ACs5Bi4(PO4)2(P2O7)3 (A = K, Rb and Cs) with two kinds of isolated P–O groups designed by dimensional reduction theory

The dimensional reduction theory is applied to an A2O–P2O5–Bi2O3 system where P2O5 serves as a binary parent while A2O and Bi2O3 are regarded as dimensional reduction agents. Thus, three novel phosphates ACs5Bi4(PO4)2(P2O7)3 (A = K, Rb and Cs) have been prepared through the high-temperature solution method. Single-crystal X-ray diffraction measurements reveal that they all crystallize in the monoclinic space group P21/c (no. 14) and in their structures, two different kinds of P–O units, i.e. isolated PO4 tetrahedra and P2O7 dimers, are interconnected by Bi–O groups to construct a 3∞[Bi4(PO4)2(P2O7)36−] framework. Note that with the change of cationic size in their structures, the Bi3+ cations also exhibit flexible coordination, i.e. BiO5 and BiO6 polyhedra in KCs5Bi4(PO4)2(P2O7)3 and only BiO6 polyhedra in Cs6Bi4(PO4)2(P2O7)3, suggesting that combining the flexible coordination of Bi3+ cations with phosphates based on dimensional reduction theory is an effective strategy to guide the synthesis of new compounds.