First-principles And Experimental Study Of The Structure And Polarization Of Barium Strontium Titanate

Due to the highly nonlinear dielectric response to an applied electric field, moderatedielectric constant and low dielectric loss in microwave domain, barium strontium titanate(Ba_xSr_1-xTiO₃, BST) is a promising material for electric tunable devices for microwaveapplications, such as tunable dielectric filters and phase shifters. BST is chosen as researchobject in this dissertation. The structure and polarization of BST were investigated byfirst-principles calculations based on density-functional theory. Furthermore, thepolarization and domain structure of BST were studied by piezoresponse force microscopy（PFM） and transmission electron microscop （TEM）.Firstly, the first-principles calculations of the five different BST ordered structureswere performed. It is found that the lattice constants show a linearly increase with x, whichis consistent with Vegard’s law. The off-center displacement of Ti atoms and O atomsresults in the distortion of Ti-O octahedron, and four pairs antiparallel spontaneouspolarization induced in Ba_xSr_1-xTiO₃（x=0.125,0.875）. Therefore this two equilibriumordered structures are both in cubic antiferroelectric phase.The analyses of the density of states and electron density difference shows that thehybridization between Ti3d and O2p is stronger than that between Ba （or Sr） and O.Especially in the high energy region near valence band maximum and conduction bandminimum. The interactions of Ba-O and Sr-O are ionic, while the Ti-O bond is covalent inBST. Based on above analysis, it concludes that the hybridization between Ti3d and O2pplays primary role in the generation of spontaneous polarization and ferroelectricity.Secondly, every three different ordered structures of Ba_0.25Sr_0.75TiO₃, Ba0.5Sr0.5TiO₃and Ba_0.75Sr_0.25TiO₃were constructed, and nine in total. The results of the first-principlescalculations of the nine different BST ordered structures indicate that all of the equilibriumordered structures of BST0.25<010>, BST0.25<011>, BST0.5{100}, BST0.75<010> andBST0.75<011> are in tetragonal antiferroelectric phase. And the BST0.5{110} orderedstructure is in tetragonal paraelectric phase, all of the ordered structures of BST0.25<111>,BST0.5{111} and BST0.75<111> are in cubic paraelectric phase. The results of the presentwork implied that the local order of A site cations significantly influence the structuralphase transition of disordered BST solid solution.Thirdly, based on the three different ordered structures of Ba_0.5Sr_0.5TiO₃, namely BST{100}, BST{110} and BST{111}. An off-center displacement of Ti atoms along thefour fold axis of Ti-O octahedron introduce into BST by0.05in fractional coordinate. Andthen the first-principles calculations of the five different structures with the off-centerdisplacement of Ti atoms were run. All the three structures BST{100}, BST{110} andBST{111} with Ti atoms respectively moving along the x, z and z are in tetragonalantiferroelectric phase. And the other two structures BST{100} and BST{110} with Tiatoms respectively moving along the z and x are in orthorhombic antiferroelectric phase. Inconclusion, the off-center displacement of Ti atoms could cause the distortion of BST latticsand Ti-O octahedron, and the structure phase transition of BST occurs.Lastly, The character of the domain structures in Ba_xSr_1-xTiO₃（x=0.3,0.6,0.9）ceramics prepared by the solid-state reaction was investigated by piezoresponse forcemicroscopy （PFM） and transmission electron microscopy （TEM）. The PFM results indicatethat the domains could be induced in paraelectric phase Ba0.6Sr0.4TiO₃under dc voltageapplied between the tip and the bottom electrode. And when the Curie temperature of BSTis higher the induction of the domains is easier. The polarization reversal occurs inBa0.9Sr0.1TiO₃after the application of reverse dc bias voltage. Furthermore, the patterns ofdomains in BST could change with the value of the dc bias voltage. And the dynamicdevelopment of domains in BST would cause the nonlinear dielectric properties ofparaelectric phase BST ceramic.