Dynamic Lone Pairs and Fluoride-Ion Disorder in Cubic-BaSnF₄

Area of Science:

• Solid-state chemistry
• Materials science
• Ionic conductors

Background:

• Disorder in crystalline solid electrolytes, particularly fluorites like (M,Sn)F2, is key to enhancing ionic conductivity.
• Two types of disorder are known: cation occupational disorder and Sn(II) lone pair orientational disorder.

Purpose of the Study:

• To characterize the structure and fluoride ion dynamics in cubic barium tin fluoride (BaSnF4).
• To investigate the interplay between cation disorder, Sn lone pairs, and fluoride ion mobility.

Main Methods:

• Rietveld refinement of X-ray diffraction (XRD) data.
• 119Sn Mössbauer spectroscopy.
• X-ray total-scattering PDF analysis.
• Ab initio molecular dynamics (AIMD) simulations.
• Variable-temperature 19F NMR spectroscopy.

Main Results:

• Cubic BaSnF4 exhibits average fluorite structure with {Ba,Sn} cation disorder and stereoactive Sn(II) lone pairs.
• Significant intrinsic fluoride ion disorder exists, with 1/3 of ions in octahedral sites due to Sn lone pair repulsion.
• Fluoride ion dynamics are highly inhomogeneous, with higher mobility near Sn-rich environments.
• Dynamical reorientation of Sn lone pairs is coupled to local cation configuration and ion dynamics.

Conclusions:

• Intrinsic fluoride ion disorder and inhomogeneous dynamics are critical features of cubic BaSnF4.
• The interplay between cation disorder, Sn lone pairs, and fluoride ion mobility governs ionic conductivity.
• Understanding these local structural and dynamic effects is essential for designing advanced solid electrolytes.