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

Abstract

Low-melting methylammonium phosphate glasses are synthesized from crystalline starting agents. To this end crystalline tris(methylammonium) cyclotriphosphate [CH₃NH₃]₃P₃O₉, was synthesized by a novel and simple synthesis route from P₄O₁₀ and N-methylformamide. It, undergoes an irreversible phase transition to methylammonium catena-polyphosphate [CH₃NH₃]PO₃. 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 ³¹P and ¹H 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 P₄O₁₀ 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 [CH₃NH₃]PO₃ serves as an approximation for the short-range order of the glass.