Polyphosphate composite: Conductivity and NMR studies

A polyphosphate composite [NH₄PO₃] ₆[(NH₄)₂SiP₄O₁₃] with high ionic conductivity was prepared and characterized by chemical analysis, X-ray diffraction, thermal gravimetry, impedance spectroscopy, and different NMR techniques. Experiments were carried out in dry and humid atmospheres and in temperatures ranging between 20°C and 300°C. During initial heating (activation) of the composite to 300°C, a weight loss of 7% due to release of ammonia occurs resulting in a material of the formal composition of [HPO ₃]₃[NH₄PO₃]₃[(NH ₄)₂SiP₄O₁₃]. This material is thermally stable between room temperature and 300°C. The conductivity was found to be remarkably high (0.1 S cm^-1) in a water-rich environment. Mainly ammonium species could be found in the ¹H MAS NMR spectra of the non-activated composite, whereas during the activation process another signal due to bridging hydrogen increases. Temperature-dependent 2D and 1D exchange NMR spectroscopy, PFG (pulsed field gradient) NMR, and SFG (stray field gradient) NMR measurements were performed in order to compare diffusion coefficients and exchange rates with dc conductivities. The small differences between experimentally obtained conductivities and those determined from the measured self-diffusion coefficients by means of the Nernst-Einstein equation hints to an ammonium vehicle as charge carrier, but can be also explained by H⁺ conductivity. The slow NMR exchange rates between hydroxyl groups and ammonium ions exclude proton conductivity via hydroxyl groups.