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Cation-, Dipole-, and Spin-Order in Perovskite Oxides

Xiaoli Tan
Department of Materials Science and Engineering, Iowa State University, Ames, IA50011, USA
Ceramics are the highest volume and tonnage materials produced and used by humankind. Among all the ceramic structures, the ABX3 perovskite is the most versatile structure that produces by far the most inorganic compounds with completely different functions including dielectric insulators, ferroelectrics and piezoelectrics, high-Tc superconductors, giant magnetoresistors and magnetoelectric multiferroics. This talk will be focused on ferroelectric and multiferroic perovskite oxides. When two or more cation species occupy the B-site on the ABO3 structure, there is a tendency for the cation species to sit on different B-site sublattices. This leads to the formation of chemically ordered perovskite oxides and most of time, the doubling of the unit cell. It has been speculated that the cation ordering directly influences the dielectric and ferroelectric properties of these compounds.
In this talk, I will present our results on the correlations of cation order with electrical dipole order in Pb(Mg1/3Nb2/3)O3-based ferroelectric oxides and with electron spin order in Pb(Fe2/3W1/3)O3-based multiferroic compounds. The degree of B-site cation order in both Pb(Mg1/3Nb2/3)O3 and Pb(Fe2/3W1/3)O3 is extremely weak and the ordered domains (<5nm) do not change with long time annealing at any high temperature. With combined chemical substitution and extended heat treatment, we successfully enhanced the cation order in Pb(Mg1/3Nb2/3)O3. The ordered Pb(Mg1/3Nb2/3)O3 ceramics with different dopants, however, show distinct dielectric and ferroelectric behavior. In situ TEM technique was used to investigate the interactions of cation ordered domains and electrical dipole ordered domains.
For the Pb(Fe2/3W1/3)O3 compound, long range B-site cation order is developed by incorporating Sc3+ cations into the compound. Initial results have demonstrated a direct correlation between the degree of cation order and the saturation magnetization. To further optimize the magnetic and ferroelectric properties, compromises were made to reach subtle balances between cation-, dipole-, and spin-order in this compound.