Compared to mixed anion compounds containing halides, these are quite rare.
The reason for the difficulty of formation is that when entering a crystal lattice, the anions have to be correctly located, and correctly oriented.
Max birefringence: δ = 0.067 Max birefringence δ = 0.027 Uniaxial (+) nω = 1.805 - 1.810 nε = 2.015 - 2.020 Max Birefringence:δ = 0.210 β = 102.42° Z=2 Biaxial (-) nα=1.618 nβ=1.658 nγ=1.672
Max birefringence: δ = 0.054 α = 80.365°, β = 75.816°, γ = 59.870° Z=2
Max birefringence δ = 0.007 α = 83.295°, β = 83.308°, γ = 60.023° Z=1