Polar catastrophe and the structure of KTa sub(1-x) Nb sub(x) O sub(3) surfaces: Results from elastic and inelastic helium atom scattering

The structure and dynamics of cleaved (001) surfaces of potassium tantalates doped with niobium, KTa sub(1-x) Nb sub(x) O sub(3)(KTN), with x ranging from 0% to 30%, were measured by helium atom scattering (HAS). Through HAS time-off-light (TOF) experiments, a dispersionless branch (Einstein phonon branch) with energy of 13 - 14 meV 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 Ta O sub(2)/ Nb O sub(2) terraces. The data, however, suggest that K super(+) and O super(2-) ions migrate from the bulk to the surface, forming a charged KO lattice that is neutralized primarily by additional K super(+) 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 Ta O sub(2)/ Nb O sub(2) (+1) layers and avoids a "polar catastrophe." This behavior is contrasted with the (001) surface behavior of the fluoride perovskite KMn F sub(3) with its electrically neutral KF and Mn F sub(2) layers.