Size dependence of the pressure-induced γ to α structural phase transition in iron oxide nanocrystals

The size trend for the pressure-induced gamma-Fe2O3 (maghemite) to alpha-Fe2O3 (haematite) structural phase transition in nanocrystals has been observed. The transition pressure was found to increase with decreasing nanocrystal size: 7 nm nanocrystals transformed at 27 +/- 2 GPa, 5 nm ones at 34 +/- 3 GPa and 3 nm ones at 37 +/- 2 GPa. Annealing of a bulk sample of gamma-Fe2O3 was found to reduce the transition pressure from 35 +/- 2 to 24 +/- 2 GPa. The bulk modulus was determined as 262 +/- 6 GPa for 7 nm nanocrystals of gamma-Fe2O3, which is significantly higher than the value of 190 +/- 6 GPa that we measured for bulk samples. For alpha-Fe2O3, the bulk moduli for 7 nm nanocrystals (336 +/- 5) and the bulk (300 +/- 30) 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% to be: 33.0 cm(3) mol(-1) for bulk gamma-Fe2O3; 32.8 cm(3) Mol(-1) for 7 nm diameter gamma-Fe2O3 nanocrystals; 30.7 cm(3) mol(-1) for bulk alpha-Fe2O3; and 30.6 cm(3) mol(-1) for alpha-Fe2O3 nanocrystals.