Defect induced sodium disorder and ionic conduction mechanism in Na1.82Mg1.09P2O7
Phase pure sodium magnesium pyrophosphate Na1.82Mg1.09P2O7 was synthesized by a solid state reaction, and used as a model system to understand the Na+ dynamics in the related cathode materials Na2−xM1+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  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.