Structural changes within and between the two isotypic series ABiO2 (A = Na, K, Rb, Cs) and ASbO2 (A = K, Rb, Cs)

Structural changes within the isotypic series ABiO(2)) (A = Na, K, Rb, Cs) and ASbO(2) (A = K, Rb, Cs) and between corresponding members of the two series are analyzed after re-determination of the structure of NaBiO2. Despite the low valence of A-O bonds as compared to E-O bonds (E = Sb, Bi), the A atoms of different sizes are largely able to satisfy their individual spatial requirements. Small deviations between expected and experimentally determined Na/K size differences in the ABiO(2) series are explained by a change in the bonding scheme in the transition from NaBiO2) to KBiO2. The results of DFT calculations show that the unexpected increase of the lattice parameters b in the three transitions ABiO(2),) --> ASbO(2)) is not caused - as assumed before - by the different stereochemical activities of the Sb and Bi lone electron pairs. Instead, this increase is caused by cation-cation repulsions owing to the observed decrease in c.