Cd2Cu(PO4)2

During an investigation of the insufficiently known systemM1O-M2O-X2O5-H2O (M1 = Cd2+, Sr2+ and Ba2+; M2 =Cu2+, Ni2+, Co2+, Zn2+ and Mg2+; X = P5+, As5+ and V5+),single crystals of the novel compound dicadmium copper(II)bis[phosphate(V)], Cd2Cu(PO4)2, were obtained. Thiscompound belongs to a small group of compounds adoptinga Cu3(PO4)2-type structure and having the general formulaM12M2(XO4)2 (M1/M2 = Cd2+, Cu2+, Mg2+ and Zn2+; X =As5+, P5+ and V5+). The crystal structure is characterized bythe interconnection of infinite [Cu(PO4)2]n chains and[Cd2O10]n double chains, both extending along the a axis.Exceptional characteristics of this structure are its novelchemical composition and the occurrence of double chains ofCdO6 polyhedra that were not found in related structures. Incontrast to the isomorphous compounds, where theM1 cationsare coordinated by five O atoms, the Cd atom is coordinatedby six. The dissimilarity in the geometry of M1 coordinationbetween Cd2Cu(PO4)2 and the isomorphous compounds ismostly due to the larger ionic radius of the Cd cation incomparison with the Cu, Mg and Zn cations. Sharing acommon edge, two CdO6 polyhedra form Cd2O10 dimers. Eachsuch dimer is bonded to another dimer sharing commonvertices, forming [Cd2O10]n double chains in the [100]direction. The Cu atoms, located on an inversion centre (sitesymmetry 1), form isolated CuO4 squares interconnected byPO4 tetrahedra, forming [Cu(PO4)2]n chains similar to thosefound in related structures. Conversely, the [Cd2O10]n doublechains, which were not found in related structures, are anexclusive feature of this structure.