Synthesis and characterization of methylammonium phosphates as crystalline approximants for anhydrous, low melting phosphate glasses

Low-melting methylammonium phosphate glasses are synthesized from crystalline starting agents. To this end crystalline tris(methylammonium) cyclotriphosphate [CH(3)NH(3)](3)P(3)O(9), was synthesized by a novel and simple synthesis route from P(4)O(10) and N-methylformamide. It, undergoes an irreversible phase transition to methylammonium catena-polyphosphate [CH(3)NH(3)]PO(3). The crystal structure of the catena-polyphosphate was solved and refined from X-ray powder diffraction data by the Rietveld method using constraints obtained by solid-state (31)P and (1)H NMR spectroscopy. This compound crystallizes in a triclinic space group with a = 13.2236(9), b = 7.8924(6), c = 4.6553(2) Å, α = 91.068(4), β = 87.840(5) and γ = 106.550(3)°. Quantum chemical calculations confirm that the obtained structure lies at an energetic minimum. Finally the reaction of tris(methylammonium) cyclotriphosphate and P(4)O(10) into methylammonium phosphate glass is presented. The synthesized, water-free phosphate glass shows a very low glass transition temperature T(g) of 33 °C, which was verified by dynamic scanning calorimetry and NMR. The chain-like crystal structure of the high-temperature methylammoniumphosphate [CH(3)NH(3)]PO(3) serves as an approximation for the short-range order of the glass.