Crystal structures of the alkali aluminoboracites A4B4Al3O12Cl (A = Li, Na)

Abstract

Single crystals of alkali aluminoboracites, A ₄B₄Al₃O₁₂Cl (A = Li, Na), were grown using the self-flux method, and their isotypic cubic crystal structures were determined by single-crystal X-ray diffraction. Na₄B₄Al₃O₁₂Cl is the first reported sodium boracite, and its lattice parameter [13.5904 (1) Å] is the largest among the boracites consisting of a cation-oxygen framework reported so far. For both crystals, structure models refined in the cubic space group F 3c, which assume that all cubic octant subcells in the unit cell are equivalent, converged with R1 factors of ∼0.03. However, the presence of weak hhl reflections with odd h and l values indicates that refinements in the space group F23, which presume a checkerboard-like ordering of two types of subcells with slightly different atomic positions, are more appropriate.

Keywords: alkali aluminoboracite; crystal structure; isotypism; self-flux method; weak structure ordering.