RbMT 3 (BO 3 ) 2 O 3 (M=Ba, Sr; T=Al, Ga): New Double‐Layered Oxyborates Constructed from [BO 3 ] Triangles and [TO 4 ] Tetrahedra

Four new alumino‐/galloborates RbMT 3 (BO 3 ) 2 O 3 (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 Sr 2 Be 2 B 2 O 7 ‐like structures, in which the [BO 3 ] triangles and [TO 4 ] 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 Sr 2 Be 2 B 2 O 7 to RbMT 3 (BO 3 ) 2 O 3 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.