Study of the trivalent elements polyphosphates by thermal analysis

Mechanisms of formation of polyphosphates Me^III(PO₃)₃, where M ^III=La, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y, Sc, Fe, Ga, Al and Cr has been simulated by thermal analysis technique. Me^III oxides and ammonium dibasic phosphate (NH₄)₂HPO₄ were used as starting materials. For M ^III=La-Lu, Y and Fe three main stages were observed: 1. elimination of water and ammonia leading to the formation of ammonium tripolyphosphate (NH₄)₅P₃O₁₀; 2. reaction of the latter with Me₂^IIIO₃ and formation of acidic polyphosphates Me^IIIH₂P₃O₁₀ or their isomers Me^III(PO₃)₃·H₂O; 3. final loss of water and formation of Me^III(PO₃)₃. For Me ^III=Sc and Ga the second stage is prolonged and the polyphosphates form at higher temperatures. Aluminum and chromium polyphosphates are unstable. It is suggested that thermal behavior of the compounds is determined by Me^III ionic radii.