Polycrystalline materials of the Sodium (Na) Super-Ionic CONductors (NaSICON) class with kosnarite structure have shown high ionic conductivities, which are a prerequisite for the electrolyte material in All-Solid State Batteries (ASSBs) [1]. In particular, sodium-based ASSBs have garnered attention as they combine abundant and low cost educts and potential high capacities with the advantages of ASSBs like increased safety and durability.

The processing and stoichiometric parameters have a major impact on the microstructural properties of the NaSICON, affecting their ionic conductivity. Therefore the impact of the Zr deficiency on the relative density, sodium concentration and crystallographic structure was investigated.

The NaSICON material (Na_3.4Zr₂Si_2.4P_0.6O₁₂) synthesized in our group shows one of the highest ionic conductivities reported in literature at room temperature (up to 5 mS/cm) [2,3]. Here Zr-deficient NaSICONs with hypothetical O^2- vacancies were studied in relation to Na_3.4Zr₂Si_2.4P_0.6O₁₂ and the impact on the phase formation, glassy parts and associated properties was investigated.

A series with constant Si+P=3 content (Na_3.4Zr_2-3x/4Si_2.4-x/4P_0.6+x/4O_12-11x/8; 0 < x < 0.8) and a second series with increasing Si+P fraction (Na_3.4Zr_2-3x/4Si_2.4+x/4P_0.6+1.5x/4O_12-x/16; 0 < x < 0.8) and O^2- vacancies to compensate for Zr deficiency were synthesized and sintered. The microstructure was studied using scanning electron microscopy (SEM) identifying the growing fraction of glassy phase with increasing x. The formation of this glassy phase improved the shrinking behavior of the samples up to x = 0.4. The primarily monoclinic phase with good ionic conductivity and rhombohedral secondary phase were identified by X-ray diffraction (XRD) and the lattice parameters were determined by Rietveld refinement. The ionic conductivity of the samples was determined by impedance spectroscopy and the first series showed good ionic conductivities until the resistive effect of the glassy phase between the crystalline grains started to dominate. The synthesized powders had a fixed stoichiometry, but due to the segregation of glassy phases the stoichiometry of the crystalline grains is not accessible that easily. The lattice parameters determined by Rietveld refinement were constant showing that the Na content did not change significantly with Zr deficiency. This lead to the conclusion that an equilibrium phase crystallized and that the additional atoms separated into the glassy phase. The lattice parameters and the Na content in the formula from literature were used to approximate the Na content from the lattice parameters determined by XRD and comparing them to the values obtained by energy dispersive X-ray spectroscopy. In summary, the ionic conductivity was related to the different compositions. The influence of temperature on the evolving crystalline [4] and glassy phase formation was investigated.