Synthesis, structure, and properties of perovskite materials for multiferroism and piezoelectricity

The structure and properties of the perovskite multiferroic (1-x)BiFeO3-xPbTiO3 ceramics prepared by solid state reaction and single crystals grown from high temperature solution have been systematically investigated. The x-ray phase analysis revealed the existence of a new phase with orthorhombic symmetry, together with the rhombohedral and the tetragonal phases, in the morphotropic phase boundary (MPB) region of this solid solution, which agrees with the domain patterns observed by piezoelectric force microcopy (PFM) on the single crystal. The MPB structure of this system has been updated. The electric study of the (1-x)BiFeO3-xPbTiO3 ceramics by means of impedance spectroscopy suggested that the electric conduction mechanism in this system can be attributed mainly to the hopping of electrons from Fe2+ to Fe3+ through oxygen vacancies. Based on this proposed mechanism, improved dielectric and ferroelectric properties have been realized in BiFe1-xTixO3+x/2-PbTiO3 ceramics via B site aliovalent ionic substitution of Ti4+ for Fe3+.which effectively decreases the oxygen vacancies. The magnetic properties of the (1-x)BiFeO3-xPbTiO3 ceramics were systematically studied. In addition to the antiferromagnetic ordering arising from the superexchange of Fe3+-O-Fe3+ in chemically disordered regions, a magnetic ordering with weak ferromagnetism has been found in the solid solution with composition x around 0.5 at a lower temperature TC, which is attributed to the long range superexchange through the pathway of Fe3+ƒ{Oƒ{Tiƒ{Oƒ{Fe3+ in the chemically ordered micro-regions with a ¡§super G-type¡¨ magnetic structure. Below TC, the materials show significantly enhanced magnetization from that of bulk BiFeO3. The unusual dependence of TC on composition is explained based on the competition between the effect of the concentration of chemically ordered micro-regions and that of the coupling distance. The magnetic phase diagram of this system has been established. We have also investigated the (1-x-y)Pb(Yb1/2Nb1/2)O3-xPbZrO3-yPbTiO3 ternary system as new high temperature high performance piezoelectric materials. The room temperature MPB phase diagram of the solid solution has been established by x-ray phase analysis. The excellent piezoelectric performance (d33=1247pC/N), and the high TC (370 oC) of the ceramics with MPB composition shows the potential for high temperature transduction, actuation, and micro-electro-mechanical system (MEMS) applications.