Epitaxial chromium oxide (alpha-CrA) films grown by atomic layer deposition at 375 degrees C from CrO2Cl2 and CH3OH on (I T 0 2) oriented alpha-Al2O3 have been studied by reflection high-energy electron diffraction (RHEED), X-ray diffraction (XRD) and X-ray reflection (XRR). The thickness of the films ranged from 10 to 3 10 nm, and the average growth rate was 0.1 nm per deposition cycle. According to the XRD analysis, the orientation relationship in thinner films was (1102)[11O]Cr2O3 vertical bar vertical bar(1102)[110]Al2O3. Confirmed by the RHEED and XRD analyses, (T 10 2) became the preferred growth plane at the thicknesses above 40 nm. This change has been interpreted as the appearance of an asymmetric rhombohedral twin with the orientation relationship between the layers (1102)[110]top vertical bar vertical bar(1102)[110]bottom and (1102)[111]top I I (I T 0 2)[T 1 1]bottom. The match of the anion and cation sublattices of both layers was characterized in terms of the structural model of the twin interface. (c) 2006 Elsevier B.V. All rights reserved.