Infrared absorption in NiO microcrystals

Infrared spectra in the 350-650 ${\mathrm{cm}}^{-1\mathrm{}}$ region of different samples of NiO in a powder of submicronic particles have been analyzed in terms of bulk and surface modes and connected to the size and shape of the particles and to solvent effects. Two surface modes observed for cubic particles in air at 510 and 563 ${\mathrm{cm}}^{-1\mathrm{}}$ were interpreted with the cubic shape model, excluding the usual spheric model. For cubes of 1000 Å side an important absorption due to the bulk mode ${\omega }_T$ is observed. For very small particles (∼ 100 Å) an aggregation effect produces a noticeable absorption at 475 ${\mathrm{cm}}^{-1\mathrm{}}$ (in air).