Local Tetragonal Structure of the Cubic Superionic Conductor Na₃PS₄

Area of Science:

• Solid-state chemistry
• Materials science
• Electrochemistry

Background:

• Sodium superionic conductor Na3PS4 exhibits two room-temperature polymorphs: cubic and tetragonal.
• Experimentally, cubic Na3PS4 shows higher ionic conductivity than tetragonal, contradicting theoretical predictions.
• The influence of crystal structure on ionic transport in Na3PS4 remains an open question.

Purpose of the Study:

• To investigate the relationship between crystal structure and ionic transport in Na3PS4.
• To reconcile experimental observations with theoretical predictions regarding conductivity differences.
• To elucidate the factors governing the high ionic conductivity of Na3PS4.

Main Methods:

• Rietveld analysis
• Pair distribution function (PDF) analysis
• Electrochemical impedance spectroscopy (EIS)

Main Results:

• Both ball-milled (cubic average) and high-temperature (tetragonal average) Na3PS4 exhibit tetragonal structural motifs on the local scale, as shown by PDF analysis.
• The high ionic conductivity of ball-milled Na3PS4 is confirmed to be independent of its average crystal structure.
• Differences in defect concentration, rather than crystal structure, are identified as the primary reason for high ionic conductivity.

Conclusions:

• Local and average crystal structures can differ in ionic conductors like Na3PS4.
• The high ionic conductivity of Na3PS4 is primarily attributed to defect concentration, not the specific crystal polymorph.
• This study clarifies the mechanism behind Na3PS4's conductivity, impacting future solid electrolyte development.