Lone pairs in insulating pyrochlores:: Ice rules and high-k behavior

Pyrochlore dielectric materials Such as (Bi1.5Zn0.5)(Nb1.5Zn0.5)O-7 (BZN) have generated interest because they combine high dielectric constants with small dielectric loss tangents and yet are cubic at all temperatures. The recent tow-temperature preparation and structural characterization of Bi2Ti2O7, which remains cubic down to 2 K, has provided a good model system for understanding the properties of Bi-based pyrochlores. In this contribution, the electronic structure of cubic Bi2Ti2O7 is visualized and compared with the electronic structure of the An-rivillius phase ferroelectric SrBi2Ta2O9 (SBT), which displays a ferroelectric distortion below 608 K associated with the tendency of lone pair active Bi3+ to move off-center. Such coherent off-centering distortions are frustrated on the pyrochlore lattice, and this prevents a ferroelectric-paraelectric phase transition in Bi2Ti2O7. Instead, Bi3+ ions in Bi2Ti2O7 are obliged to off-center in all incoherent manner, that is compatible with the Cubic structure being retained. Frustrated lone pair behavior in the defect pyrochlore Pb2Sn2O6 is also described. Parallels between the well-studied frustration of certain types of magnetism in pyrochlore compounds (spin-ice) and the striking paucity of ferroelectric pyrochlores, arising from the corner-connected tetrahedral topology of the pyrochlore lattice are pointed Out. (c) 2006 Elsevier SAS. All rights reserved.