Showing papers by "Leif Gerward published in 1998"

Enhanced bulk modulus and reduced transition pressure in gamma-Fe2O3 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.

High-pressure phase of the cubic spinel NiMn2O4

Abstract: It has been observed that the fcc spinel ${\mathrm{NiMn}}_{2}{\mathrm{O}}_{4}$ transforms to a tetragonal structure at about 12 GPa. The tetragonal phase does not revert to the cubic phase upon decompression and its unit-cell constants at ambient pressure are ${a}_{0}=8.65(8)$ and ${c}_{0}=7.88(15)\AA{}$ (distorted fcc). Within the experimental uncertainty, there is no volume change at the transition. The $c/a$ ratio of the tetragonal spinel is almost independent of pressure and equal to 0.91. The phase transition is attributed to the Jahn-Teller-type distortion and the ionic configuration can be assumed as ${(\mathrm{M}\mathrm{n}}^{3+}{)}_{\mathrm{tetr}}{[\mathrm{Ni}}^{2+}{\mathrm{Mn}}^{3+}{]}_{\mathrm{oct}}.$ The bulk modulus of the cubic phase is 206(4) GPa.