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M. Setvin, M. Reticcioli, F. Pötzleitner, J. Hulva, M. Schmid, L. Boatner, C. Franchini, U. Diebold:
"Polarity compensation mechanisms on the perovskite surface KTaO₃(001)";
Science, 359 (2018), 572 - 575.

The stacking of alternating charged planes in ionic crystals creates a diverging electrostatic
energy-a "polar catastrophe"-that must be compensated at the surface.We used scanning
probe microscopies and density functional theory to study compensation mechanisms
at the perovskite potassium tantalate (KTaO3) (001) surface as increasing degrees of freedom
were enabled.The as-cleaved surface in vacuum is frozen in place but immediately responds
with an insulator-to-metal transition and possibly ferroelectric lattice distortions. Annealing in
vacuum allows the formation of isolated oxygen vacancies, followed by a complete
rearrangement of the top layers into an ordered pattern of KO and TaO2 stripes.The optimal
solution is found after exposure to water vapor through the formation of a hydroxylated
overlayer with ideal geometry and charge.