Abstract
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 timeofflight
(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 singlecrystal
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.” This behavior is
contrasted with the (001) surface behavior of the fluoride perovskite KMnF3 with its electrically neutral KF and
MnF2 layers.