Polar catastrophe and the structure of KT a 1 − x N b x O 3 surfaces: Results from elastic and inelastic helium atom scattering

In this paper, the structure and dynamics of cleaved (001) surfaces of potassium tantalates doped with niobium, KTa1-xNbxO3 (KTN), with x ranging from 0% to 30%, were measured by helium atom scattering (HAS). Through HAS time-of-flight (TOF) experiments, a dispersionless branch (Einstein phonon branch) with energy of 13-14meV was observed across the surface Brillouin zone in all samples. When this observation is combined with the results from earlier experimental and theoretical studies on these materials, a consistent picture of the stable surface structure emerges: After cleaving the single-crystal sample, the surface should be composed of equal areas of KO and TaO2/NbO2 terraces. The data, however, suggest that K+ and O2- ions migrate from the bulk to the surface, forming a charged KO lattice that is neutralized primarily by additional K+ ions bridging pairs of surface oxygens. This structural and dynamic modification at the (001) surface of KTN appears due to its formally charged KO(-1) and TaO2/NbO2(+1) layers and avoids a “polar catastrophe.” Finally, this behavior is contrasted with the (001) surface behavior of the fluoride perovskite KMnF3 with its electrically neutral KF and MnF2 layers.