A new mechanism of anionic substitution in fluoride borates

Abstract

A comprehensive study of the BaF2–Ba3(BO3)2 phase diagram has revealed a significant difference between the two intermediate phases Ba5(BO3)3F and Ba7(BO3)4−y F2+3y . The latter exhibited (BO3)3− ↔ 3F− anionic substitution which, unusually, strongly influences the solidus temperature. A comparison of the Ba5(BO3)3F and Ba7(BO3)4−y F2+3y crystal structures, along with consideration of other compounds demonstrating (BO3)3− ↔ 3F− isomorphism, allows for the disclosure of the mechanism of (BO3)3− ↔ 3F− heterovalent anionic substitution in fluoride borates via [(BO3)F]4− tetrahedral groups being replaced by four fluoride anions. No exception to this mechanism has been discovered among all known phases with (BO3)3− ↔ 3F− substitution.