First - Principles Study Of Ordered Superlattices Of BaTiO <3> / SrTiO 3 (1: 1)

Ferroelectric materials are the subjects of extensive investigation both because of their technical importance and because of the fundamental interest in the physics of their phase transition. In recent years, with the development of computer technology, first principles calculation has become one important tool in studying ferroelectrics. Due to the highly nonlinear dielectric response to an applied electric field and low dielectric loss in microwave domain, barium strontium titanate (BaxSr1-xTiO3, BST) is a promising material for electric tunable devices for microwave applications, such as tunable dielectric filters and phase shifters. BaTiO3/SrTiO3(BTO/STO)(1:1) superlattice is chosen as research object in this dissertation. The structure, dielectric and piezoelectric responses of BTO/STO (1:1) superlattice were investigated by first-principles calculations based on density-functional theory.The zone-center phonon of BTO/STO (1:1) superlattice with Ba/Sr [001],[110] and [111] ordering were investigated by first-principles calculations. The lattice dynamics, electronic and lattice contributions to the dielectric and piezoelectric responses of BTO/STO (1:1)10-atom superlattice are calculated. For the BTO/STO (1:1) superlattice with Ba/Sr [001] ordering, we explore the ground structure from the highest P4/mmm phase by successively freezing in the unstable polar modes. We find that the ground structure is in the Cm symmetry. For the BTO/STO (1:1) superlattice with Ba/Sr [110] ordering, the ground structure is in the Pm symmetry.And for the BTO/STO (1:1) superlattice with Ba/Sr [111] ordering, the ground structure is in the R3m symmetry.The analyses of the density of states and charge density shows that the hybridization between Ti3d and O2p is stronger than that between Ba (or Sr) and O. In BTO/STO superlattice the interactions of Ba-O and Sr-O are ionic, and the Ti-O bond is covalent. Based on above analysis, we conclude that the hybridization between Ti3d and O2p plays primary role in the generation of spontaneous polarization and ferroelectricity.The lattice contributions to dielectric and piezoelectric tensor coming from individual atoms and individual modes are explored. Detailed analysis shows that for the BTO/STO (1:1) superlattice with Ba/Sr [001] ordering, the ε22and e26are mainly due to the A" phonons with ωλ=197and146cm-1, while the A" phonons with ωλ=97cm-1also make relatively large contributions. The ε33and e33are mainly due to the A phonons with ωλ=189and139cm-1. The ε11and e11are mainly due to the A phonons with ωλ;=246cm-1. For the BTO/STO (1:1) superlattice with Ba/Sr [110] ordering, the ε22and e26are mainly due to the B1phonons with ωλ=29cm-1. For the BTO/STO (1:1) superlattice with Ba/Sr [111] ordering, the ε11, e15, ε22and e26are mainly due to the E phonons with ωλ=154,103and97cm-2. The ε33and e33are mainly due to the A1phonons with ωλ=199and116cm-1, while the A1phonons with ωλ=97cm-1also afford relatively large contributions. On the other hand, the O and Ti atoms provide extraordinary large contribution to the lattice dielectric and piezoelectric responses.