Tin-, titanium-, and magnesium-doped α-Cr₂O₃: Characterisation and rationalisation of the structures

Tin- and titanium-doped α-Cr₂O₃ have been prepared by the calcination of precipitates. Rietveld structure refinement of the X-ray powder diffraction patterns shows that the dopant metals occupy interstitial sites and partially substitute on octahedral chromium sites in the corundum-related α-Cr₂O₃ structure. Interatomic potential calculations show that such defects are energetically more favourable than the exclusive substitution of chromium or the occupation by the dopant of interstitial sites in α-Cr₂O₃ with charge balance being achieved by an appropriate number of cation vacancies. However, the attempted incorporation of magnesium within α-Cr₂O₃ results in the formation of spinel-related MgCr₂O₄. The observation is rationalised in terms of the greater thermodynamic stability of the spinel-related structure.