Structural field of K 2 Al 2 B 2 O 7 -family crystals

The known crystal structures of borate compounds with general composition A I 2 MIII 2 B 2 O 7 (AI: Na, K, Cs, Rb) have been considered. The structures in space groups P63/m, P-3 1 c, P321 and P21/c have been found depending on cation combination. Criterion k = (r(M III )+r(B)+2r(O))/r(A I ) has been formulated and it controls the formation of trigonal K2Al2B2O7 (KABO) type structure. The existence of noncentrosymmetric KABO-type borates is possible over the range of k = 2.7 – 2.06. This range is promising for searching new borate crystals and solid solutions with pronounced nonlinear optical properties.