Compounds and Solid Solutions of Cobalt, Copper Phosphates

Abstract

Cobalt phosphates have been synthesized and their thermal behavior examined. The formation of compounds and solid solutions among cobalt and copper phosphates is studied by X-ray diffraction. Unlimited solubility exists between the pyrophosphates Co2P2O7 and Cu2P2O7, the orthophosphates Co3(PO4)2 and Cu3(PO4)2, and Co3(PO4)2 and Cu2P2O7. X-ray diffraction indicates that the structures of cobalt and copper pyrophosphate solid solutions vary gradually from one end member to the other. The structures of orthophosphate solid solutions vary in a more complex manner: the change is gradual from Co3(PO4)2 to Co2Cu(PO4)2. At Co1.5Cu1.5(PO4)2, a new structure (L) occurs. At 950°C, CoCu2(PO4)2 also exhibits this structure, which is similar to that of graftonite, Fe3(PO4)2. CoCu2(PO4)2 is dimorphic, changing reversibly at 1000°C to a triclinic structure (H) related to that of Cu3(PO4)2. From CoCu2(PO4)2 (H) to Cu3(PO4)2, the structure appears to vary gradually. A new phosphate, Cu+Cu2+Co3(PO4)3, has also been synthesized.