Enhanced bulk modulus and reduced transition pressure in γ-Fe ₂ O ₃ nanocrystals

Abstract

Synchrotron radiation X-ray diffraction studies of γ-Fe2O3 have been performed with emphasis on the pressure-induced phase transformation γ(maghemite) → α(hematite) and the equation of state of the nanophase material. For γ-Fe2O3 the bulk modulus has been found to increase from 203 GPa for the bulk material to 305 GPa for 9 nm size crystals. At the same time the transition pressure decreases from 35 to 27 GPa. The reduced transition pressure is explained in terms of nucleation and growth, the larger volume change upon transition in the nanocrystalline material being the main factor.

Pressure- and temperature-dependent X-ray diffraction studies of NdCrO3

“…Neodymium is substantially less compressible than other lanthanide cations and the larger bulk modulus of NdFeO 3 compared to GdFeO 3 demonstrates the influence of Nd 3+ . Also, the binary oxide Cr 2 O 3 has a bulk modulus of 240(8) GPa [41] whereas Fe 2 O 3 has a bulk modulus of 203 GPa [42]. The large value of the bulk modulus of NdCrO 3 compared to other simple perovskites indicates it is one of the least compressible simple perovskites yet determined.…”

Phase transformation and conductivity in nanocrystal PbS under pressure

“…Similar behavior of the resistivity of the B16 phase versus pressure was also detected in nanocrystal PbS. The data obtained from a pressure range of 4-14 GPa for bulk PbS and of [8][9][10][11][12][13][14][15][16][17][18] GPa for nanocrystal PbS were fitted by d(ln R)/dPϭC, where C is constant. It found CϭϪ0.64 GPa Ϫ1 for bulk PbS and CϭϪ0.34 GPa Ϫ1 for nanocrystal PbS, as is shown by the solid lines in Fig.…”

“…8 compared with corresponding bulk materials. However, Jiang et al 9 reported that for nanometer-sized ␥-Fe 2 O 3 particles the phase transition pressure ͑from ␥-Fe 2 O 3 to ␣-Fe 2 O 3 ͒ is lower than that for bulk material. They suggested that the larger volume change upon transition in the nanocrystals is the main factor in this case.…”

Grain-size effect on pressure-induced semiconductor-to-metal transition in ZnS

“…7 Qadri et al 8 have also reported that the effect of reduced particle size in PbS nanocrystals is to elevate the transition pressure while the compressibility increases with decreasing particle size. However, Jiang et al 9 reported that for nanometer-sized ␥-Fe 2 O 3 particles the phase transition pressure ͑from ␥-Fe 2 O 3 to ␣-Fe 2 O 3 ) is lower than for bulk material. A similar result was also reported in TiO 2 for the rutile-to-␣-PbO 2 transition.…”