Ultraviolet to infrared refractive indices of tetragonal double tungstate and double molybdate laser crystals

Sellmeier parameters of thirteen tetragonal (space group \(I\overline{4}\)) double tungstate and double molybdate laser crystals with M+T3+(X6+O4)2 composition have been calculated using the room temperature refractive indices determined from the ultraviolet band gap of the crystals to λ≈2 μm. All considered crystals are uniaxial but only crystals with Bi in their composition show a significant birefringence (Δn>10−2). The refractive index value increases for the sequence T3+=La, Y, Gd, Lu, and Bi independently of the M+ and X6+ cation pair. Implications for the design of laser waveguides and laser pulse dispersion are discussed.