Alkali Manganese Oxynitridophosphates A1.1Mn2.9P3O8N2 (A = Li, Na): Direct Synthesis, Crystal Chemistry, and Ion-Transport Limitations

Alkali manganese oxynitridophosphates A1.1Mn2.9P3O8N2 (A = Na, Li) were prepared by one-step solid-state ammonolysis, where the lithium analogue is obtained directly rather than by alkali-ion exchange from the sodium compound. Both compositions crystallize in a previously unreported orthorhombic Pbcn framework, in which ordered O/N environments, unusual manganese coordination in MnO5N and MnO6N2 polyhedra, and alkali–manganese disorder are accommodated within the same structure. The proposed structural model is supported by complementary synchrotron X-ray and neutron powder diffraction, electron microscopy, solid-state NMR and XAS data, as well as density functional theory calculations. Magnetic susceptibility is consistent with predominantly Mn2+ and antiferromagnetic ordering below 35.1 K for the Li phase and 32.6 K for the Na phase. Although the Mn2+/Mn3+ redox couple lies within the accessible potential window of nonaqueous M-ion batteries, reversible alkali extraction is limited by the restricted migration pathways imposed by the framework. Overall, this work expands the compositional and structural landscape of oxynitridophosphates and highlights nitrogen incorporation in phosphate crystal chemistry as a route to unconventional coordination environments, new framework connectivities and functional properties.