We report the discovery of a face-centered cubic (Al_1−xCr_x)₂O₃ solid solution [0.60bxb0.70] in films grownonto Si substrates using reactive radio frequency magnetron sputtering from Al and Cr targets at 400 °C. Theproposed structure is NaCl-like with 33% vacancies on the metal sites. The unit cell parameter is 4.04 Å asdetermined by X-ray diffraction. The films have a b100N preferred crystallographic orientation and exhibithardness values up to 26 GPa and an elastic modulus of 220–235 GPa.

Thermal stability of fcc-(Al_0.32Cr_0.68)₂O₃ thin film is investigated by ex-situ annealing of films in the temperature range of 500-1100 °C and annealing times of 2-10 h. X-ray diffraction indicates that the phase transformation from fcc to corundum crystal structure starts at 925 °C and completes at 1000 °C. This is further confirmed by high resolution transmission electron microscopy and selected area electron diffraction. The kinetics of phase transformation is analyzed by Johnson-Avrami-Mehl model. The phase transformation from fcc to corundum in (Al_0.32Cr_0.68)₂O₃ thin films is a two-stage thermally activated process. The first stage is phase transformation and has the apparent activation energy of 213±162 kJ/mol (0.44±0.34 eV/atom). This is followed by a second stage with a larger apparent activation energy of 945±27 kJ/mol (1.96±0.06 eV/atom). This is likely associated with the grain growth of a-phase upon completion of the phase transformation.