High temperature stability of maghemite (γ‐Fe2O3)

Abstract

The temperature stability of acicular maghemite (γ‐Fe2O3) was studied between 20°C and 600°C by x‐ray diffraction and magnetic measurements. Synthetic monodomain maghemite was found to be stable with respect to hematite (α‐Fe2O3) at temperatures as high as 500°C, inverting to α‐Fe2O3 between 510‐660°C. However, a complete inversion of the γ to α form for the sample was not achieved even at 660°C, the final product was a two‐phase mixture, with 65% of the original γ‐Fe2O3 unchanged. Two conclusions are made: (a) the absence of maghemite in a natural sample cannot be proven if heating has been restricted to moderate temperatures (300°‐400°C) and (b) the true Curie point of pure maghemite is close to 645°C.