Crystal structures of the alkali aluminoboracites A 4 B 4 Al 3 O 12 Cl ( A = Li, Na).

Single crystals of alkali aluminoboracites, A 4 B 4 Al 3 O 12 Cl ( A = Li, Na), were grown using the self-flux method, and their isotypic cubic crystal structures were determined by single-crystal X-ray diffraction. Na 4 B 4 Al 3 O 12 Cl 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 3 c , which assume that all cubic octant subcells in the unit cell are equivalent, converged with R 1 factors of ∼0.03. However, the presence of weak hhl reflections with odd h and l values indicates that refinements in the space group F 23, which presume a checkerboard-like ordering of two types of subcells with slightly different atomic positions, are more appropriate.