Defect induced sodium disorder and ionic conduction mechanism in Na_1.82Mg_1.09P₂O₇

Phase pure sodium magnesium pyrophosphate Na_1.82Mg_1.09P₂O₇ was synthesized by a solid state reaction, and used as a model system to understand the Na⁺ dynamics in the related cathode materials Na_2−xM_1+x/2P₂O₇ (M = Fe, Mn, and Co). We found a three-dimensional framework structure formed by interconnecting Mg₂O₁₁ dimers with P₂O₇ pyrophosphate groups, where Na cages align along the [100] direction and are kinetically separated from each other. Diffuse distribution of Na was found inside the cage, and explained using a vacancy induced disorder model. Ionic conductivity is limited by inter-cage diffusion and measured to be 1.10 × 10⁻¹¹ S cm⁻¹ at room temperature with an activation energy of 0.77 eV. These features were further supported by bond-valence sum and lattice mechanics calculations.