Compensating a polar surface An ionic crystal surface can be electrostatically unstable, and the surface must reconstruct in some way to avoid this “polar catastrophe.” Setvin et al. used scanning probe microscopies and density functional theory to study the changes in the polar surface of the perovskite KTaO ₃ . They observed several structural reconstructions as the surface cleaved in vacuum was heated to higher temperatures. These ranged from surface distortions to the formation of oxygen vacancies to the development of KO and TaO ₂ stripes. Hydroxylation after exposure to water vapor also stabilized the surface. Science , this issue p. 572