Structural Characterization and Study of the Mixed-Ion Effect in K–Li Metaphosphate Glasses

This study investigates potassium-lithium metaphosphate glasses using Differential Scanning Calorimetry, Complex Impedance Spectroscopy, Nuclear Magnetic Resonance, and Raman Spectroscopy to elucidate the structural mechanisms underlying the Mixed Ion Effect. Thermal analyses reveal a systematic decrease in the glass transition temperature with increasing potassium content, which is dictated not solely by ionic size mismatch but also by structural reorganization within the glass network. The redistribution of nonbridging oxygens and the reduction of phosphate cross-links contribute to this behavior. Impedance spectroscopy shows a pronounced nonlinear reduction in ionic conductivity, decreasing by over 6 orders of magnitude at room temperature for the intermediate composition. NMR analysis indicates a nearly linear evolution of ³¹ <i>P</i> and ⁷ <i>Li</i> chemical shifts and full width at half-maximum, confirming the absence of phase segregation. Raman spectroscopy reveals a consistent <i>PO</i> ₂ symmetric mode shift, indicative of solid solution behavior and random cation mixing. These findings validate two key hypotheses of the Random Ion Distribution Model: the structural specificity of each cation site and their random distribution within the glass network, while also demonstrating that structural reorganization plays a critical role in modulating <i>T</i> _g.