In the transparent region ( > 500 nm), the refractive index of a film (nf) can be estimated from the minima in the transmittance spectra. For CeO2 films (Fig. 3a), nf = 1.82 at = 611 nm. The bulk refractive index (nb) of cerium oxide is 2.5 [6]. The variation of the film density (f) with the refractive index is given by the Lorentz–Lorenz law [13]: