Title : Rb+-doped [Tl1-x-yAgyRbx)2Cd0.5I5] as a high-performance cathode material for supercapacitor applications: Structural disordering and ion transport
Abstract:
Fast ion conductors based on thallium cadmium iodide are emerging candidates for solid-state electrolytes due to their flexible tetragonal framework. However, controlled lattice disordering to stabilize superionic phase remains unexplored. Herein, we report synthesis and structural evolution of [Tl?????Ag?Rb?)?Cd?.?I?] (x = 0.2, 0.4, 0.6, 0.8 mol.%) via solid-state reaction route. Rietveld refinement confirms retention of tetragonal I4/mcm (No.140) symmetry across the series. Structural analysis via VESTA reveals systematic substitution at 4b Wyckoff position: Tl occupancy decreases from 0.60 (x = 0.2) to 0.15 (x = 0.8) with concomitant increase in Rb from 0.20 to 0.80, while Cd at 4c retains 0.5 vacancy indicating intrinsic defect structure. The incorporation of larger Rb? (1.52 Å) induces lattice distortion, evidenced by polyhedral rotation of (Tl/Ag/Rb)I? yellow dodecahedra and widening of I-I bottleneck from 3.82 Å (x = 0.2) to 4.35 Å (x = 0.8), beyond the minimum required for Ag? hopping (rAg? + rI? = 3.35 Å). Williamson-Hall analysis shows crystallite size reduction from 42.5 nm to 31.8 nm and micro-strain increase from 1.82 × 10?³ to 3.62 × 10?³, while Goldschmidt tolerance factor increases from 0.892 to 0.927 approaching stability. Tetrahedral strain Δ increases from 0.0182 to 0.0362, correlating with bottleneck expansion. Theoretical evaluation predicts optimum percolation threshold at x = 0.4-0.6 where pathway enlargement and sufficient Ag? carrier concentration coexist.
Keywords: Superionic conductor, Tl?CdI?, RbI doping, VESTA polyhedral, Williamson-Hall, bottleneck
