Compressibility of nanostructured alumina phases determined from synchrotron x-ray diffraction studies at high pressure

Synchrotron x-ray diffraction studies up to pressures of 30 GPa have been performed on both nanostructured γ-Al₂O₃ and α-Al₂O₃. The P-V equation of state has been calculated from a fit to the diffraction data to obtain the compressibility for each of the compounds. The bulk modulus B₀ of nanostructured γ-Al₂O₃ having grain-size diameters in the range 4 - 8 nm is B₀ =152(8) GPa, appreciably lower than the value B_0≈250 GPa of its bulk structural analog. The bulk-modulus of nanophase α-Al₂O₃ of 25 nm average crystallite diameter, which is difficult to stabilize at smaller dimensions because of thermodynamic considerations, is 252(9) GPa and therefore similar to that of bulk α-Al₂O₃. These studies suggest that only if the grain-size of alumina is below a critical value of several nanometers can substantial changes be expected in the isothermal bulk modulus compared to that of the isostructural bulk compound.