Trigonal structures of ABe2BO3F2 (A = Rb, Cs, Tl) crystals.

Several interesting fluoroberyllium borates were synthesized hydrothermally and characterized by single-crystal X-ray diffraction. The crystal structures of RbBe(2)BO(3)F(2) (RBBF; rubidium fluoroberyllium borate) and CsBe(2)BO(3)F(2) (CBBF; caesium fluoroberyllium borate), previously determined in the space group C2, were reinvestigated for higher symmetry and found to have more suitable solutions in the space group R32. TlBe(2)BO(3)F(2) (TBBF; thallium fluoroberyllium borate) was synthesized as a novel compound also having this trigonal structure type. Details of the space-group determination and unique structural features are discussed. These crystal structures were compared with that of KBe(2)BO(3)F(2), revealing interesting structural trends within this family of compounds that are also discussed. A crystallographic explanation of the physical morphology is postulated.