In recent years sodium-based All-Solid-State-Batteries (ASSBs) have attracted a lot of attention, as they have the potential to combine the advantages of ASSBs like increased safety with abundant raw materials [1]. In 2019 Ma et al. identified the composition Na_3.4Zr_2.0Si_2.4P_0.6O_12.0 to be the best Na ion conductor within the crystal series Na_1+xZr₂Si_xP_1-xO₁₂, with a total ionic conductivity of 5 x 10^-3 S/cm at 25 °C [2].

The most obvious approach to maximise the ionic conductivity of a ceramic ion-conductor would be to increase the material density by raising the sintering temperature. In this context the question arises, what criterion actually defines an upper limit for the sintering temperature in the given material system. For these ceramics different authors suspect a beginning melt formation even at relevant sintering temperatures [3]. Although this hypothesis would strongly affect the sintering and densification behaviour of these materials, this phenomenon hasn‘t been investigated in more detail so far. To clarify to what extent a potential liquid phase formation might take place at relevant sintering temperatures and therefore might influence the densification of these materials, the melting behaviour of the given material is investigated in this study.

Na_3.4Zr_2.0Si_2.4P_0.6O_12.0 was synthesized by a solution-assisted solid-state reaction described by Naqash et al. [4]. Samples were sintered at temperatures of 1230 °C, 1280 °C, 1300 °C and 1330 °C for 5h. Based on X-Ray Diffraction (XRD) data the NaSICON phase crystallizes in the space group C 2/c. No second phase is found for the sintering temperature 1280 °C, resulting in a single phase material. The material density passes through a maximum for the sintering temperature 1300 °C, as does the total ionic conductivity. The total ionic conductivity was determined by impedance spectroscopy to be 2.7 x 10^-3 S/cm at 25 °C for the material sintered at 1300 °C.

Cross sections of samples, that were quenched from 1300 °C and 1330 °C, were analysed by Scanning Electron Microscopy (SEM). First evidence for the existence of a melt phase in the microstructure at the sintering temperature 1330 °C could be found. This melt phase could be part of an explanation for the decrease of the material density and ionic conductivity from 1300 °C to 1330 °C. 

In this context Differential Thermal Analysis (DTA) was used to study the melting behaviour of the material. The DTA signal showed an endothermic event during heating and an exothermic event during cooling, that can be interpreted as melting and resolidification, respectively. The endothermic event, that is interpreted as beginning melt formation, starts at a temperature of around 1320 °C.

References
[1] M. Rohde, I. U. Mohsin, C. Ziebert, H. J. Seifert, International Journal of Thermophysics, 2021, 42, 1-15
[2] Q. Ma, C. Tsai, X. Wei, M. Heggen, F. Tietz, J. Irvine, Journal of Materials Chemistry A, 2019, 7(13), 7766-7776
[3] A. K. Kuriakose, T. A. Wheat, A. Ahmad, J. Dirocco, Journal of the American Ceramic Society, 1984, 67(3), 179-183
[4] S. Naqash, Q. Ma, F. Tietz, O. Guillon, Solid State Ionics, 2017, 302, 83-91