First-order Raman scattering in MgO microcrystals

Raman scattering is observed in MgO microcrystals with three different particle sizes (105, 103, and 102 Å). The second-order Raman lines in the bulk crystals of MgO disappear in all the microcrystals, while new peaks emerge at 280, 446, and 1088 cm−1 except for specimen prepared by grinding a large single crystal. Raman lines with lower frequencies, which are enhanced with decrease of the particle size, are discussed as due to the first-order Raman scattering forbidden in the bulk crystals. The assignment is confirmed in terms of photon density of states in microcrystals. Another line with the highest frequency is coincident with a previously reported Raman line whose origin has been attributed to a surface phonon mode.