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

Pyrochlore dielectric materials such as (Bi1.5Zn0.5)(Nb1.5Zn0.5)O7 (BZN) have generated interest because they combine high dielectric constants with small dielectric loss tangents and yet are cubic at all temperatures. The recent low-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 Aurivillius 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 an 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.

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