Phase transitions in BaTiO3 thin films and BaTiO3/BaZrO3 superlattices

Using pulsed laser deposition, we grew a ferroelectric [BaTiO3]Λ/2/[BaZrO3]Λ/2 (BT/BZ) superlattice with a stacking periodicity of Λ = 256 Å and a BT single film on a La1/2Sr1/2CoO3-buffered (001)MgO substrate, and then studied the phase transitions of these materials. At room temperature, the polarized Raman spectra of the BT film corresponded to a ferroelectric orthorhombic C2V14 phase with the polar axis oriented in the plane of the substrate. A ferroelectric-paraelectric phase transition in the BT film occurred at ∼450 K. Upon cooling to ∼300 K, a phase transition to the monoclinic Cs3 phase occurred. These experimental results agree well with a theoretical “temperature-misfit strain” phase diagram of the BT film. We found no evidence of phase transitions in the BT/BZ superlattice below room temperature. The phase transition to the paraelectric phase in the BT/BZ superlattice increased in temperature because of lattice mismatch between the BT and BZ layers. A desirable Curie temperature can be tailored rather precisely by varying the layer thicknesses or the BT/BZ ratio in the superlattice. The BT/BZ superlattices are very good for device applications because their ferroelectric phase with in-plane polarization can remain stable at least from 0 K up to 723 K, and even at higher temperatures in superlattices with smaller periodicities.