The possibility of making multilayer reflecting coatings by using materials whose dielectric susceptibility is very near to unity at the frequency under consideration is discussed. For the pair of substances with a given Ree and Ime the explicit expressions are obtained for the value of reflection coefficient and for the optimum thickness of layers. The published data on Re∊ and Im∊ allowed us to choose the pairs of substances for which one may hope to create (in principle) the mirrors for normal incidence with R ~ 50% at λ ≳ 30 Å.
A. V. Vinogradov and B. Ya. Zeldovich, "X-ray and far uv multilayer mirrors: principles and possibilities," Appl. Opt. 16, 89-93 (1977)