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We review the concept of surface charge, first in the context of thepolarization in ferroelectric materials, and second in the context of layers ofcharged ions in ionic insulators. While the former is traditionally discussedin the ferroelectrics community, and the latter in the surface sciencecommunity, we remind the reader that the two descriptions are convenientlyunified within the modern theory of polarization. In both cases, the surfacecharge leads to electrostatic instability - the so-called "polar catastrophe" -if it is not compensated, and we review the range of phenomena that arise as aresult of different compensation mechanisms. We illustrate these concepts usingthe example of the prototypical multiferroic bismuth ferrite, BiFeO3, which isunusual in that its spontaneous ferroelectric polarization and its layercharges can be of the same magnitude. As a result, for certain combinations ofpolarization orientation and surface terminationits surface charge isself-compensating. We use density functional calculations of BiFeO3 slabs andsuperlattices, analysis of high-resolution transmission electron micrographs aswell as examples from the literature to explore the consequences of thispeculiarity.

Layer and spontaneous polarizations in perovskite oxides and their interplay in multiferroic bismuth ferrite