Magnesium doped Gallium Phosphonates Ga 1– x Mg x [H 3+ x (O 3 PCH 2 ) 3 N] ( x = 0, 0.20) and the Influence on Proton Conductivity

In our contribution to the development of new proton conductive coordination polymers (CPs) we focus on the impact of a partial replacement of Ga 3+ by Mg 2+ . This approach should come along with the introduction of additional protons due to charge balances. In a first step we have synthesized an isostructural compound to the literature known compound AlH 3 P3N [H 6 P3N = nitrilotris(methylene)triphosphonic acid], where Al 3+ is replaced by Ga 3+ , since all attempts to incorporate Mg 2+ ions directly into AlH 3 P3N were not successful. The relative amount of Mg 2+ and Ga 3+ was established by EDX analysis. Rietveld refinement of the synchrotron data located the Ga 3+ and Mg 2+ ions on a split position, proving the disordered incorporation of the Mg 2+ ions. Solid‐state NMR spectroscopy confirms a disordered protonation of the phosphonate groups as well and shows that all amine groups are protonated. In order to investigate the effect on the proton conductivity the compounds Ga[H 3 (O 3 PCH 2 ) 3 N], denoted GaH 3 P3N as well as Ga 0.80 Mg 0.20 [H 3.20 (O 3 PCH 2 ) 3 N], denoted GaMgH 3.20 P3N, were characterized by electrochemical impedance spectroscopy (EIS). Arrhenius behavior in the investigated temperature range (70–130 °C) was found for both compounds (activation energies of E a = 0.15 eV for GaH 3 P3N and 0.17 eV for GaMgH 3.20 P3N). The GaMgH 3.20 P3N sample shows a reduced proton mobility ( σ = 1.2 × 10 –4 S · cm –1 ) of about one order of magnitude in comparison to GaH 3 P3N ( σ = 1.0 × 10 –3 S · cm –1 ).