• Borates;
  • Semiconductors;
  • ­Solid-state structures;
  • Alkali metals


Two new isostructural compounds, M2Cd3B16O28 (M = Rb, Cs), have been synthesized by a high-temperature solid-state reaction for the first time. M2Cd3B16O28 (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 Rb2Cd3B16O28 and a = 21.621(3) Å, b = 7.6820(11) Å, c = 13.4170(18) Å, β = 91.068(8)°, Z = 4 for Cs2Cd3B16O28. The compounds crystallize with (B16O34) 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 M2Cd3B16O28 (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.