Anion Distribution in Superionic Ag₃PO₄–AgI Glasses Revealed by Dipolar Solid-State NMR

The structure of roller-quenched fast ion conductive glasses (FICs) (Ag₃PO₄)_x(AgI)_1–x (0.15 ≤ x ≤ 0.50) is investigated by ¹⁰⁹Ag and ³¹P solid-state NMR spectroscopies. Monotonic linear dependences of ¹⁰⁹Ag and ³¹P chemical shifts on x are consistent with a statistical distribution of the phosphate and iodide anions. This conclusion is quantitatively confirmed by a new ³¹P homonuclear magnetic dipolar recoupling method, termed double-quantum-based dipolar recoupling effects nuclear alignment reduction (DQ-DRENAR), which numerically proves a random spatial distribution of the phosphate anions. Altogether these results give the final answer to a long-standing debate on the structure of silver in AgI-based (FIC) glasses, proving the absence of previously postulated silver iodide cluster domains.