Size dependence of the pressure-induced γ to α structural transition in iron oxide nanocrystals S.M. Clark* 1 , S.G. Prilliman* 2 , C.K. Erdonmez 2 , J. Rockenberger 2 , D.J. Zaziski 2 , J. Kwong 2 , and A.P. Alivisatos 2 1 Advanced Light Source, Lawrence Berkeley National Laboratory, MS6R2100, Berkeley, CA 94720 2 Department of Chemistry, University of California, Berkeley, CA 94720 and Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 * These authors contributed equally The size trend for the pressure-induced γ-Fe 2 O 3 (maghemite) to α-Fe 2 O 3 (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 27GPa, 5 nm at 34GPa and 3 nm at 37GPa. Comparison with literature data on larger γ-Fe 2 O 3 nanocrystals suggests that the effect saturates at a particle size of roughly 7 nm, indicating a structural or electronic transition at that size. Annealing of a bulk sample of γ-Fe 2 O 3 was found to greatly reduce the transition pressure from 35 to 24GPa. The bulk modulus was determined to be ~ 260GPa for 7 nm nanocrystals of γ-Fe 2 O 3 , which is significantly higher than for the value of 190GPa we measured for bulk samples. For α-Fe 2 O 3 , the bulk moduli for 7 nm nanocrystals and bulk were almost the same within error. The bulk modulus for the γ 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 cc/mol for bulk γ-Fe 2 O 3 , 32.8 cc/mol for 7 nm diameter γ-Fe 2 O 3 nanocrystals, 30.7 cc/mol for bulk α-Fe 2 O 3 and 30.6 cc/mol for α- Fe 2 O 3 nanocrystals. As the size of γ-Fe 2 O 3 nanocrystals is decreased, the onset of the softening of the lattice precedes the onset of the change in transition pressure. The change in transition pressure is argued to be a result of the lattice softening. PACS numbers 61.10.-I – X-ray diffraction and scattering 62.50.+p – High-pressure and shock wave effects in solids and liquids 64.70.Kb – Solid-solid transitions I. INTRODUCTION The properties of nanoscale materials can be quite different from bulk 1 . Some properties have been found to vary continuously with particle size, the most famous example being the reduction in melting temperature in smaller sizes 2 . The reduction is driven by the fact that the liquid generally has a lower surface energy than the solid, and is observed almost universally (although