• anionic isomorphism;
  • fluoride borates;
  • fluorine–fluorine interaction

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−yF2+3y. The latter exhibited (BO3)3−[LEFT RIGHT ARROW] 3F anionic substitution which, unusually, strongly influences the solidus temperature. A comparison of the Ba5(BO3)3F and Ba7(BO3)4−yF2+3y crystal structures, along with consideration of other compounds demonstrating (BO3)3−[LEFT RIGHT ARROW] 3F isomorphism, allows for the disclosure of the mechanism of (BO3)3−[LEFT RIGHT ARROW] 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−[LEFT RIGHT ARROW] 3F substitution.