RbMT3 (BO3 )2 O3 (M=Ba, Sr; T=Al, Ga): New Double-Layered Oxyborates Constructed from [BO3 ] Triangles and [TO4 ] Tetrahedra.

Four new alumino-/galloborates RbMT3 (BO3 )2 O3 (M=Ba, Sr; T=Al, Ga) have been synthesized for the first time by using a high-temperature solution method. All the title compounds have Sr2 Be2 B2 O7 -like structures, in which the [BO3 ] triangles and [TO4 ] tetrahedra form the final double-layered configurations with the M- and Rb-site atoms located between and in the double layer, respectively. The structure evolution from Sr2 Be2 B2 O7 to RbMT3 (BO3 )2 O3 series is discussed. The broader energy bandgaps in Al-based borates when compared with Ga-based ones can be entirely attributed to the location of Al/Ga s orbitals near the Fermi surface. Both experimental and computational approaches were used to study their structure-property relationships.