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P. Lackner, Z. Zou, S. Mayr, J. Choi, U. Diebold, M. Schmid:
"Surface structures of ZrO₂ films on Rh(111): From two layers to bulk termination";
Surface Science, 679 (2019), 180 - 187.

We have studied zirconia films on a Rh(111) substrate with thicknesses in the range of 2-10 monolayers (ML)
using scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). Zirconia was deposited
using a UHV-compatible sputter source, resulting in layer-by-layer growth and good uniformity of the films. For
thicknesses of 2-4 ML, a layer-dependent influence of the substrate on the structure of the thin films is observed.
Above this thickness, films show a (2×1) or a distorted (2×2) surface structure with respect to cubic
ZrO2(111); these structures correspond to tetragonal and monoclinic zirconia, respectively. The tetragonal phase
occurs for annealing temperatures of up to 730 °C; transformation to the thermodynamically stable monoclinic
phase occurs after annealing at 850 °C or above. High-temperature annealing also breaks up the films and exposes
the Rh(111) substrate. We argue that the tetragonal films are stabilized by the interface to the substrate
and possibly oxygen deficiency, while the monoclinic films are only weakly defective and show band bending at
defects and grain boundaries. This observation is in agreement with positive charge being responsible for the
grain-boundary blocking effect in zirconia-based solid electrolytes. Our work introduces the tetragonal and
monoclinic 5 ML-thick ZrO2 films on Rh(111) as a well-suited model system for surface-science studies on ZrO2,
as they do not exhibit the charging problems of thicker films or the bulk material and show better homogeneity
and stability than the previously-studied ZrO2/Pt(111) system.

Zirconium oxide Monoclinic and tetragonal zirconia Thin film growth Phase transformation Scanning tunneling microscopy Low-energy electron diffraction