INTERGRANULAR AND INTERPHASE BOUNDARIES IN MATERIALS First principles study of three-component SrTiO3/BaTiO3/PbTiO3 ferroelectric superlattices

The geometrical, chemical and ferroelectric properties of a new nanoscale short-period three-component SrTiO3/BaTiO3/PbTiO3 perovskite superlattice are investigated using a first principles density functional approach. The study focuses on varying the thickness of each component in the superlattice and determining the resulting lattice distortion and total polarization. Thicknesses of up to three unit cells in a single component are considered and the in-plane lattice constants normal to the [001] stacking direction are fixed to the bulk SrTiO3 values to simulate a rigid substrate. It is found that the PbTiO3 layers play a key role in strain and polarization enhancement. By increasing the amount of PbTiO3 in the superlattices the strain in the other components increases significantly resulting in an enhanced total polarization of the superlattice relative to bulk BaTiO3. Increasing the number of BaTiO3 layers also improves the overall polarization. All the SrTiO3 layers in each superlattice are found to be highly polarized. Many of the calculated features are similar to those found previously by others for the SrTiO3/ BaTiO3/CaTiO3 superlattice, although in the present study significantly greater enhancement factors and polarization values are found. The predicted enhancement of the polarization is mostly attributed to lattice strain due to mismatch of the in-plane lattice constant of the threecomponent materials. Introduction There is considerable research activity into the design and characterization of ferroelectric superlattices (SL) since experimental measurements and theoretical investigations have shown that they can display superior functional properties relative to single-component ferroelectric materials in either bulk or thin-film form. Ferroelectric SLs are artificially structured multilayers composed of two or more thin-film perovskite components such as BaTiO3 (BT), PbTiO3 (PT) and CaTiO3 (CT). The components themselves need not all be ferroelectric or chemically uniform. Incipient ferroelectrics such as SrTiO3 (ST), metallic oxides such as SrRuO3 and solid solutions have been used with similar effects obtained. By varying the number of components, their thickness, composition and sequence in the multilayer it may be possible to achieve a significant enhancement in dielectric properties such as spontaneous polarization which will undoubtedly benefit future device applications. The polarization increase is clearly related to the lattice strain induced in the multilayer by interfacing components with different structural parameters, ferroelectric distortions and ionic charges. The optimal combination of parameters which maximizes the functional properties is not obvious and understanding this has been the focus of many of the studies. The experimental fabrication of ferroelectric SLs with controlled composition and structure requires the use of epitaxial deposition techniques such as MBE and PLD. With these techniques it is possible to engineer the SLs while minimizing the introduction of lattice defects, dislocations and interfacial steps that may degrade the polarization effects. The structural quality of the multilayers can be assessed using various electron microscopy and diffraction techniques [1]. The simplest type of SL involves just two S. H. Shah P. D. Bristowe (&) Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 3QZ, UK e-mail: [email protected] A. M. Kolpak A. M. Rappe Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104-6323, USA 123 J Mater Sci (2008) 43:3750–3760 DOI 10.1007/s10853-007-2212-7