Phase transitions and local ordering in the Na3Sb1-xWxS4 system

Compounds with general formula Na3MS4 (M=P,Sb) are solid state electrolytes for sodium-ion battery. Depending on composition and doping on the M site, they are tetragonal or cubic at rt. W substitution stabilizes the high temperature cubic phase at rt. We aim at investigating the average and local structure of Na3Sb1-xWxS4 (x=0, 0.075; 0.10) below rt, to identify the temperature of the cubic to tetragonal phase transition and gain an insight into the mechanism of transformation. This study will provide a better understanding of the correlation between structure and properties by: i) establishing the stability range of the cubic phase; ii) identifying deviations between the average and local structure of the W substituted systems; iii) helping to rationalize the existence of structural guidelines for the preparation of cubic phases at rt; iv) support the comprehension of the Na-ion diffusion mechanism. Data will be analysed using Rietveld and PDF analysis, supported by DFT calculations.