a phosphate - phosphonate compound which is lowly volatile , has a high phosphorus content , and does not influence various properties of a product when used as a material therefor . it is reduced in environmental burden in view of suitability for recycling and contains no halogen atoms including chlorine and bromine . the phosphorus compound has a phosphate - phosphonate bond within the molecule and has a specific ring structure . because of this , the compound has excellent flame retardancy . addition of the phosphate - phosphonate compound as a flame retardant for polyurethanes or polyesters imparts satisfactory plasticity and flame retardancy . the flame retardant is reduced in the exertion of adverse influences on the material properties of , in particular , polyurethane foams , polyester fibers , and the like .