Infrared active phonon modes and ionicity of single crystal MgAl₂O₄

Near-normal incident infrared reflectivity spectra of (100) MgAl₂O₄ spinel single crystal have been measured at different temperatures in the frequency region between 50 and 6000cm^-1. Eight infrared-active phonon modes are identified, which are fitted with the factorized form of the dielectric function. The dielectric property and optical conductivity of the MgAl₂O₄ crystal are analysed. From TO/LO splitting, the effective Szigeti charges and Born effective charges at different temperatures are calculated for studying the ionicity and the effect of polarization. Based on the relationship between the (LO--TO)₁ splitting, which represents the transverse and longitudinal frequencies splitting of the highest energy phonon band in the reflectivity spectrum, and the ionic-covalent parameter, the four main phonon modes are assigned. MgAl₂O₄can be considered as a pure ionic crystal and its optical characters do not change with decreasing temperature, so it may be used as a suitable substrate for high-T_c superconducting thin films.