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Sodium Site Exchange and Migration in a Polar Stuffed-Cristobalite Framework Structure
Alberto J Fernández-Carrión1, Aydar Rakhmatullin2, Li Yang1
1MOE Key Laboratory of New Processing Technology for Nonferrous Metals and Materials, Guangxi Key Laboratory of Optical and Electronic Materials and Devices, College of Materials Science and Engineering, Guilin University of Technology, Guilin 541004, P. R. China.
This study reveals that Na₂MgSiO₄ exhibits pure sodium ionic conductivity due to Na vacancies. Ionic conduction is facilitated by hopping between Na sites, forming 3D zigzag paths, and a Na site exchange phenomenon is observed at room temperature.
Area of Science:
- Solid-state chemistry and materials science.
- Energy storage and conversion technologies.
Background:
- Ionic dynamics in solids are critical for modern energy technologies.
- Orthorhombic Na₂MgSiO₄ possesses a polar stuffed-cristobalite-type structure with two Na sites.
Purpose of the Study:
- To investigate the ionic dynamics and conduction mechanisms in Na₂MgSiO₄.
- To characterize the structure-property relationships governing sodium ion transport.
Main Methods:
- Solid-state reaction synthesis.
- Impedance spectroscopy and molecular dynamics simulations for macroscopic migration.
- Solid-state ²³Na magic-angle-spinning (MAS) NMR spectroscopy for microscopic dynamics.
Main Results:
- Optimized ionic conductivity of ~10⁻⁵ S cm⁻¹ at 200 °C due to ~2% Na vacancies.
- Identification of 3D zigzag migration paths between Na1 and Na2 sites.
- Observation of Na site exchange at room temperature via variable-temperature ²³Na MAS NMR and 2D exchange spectroscopy.
Conclusions:
- Na₂MgSiO₄ functions as a pure sodium ionic conductor.
- Na site exchange at room temperature is a key factor triggering ionic conduction at higher temperatures.
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