Two
metal borate–carbonates, M6[Cd2(CO3)2(B12O18)(OH)6] [M = K (1), Rb (2)], were obtained
under surfactant-thermal conditions. In 1 and 2, each cyclic [(B12O18)(OH)6]6– anion captures two CdCO3 in two sides
of the rings and finally forms the unusual (CdCO3)2@[(B12O18)(OH)6] cluster.
Both 1 and 2 show moderate birefringence.
Density functional theory calculations indicate that carbonate groups
have a major contribution to electron-related optical transition.