Q₁₈Mg₆(B₅O₁₀)₃(B₇O₁₄)₂F (Q=Rb and Cs): New Borates Containing Two Large Isolated Polyborate Anions with Similar Topological Structures

Abstract

Two novel isomorphic borates, Q₁₈Mg₆(B₅O₁₀)₃(B₇O₁₄)₂F (Q=Rb and Cs), were synthesized from high‐temperature solutions. These borates are created from two kinds of isolated polyborate anionic groups, [B₅O₁₀] and [B₇O₁₄], and are further examples of two kinds of isolated polyborate anions coexisting in one borate structure. Although the title compounds are new structures, they are similar to the structures of the KBe₂BO₃F₂ (KBBF) series of compounds, for example, Na₂Be₄B₄O₁₁ and γ‐KBe₂B₃O₇, which can be regarded as the substitution of [BO₃] and [B₂O₅]/[B₃O₆] groups with [B₇O₁₄] and [B₅O₁₀] groups, respectively. It is found that the coexisting BO groups tend to have similar structures. Also, the isolation of [B₅O₁₀] and [B₇O₁₄] in the title compounds mainly derives from the special role of magnesium tetrahedra, which has a similar role to that of the BeO₄ tetrahedra in the KBBF series compounds. Electron‐density maps and bond‐overlap population (BOP) were calculated to illustrate this feature.