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

Abstract

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.