Abstract
The size trend for the pressure-inducedγ-Fe2O3 (maghemite)to α-Fe2O3 (haematite) structural phase transition in nanocrystals has been observed. The transition pressurewas found to increase with decreasing nanocrystal size: 7 nm nanocrystals transformed at27 ± 2 GPa, 5 nm ones at34 ± 3 GPa and 3 nm ones at37 ± 2 GPa. Annealing of abulk sample of γ-Fe2O3 was found to reduce the transition pressure from35 ± 2 to24 ± 2 GPa. The bulk moduluswas determined as 262 ± 6 GPa for7 nm nanocrystals of γ-Fe2O3, which is significantly higher than the value of190 ± 6 GPa that we measured for bulk samples. Forα-Fe2O3, the bulk modulifor 7 nm nanocrystals (336 ± 5) and the bulk (300 ± 30) were found to be almost the same within error. The bulk modulus for theγ phase was found to decrease with decreasing particle size between 10 and 3.2 nm particlesize. Values for the ambient pressure molar volume were found within 1% to be:33.0 cm3 mol−1 forbulk γ-Fe2O3;32.8 cm3 mol−1 for 7 nmdiameter γ-Fe2O3 nanocrystals; 30.7 cm3 mol−1 for bulk α-Fe2O3; and 30.6 cm3 mol−1 for α-Fe2O3 nanocrystals.
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