Phase transitions in BaTiO3 thin films and BaTiO3/BaZrO3 superlattices

Using pulsed laser deposition, we grew a ferroelectric [BaTiO3](Lambda/2)/[BaZrO3](Lambda/2) (BT/BZ) superlattice with a stacking periodicity of Lambda = 256 angstrom 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 C-2V(14) phase with the polar axis oriented in the plane of the substrate. A ferroelectric-paraelectric phase transition in the BT film occurred at similar to 450 K. Upon cooling to similar to 300 K, a phase transition to the monoclinic C-s(3) 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. (C) 2014 AIP Publishing LLC.