The influence of residual gas pressure and of deposition rate on the refractive index of Na3AlF6 and ThF4 films was investigated. Transition from high to medium deposition rates causes a distinct decrease of the refractive index, but a further reduction of the deposition rate gives practically no additional change. Increasing the residual gas pressure from 2 or 4 × 10 -6 to 2 × 10 -5 torr has no influence on the refractive Index although at 2 × 10 -4 torr a lower value was measured. The refractive indices of Na3AlF6, ThF4, and ZnSe coatings deposited on Ag and Al films were found to be essentially the same as those obtained on dielectric multilayers.
Walter Heitmann, "The Influence of Various Parameters on the Refractive Index of Evaporated Dielectric Films," Appl. Opt. 7, 1541-1544 (1968)