Multimodal Structure Solution Unravels Correlated Disorder Promoting Ionic Migration in Silicate Sodium-Ion...

Characterizing structural disorder in solids is of significant challenge, which requires new strategy on probing and describing local structures over different scales and uncovering coherent ordering hidden in the structural disorder. Herein, we demonstrate a multimodal solution for structural disorder in Na_{2(1- x )}Mg_{1- x}Si_{1+ x}O₄ sodium-ion electrolytes with a stuffed cristobalite tetrahedral network. Neutron pair distribution function analysis combined with reverse Monte Carlo simulations was employed to probe the structural disorder in nanometer-scale supercells, uncovering the hidden correlated Mg/Si disorder or local Mg/Si order forming neighboring pure Mg (or Mg-rich) and Si columns. This correlated disorder was further validated by solid state ²⁹Si nuclear magnetic resonance (NMR) spectroscopy and NMR-guided structure screenings. The sodium cations in the tunnels were proposed to be interstitial-like mobile charge carriers for ionic conduction in Na_{2(1- x )}Mg_{1- x}Si_{1+ x}O₄ referring to the parent cristobalite structure. Both sodium contents within the tunnels and structural disorder level play competing roles in the sodium migration, while local Mg/Si order may minimize distortion of tetrahedral network and therefore maximize the tunnel bottlenecks promoting sodium migration. This work provides practicable multimodal solution strategy to solve the commonly complex structural disorders and unveil inherently local order with wide applicability in functional materials, enhancing understanding of structure-property relationship.