Refractive index dispersion and anisotropy in NaGd(WO₄)₂ single crystal

Refractive index characteristics were measured in the tetragonal, scheelite-like, NaGd(WO₄)₂ (NGW) single crystal. The room temperature ordinary refractive-index dispersion was measured by the minimum-deviation method between 320 and 1342 nm. The refractive-index anisotropy was measured between 400 and 1860 nm using a combination of bireflection in an oriented single crystal prism, and birefringence modulation in an oriented plane-parallel plate. The crystal is positive uniaxial, with its optic axis parallel to the crystallographic c-axis. The dispersion was fitted to a simple two-parameter Sellemeier function. The anisotropy dispersion was also fitted to an empiric function. Taking the 632.8 nm wavelength (the He-Ne laser line) as a representative of the visible range, the ordinary refractive index is n_o = 1.989(1), and the anisotropy (n_e - n_o) = 0.00342(1). The refractive indices' changes with temperature were measured between room temperature and ∼200 °C at λ = 632.8 nm, yielding dn_o/dT = - 10.70 × 10^-5 °C^-1 and dn_e/dT = - 4.17 × 10^-5 °C^-1.