Cation distribution and local order in mixed sodium metaphosphate glasses

Abstract

Two structural properties in mixed alkali metal phosphate glasses that seem to be crucial to the development of the mixed-ion effect in dc conductivity were systematically analyzed in Na mixed metaphosphates: the local order around the mobile species, and their distribution and mixing in the glass network. The set of glasses considered here, Na1–xMxPO3 with M = Li, Ag, K, Rb, and Cs and 0 ≤ x ≤ 1, encompass a broad degree of size mismatch between the mixed cation species. A comprehensive solid-state nuclear magnetic resonance study was carried out using 31P MAS, 23Na triple quantum MAS, 87Rb QCPMG, 31P–23Na REDOR, 23Na–7Li and 7Li–6Li SEDOR, and 23Na spin-echo decay. It was observed that the arrangement of P atoms around Na in the mixed glasses was indistinguishable from that observed in the NaPO3 glass. However, systematic distortions in the local structure of the O environments around Na were observed, related to the presence of the second cation. The average Na–O distances show an expansion/compression when Na+ ions are replaced by cations with respectively smaller/bigger radii. The behavior of the nuclear electric quadrupole coupling constants indicates that this expansion reduces the local symmetry, while the compression produces the opposite effect. These effects become marginally small when the size mismatch between the cations is small, as in Na–Ag mixed glasses. The present study confirms the intimate mixing of cation species at the atomic scale, but clear deviations from random mixing were detected in systems with larger alkali metal ions (Cs–Na, K–Na, Rb–Na). In contrast, no deviations from the statistical ion mixture were found in the systems Ag–Na and Li–Na, where mixed cations are either of radii comparable to (Ag+) or smaller than (Li+) Na+. The set of results supports two fundamental structural features of the models proposed to explain the mixed-ion effect: the structural specificity of the sites occupied by each cation species and their mixing at the atomic scale.