CsNbOB2O5: the Basic Structure Type of the Borates AMOB2O5 (A = K, Rb, Cs, Tl; M = Nb, Ta)

A new anhydrous borate, CsNbOB 2 O 5 (caesium niobatodiborate), has been found in the ternary system Cs 2 O-Nb 2 O 5 -B 2 O 3 . The compound belongs to the already known AMOB 2 O 5 family (originally reported as AMB 2 O 6 ) with A = K, Rb, Cs, Tl; M = Nb, Ta. These published members exhibit superstructures, the length of the superperiod depending on the chemical composition A and M. The structure of CsNbOB 2 O 5 reported here has the smallest possible unit cell and represents the basic structural type for the family.