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

Ram Seshadri

doi:10.1016/j.solidstatesciences.2006.02.020

paper

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

arXiv:cond-mat/0507634 · doi:10.1016/j.solidstatesciences.2006.02.020

Abstract

Pyrochlore dielectric materials such as (Bi $_{1.5}$ Zn $_{0.5}$ )(Nb $_{1.5}$ Zn $_{0.5}$ )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 low-temperature preparation and structural characterization of Bi $_{2}$ Ti $_{2}$ O $_{7}$ , 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 Bi $_{2}$ Ti $_{2}$ O $_{7}$ is visualized and compared with the electronic structure of the Aurivillius phase ferroelectric SrBi $_{2}$ Ta $_{2}$ O $_{9}$ (SBT), which displays a ferroelectric distortion below 608 K associated with the tendency of lone pair active Bi $^{3 +}$ to move off-center. Such coherent off-centering distortions are frustrated on the pyrochlore lattice, and this prevents a ferroelectric-paraelectric phase transition in Bi $_{2}$ Ti $_{2}$ O $_{7}$ . Instead, Bi $^{3 +}$ ions in Bi $_{2}$ Ti $_{2}$ O $_{7}$ are obliged to off-center in an \textit{incoherent} manner, that is compatible with the cubic structure being retained. Frustrated lone pair behavior in the defect pyrochlore Pb $_{2}$ Sn $_{2}$ O $_{6}$ is also described. Parallels between the well-studied frustration of certain types of \textit{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.

15 pages in with color figures

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

Abstract

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Cited by in corpus (12)

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

Abstract

References in corpus (4)

Cited by in corpus (12)

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