Polar catastrophe and the structure ofKTa1−xNbxO3surfaces: Results from elastic and inelastic helium atom scattering
| Autor: | Rifat Fatema, Sanford A. Safron, Lynn A. Boatner, T. W. Trelenberg, J. A. Li, F. A. Flaherty, J. G. Skofronick, D. H. Van Winkle |
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| Rok vydání: | 2015 |
| Předmět: | |
| Zdroj: | Physical Review B. 92 |
| ISSN: | 1550-235X 1098-0121 |
| DOI: | 10.1103/physrevb.92.035414 |
| Popis: | The structure and dynamics of cleaved $(001)$ surfaces of potassium tantalates doped with niobium, $\mathrm{KT}{\mathrm{a}}_{1\ensuremath{-}x}\mathrm{N}{\mathrm{b}}_{x}{\mathrm{O}}_{3}$ (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 $13\ensuremath{-}14\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$ 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 $\mathrm{Ta}{\mathrm{O}}_{2}/\mathrm{Nb}{\mathrm{O}}_{2}$ terraces. The data, however, suggest that ${\mathrm{K}}^{+}$ and ${\mathrm{O}}^{2\ensuremath{-}}$ ions migrate from the bulk to the surface, forming a charged KO lattice that is neutralized primarily by additional ${\mathrm{K}}^{+}$ ions bridging pairs of surface oxygens. This structural and dynamic modification at the $(001)$ surface of KTN appears due to its formally charged $\mathrm{KO}(\ensuremath{-}1)$ and $\mathrm{Ta}{\mathrm{O}}_{2}/\mathrm{Nb}{\mathrm{O}}_{2}(+1)$ layers and avoids a ``polar catastrophe.'' This behavior is contrasted with the $(001)$ surface behavior of the fluoride perovskite $\mathrm{KMn}{\mathrm{F}}_{3}$ with its electrically neutral KF and $\mathrm{Mn}{\mathrm{F}}_{2}$ layers. |
| Databáze: | OpenAIRE |
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