KBa7Mg2B14O28F5, a new borate with an unusual heptaborate group and double perovskite unit

A new borate, potassium barium magnesium borate fluoride, KBa₇Mg₂B₁₄O₂₈F₅, with a nominal 7:1 composition of BaB₂O₄ to KMg₂F₅, has been found during the growth of BaMgBO₃F crystals with a KF flux. It crystallized in the space group C2/c and is composed of isolated heptaborate B₇O₁₄]⁷⁻ groups and double perovskite Mg₂O₆F₅]¹³⁻ units.