase transitions in (Na1/2Bi1/2)TiO3 crystals

The ferroic phase-transition behavior of two (Na1/2Bi1/2)TiO3 (NBT) crystals grown by flux and by the Czochralski method has been investigated in the present study. Although both the tetragonal and the rhombohedral phases of NBT are expected to be ferroelastic, these crystals exhibit different ferroelastic behavior. The two NBT crystals also show differences in the amount of temperature hysteresis and the thermal expansion coefficients. These differences can be attributed to nonstoichiometry and structural variations dependent on the growing method, The present investigation has revealed a second maximum at similar to 450 degrees C in dielectric constant (epsilon(T)) curves, which could indicate that the intermediate tetragonal phase is either polar or antipolar. This maximum, however, originates from space-charge polarization and interaction between the charge carrier and the electrode, such that the tetragonal phase, in fact, is paraelectric. The diffuse phase transition (DPT) of the NBT crystal, therefore, is from a paraelectric and ferroelastic tetragonal phase to a ferroelectric and ferroelastic rhombohedral phase. The crystallographic supergroup-subgroup relationships in the ferroic phase transitions of NBT crystals are discussed.