Reactive hot-pressed Na_3.4Zr₂Si_2.4P_0.6O₁₂: nanoscale grains, glass-free microstructure, high total conductivity, enhanced chemical stability

Hot-pressing of amorphous Na_3.4Zr₂Si_2.4P_0.6O₁₂ powders at 1225 °C followed by annealing for 30 min resulted in a total conductivity of 7.3 mS cm⁻¹ compared to 3 to 5 mS cm⁻¹ for sintered Na_3.4Zr₂Si_2.4P_0.6O₁₂. The higher total conductivity of the hot-pressed material is a result of its lower grain boundary resistance. The grain boundary resistance to the total resistance, R_GB/R_Total, for hot-pressed Na_3.4Zr₂Si_2.4P_0.6O₁₂ is about 0.53-0.55 compared to 0.67-0.85 for sintered Na_3.4Zr₂Si_2.4P_0.6O₁₂. The lower grain boundary resistance of the hot-pressed material is a result of its dense microstructure with nano-sized grains, which prevents grain boundary microcracking, and the absence of high resistance secondary phase(s) (e.g., glass and Na₂ZrSi₂O₇) along grain boundaries. In addition, the absence of a glass phase led to enhanced chemical stability in aqueous solutions for hot-pressed versus sintered Na_3.4Zr₂Si_2.4P_0.6O₁₂.