Pb8M(BO3)6 (M = Mg, Ca): Two new borates with large birefringence activated by the ∞[Pb8B6O18]2- double layers

The substitution strategy based on theory of acid and alkali was adopted and we successfully designed Pb8Mg(BO3)6 and Pb8Ca(BO3)6 with calculated birefringence equal to 0.12 and 0.15@1064 nm, respectively.522