A2BBi2(PO4)2(P2O7) (A = K, Rb, B = Pb, Cd): the Effect of Cation Sizes on Structural Evolution

Two homologous new mixed-metal phosphates K 2 PbBi 2 (PO 4 ) 2 (P 2 O 7 ) ( I ) and Rb 2 CdBi 2 (PO 4 ) 2 (P 2 O 7 ) ( II ) were successfully synthesized in A 2 BBi 2 (PO 4 ) 2 (P 2 O 7 ) (A = K, Rb, B = Pb, Cd) system through the traditional high-temperature solid-phase method. The single crystal structures of the two compounds are shown to be the unique three-dimensional (3D) framework composed of different types of layers, but not isomorphic. We compared series compounds A 2 PbBi 2 (PO 4 ) 2 (P 2 O 7 ) (A = K, Rb, Cs), which are isomorphic and crystallize in the same space group Pnma . It is noteworthy that they are distinct different in degree of structural disorder, which is tuned by A-site cations. The effect of A-site cations is recognized by analyzing the disorder degree in three related structures and other available 182 mixed-metal phosphates. To the best of our knowledge, such structural analysis is rarely reported in inorganic phosphates and the cation modulation strategy can be used to guide structural disorder according to the difference-value (D-value) of cation radii. Furthermore, the syntheses, thermal analysis, relevant spectroscopic measurements and theoretical calculations for I and II compounds were also discussed. Graphical Two new mixed-metal phosphates K 2 PbBi 2 (PO 4 ) 2 (P 2 O 7 ) and Rb 2 CdBi 2 (PO 4 ) 2 (P 2 O 7 ) were firstly reported and the cation size effect has substantial influence on the crystal structural disorder combining with the title compounds and 182 phosphates.