M₂Cd₃B₁₆O₂₈ (M = Rb, Cs): Two isostructural alkali cadmium borates with a new type of borate layer

Two new isostructural compounds, M₂Cd₃B ₁₆O₂₈ (M = Rb, Cs), have been synthesized by a high-temperature solid-state reaction for the first time. M₂Cd ₃B₁₆O₂₈ (M = Rb, Cs) crystallize in the monoclinic space group C2/c, with a = 21.4856(10) Å, b = 7.6852(4) Å, c = 13.3694(8) Å, β = 91.705(3)°, Z = 4 for Rb ₂Cd₃B₁₆O₂₈ and a = 21.621(3) Å, b = 7.6820(11) Å, c = 13.4170(18) Å, β = 91.068(8)°, Z = 4 for Cs₂Cd₃B₁₆O ₂₈. The compounds crystallize with (B₁₆O ₃₄)_∞ double layers bridged by Cd²⁺ cations to form three dimensional (3D) structures. The M⁺ (M = Rb, Cs) cations reside in the tunnels of the (B₁₆O₃₄) _∞ double layers. First-principle electronic structure calculations show that the calculated band gaps of M₂Cd ₃B₁₆O₂₈ (M = Rb, Cs) are 4.67 and 4.71 eV, respectively, which are in good agreement with the experimental values estimated from UV/Vis/NIR absorption spectroscopy by using the Kubelka-Munk equation, and the observed absorption peak is assigned as a charge transfer from the O 2p state to the Cd 5s state. M₂Cd₃B₁₆O ₂₈ (M = Rb, Cs) consists of two-dimensional infinite (B ₁₆O₃₄)_∞ SrBBO-like double layers, which are built up by BO₃ triangles and BO₄ tetrahedra.