LiNH4SO4: A new crystal with an isotropic point

Spectral dependences of the refractive and birefringence indices of α-LiNH4SO4 crystals have been investigated. Curves ni(λ) are found to intersect, which indicates the existence of an isotropic point (Δny = 0) for this crystal; at room temperature, this point corresponds to wavelength λ0 ≈ 683 nm. The isotropic point is blue-shifted upon heating. Temperature measurements of the angle between the optical axes indicate that the optical-axis plane changes orientation when passing through the isotropic state. The results obtained give grounds to consider LiNH4SO4 as a new crystal with an isotropic point.