Structure-property relationships in the ionic conductor NaTi2(PS4)3
Solid State and Surface Chemistry, Material Synthesis
Final Report Abstract
In order to obtain a better understanding of the Na+ substructure in NTPS, low temperature single crystal diffraction measurements were carried out. However, no ordering of Na+ was observed, also at low temperatures, alluding to the frustrated Na+ energy landscape that offers no stable coordination sites. Attempts to obtain Na-Na and Na-S distances from pair distributions functions were also not successful to gain more insights on the Na+ substructure, due to the low Na form factor and overall low molar fraction of Na per formula unit. Low temperature synchrotron diffraction data further did not help to resolve the Na+ substructure in these materials. Alongside the single crystal diffraction experiments on NTPS crystalizing in space group P6cc a new NTPS polymorph was discovered, crystalizing in space group Fddd. This newly discovered polymorph also possesses an interlocked structure. However, no wide tunnels are found in this structure and Na+ ions are well ordered, which implies low ionic conductivities. Unfortunately, it was not possible to obtain phase pure compounds and the ionic conductivities could hence not be determined in the Fddd phase. Aliovalent substitutions of Ti with Al and Ga were carried out. While Ga showed considerably higher solubility in the NTPS structure compared to Al, both substitution approaches lead only to small changes in the structure but also the ionic transport properties. Lastly, Se substitutions were investigated. While the observed changes to the diffraction patterns are yet not understood, a formation of a structure crystalizing in a different space group than NTPS is expected for high Se contents. Overall, while a promising material at the conception stage of the project the substitutional range of elements, challenging syntheses of the polymorphs and issues in properly resolving the structural characteristics made the whole project quite challenging leading to only one publication at the beginning of the project. Nevertheless, we will continue trying to isolate the new polymorph in a phase pure state within future M.Sc projects.
Publications
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Ionic Conductivity of the NASICON‐Related Thiophosphate Na1+xTi2−xGax(PS4)3. Chemistry – A European Journal, 25(16), 4143-4148.
Schlem, Roman; Till, Paul; Weiss, Manuel; Krauskopf, Thorben; Culver, Sean P. & Zeier, Wolfgang G.
