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

• Published in: RSC advances 2019-01, Vol.9 (4), p.1822-183
• Main Authors: Mangstl, Martin, Wied, Jan Konrad, Weber, Johannes, Pritzel, Christian, Trettin, Reinhard, Schmedt auf der Günne, Jörn
• Format: Article
• Language: English
• Subjects: Approximants; Chemistry; Crystal structure; Crystallinity; Glass transition temperature; High temperature; NMR spectroscopy; Organic chemistry; Phase transitions; Phosphate glass; Phosphates; Phosphorus pentoxide; Quantum chemistry; Rietveld method; Short range order; Synthesis; X ray powder diffraction
• ISSN: 2046-2069; 2046-2069
• DOI: 10.1039/c8ra07736c

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. The novel glass shows the lowest glass transition temperature known for binary phosphate glass systems without acidic protons.