Impact of Microstructural Properties on Ionic and Heat Transport in NaSICON Glass Ceramics

Two composition series of Zr-deficient NaSICON materials are investigated with respect to their ionic and thermal transport properties. The bulk conductivity varies between 1.4 and 6.6 mS cm⁻¹. The total conductivity showdecreasing values with increasing Zr deficiency due to the impact of the increasing fraction of glass phase. The calculated grain boundary conductivity is about two orders of magnitude lower than the total conductivity but does not correspond to the conductivity of any known glass composition of sodium silicates/phosphates. Nuclear magnetic resonance reveals three ²³Na relaxation rates, the fastest of which is attributed to the NaSICON phase and the two slower relaxation rates to sodium orthophosphates and the glass phase. Thermal conductivity varies between 0.9 and 1.0 W m⁻¹ K⁻¹ at 25 °C. At elevated temperatures, a clear trend is observed toward lower thermal conductivity with a higher glass fraction. In addition, atom probe tomography is applied to precisely quantify the composition of specific microstructural regions found within the glassy phase. A scanning electron microscopy study of the surfaces of sintered pellets shows an increasing amount of glass phase between the NaSICON particles with increasing Zr deficiency. Furthermore, a time-dependent phase separation is observed in relation to the dynamic formation and dissolution of Na₃PO₄ domains.