Infrared to vacuum-ultraviolet ellipsometry studies of spinel (MgAl2O4)

The dielectric function and the loss function for spinel (MgAl2O4) were determined using Fourier-transform infrared ellipsometry from 250 to 1000 cm− 1. We fit our data using two different dispersion models: (1) The Lorentz oscillator model describes the lattice optical response using a sum of independent classical harmonic oscillators with constant damping. (2) We also use a factorized oscillator model with independent broadening parameters for the transverse and longitudinal optical phonons. By fitting our data to these models, we determine the transverse and longitudinal optical phonon energies, their broadenings, and their amplitudes. The factorized model provides a better description of the data at high energies. The agreement is not so good for the lower-energy phonons, presumably due to broadenings caused by cation disorder. We also studied the Raman-active phonons by Raman spectroscopy. Using spectroscopic ellipsometry, we also determine the dispersion of the refractive index from 0.76 to 9.0 eV. Combining both data sets we find the high- and low-frequency dielectric constant. In the visible and ultraviolet region, the data are dominated by a Lorentz oscillator peak at 8.1 eV masked by surface roughness (13-16 Å̊).