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The phase stability of Mn+1AXn phase (M: early transition metal, A: A-group element, and X: C and/or N) Ti2AlN thin films reactively sputtered onto MgO(111) and Al2O3(0001) substrates has been investigated by in situ x-ray diffraction and Rutherford backscattering. High substrate temperature deposition results in epitaxial Ti2AlN growth with basal planes parallel to the substrate surface. In contrast to reported high thermal stability for bulk Ti–Al–N Mn+1AXn phases in air, Ti2AlN thin films in vacuum decompose already at ∼800 °C. The decomposition proceeds by outward Al diffusion and evaporation, followed by detwinning of the as-formed Ti2N atomic layers into cubic TiNx and intermediate phases. [ABSTRACT FROM AUTHOR]