Size dependence of the pressure-induced gamma to alpha structural transition in iron oxide nanocrystals

The size trend for the pressure-induced gamma-Fe2O3(maghemite) to alpha-Fe2O3 (hematite) structural phase transition in nanocrystals has been observed. The transition pressure was found to increase with decreasing nanocrystal size: 7 nm nanocrystals transformed at 272GPa, 5 nm at 343GPa and 3 nm at 372GPa. Annealing of a bulk sample of gamma-Fe2O3 was found to reduce the transition pressure from 352 to242GPa. The bulk modulus was determined to be 2626GPa for 7 nm nanocrystals of gamma-Fe2O3, which is significantly higher than for the value of 1906 GPa that we measured for bulk samples. For alpha-Fe2O3, the bulk moduli for 7 nm nanocrystals (3365) and bulk (30030) were found to be almost the same within error. The bulk modulus for the gamma phase was found to decrease with decreasing particle size between 10 and 3.2 nm particle size. Values for the ambient pressure molar volume were found within 1 percent to be: 33.0 cm3/mol for bulk gamma-Fe2O3, 32.8 cm3/mol for 7 nm diameter gamma-Fe2O3 nanocrystals, 30.7 cm3/mol for bulk alpha-Fe2O3 and 30.6 cm3/mol for alpha-Fe2O3 nanocrystals.