Universal trend of the Haven ratio in glasses: origin and structural evidences from neutron diffraction and small-angle neutron scattering

The Haven ratio HR exhibits a universal trend in oxide and chalcogenide glassy systems as a function of the mobile ion content x. The ion transport in extremely dilute glasses (x=30–100 ppm M+) is uncorrelated (HR≈1), but HR decreases rapidly with increasing x and remains nearly constant (HR=0.2–0.4) at x ⩾ 10 at.%. An experimentally verified interpretation of this phenomenon is lacking. Our neutron diffraction (ND) and small-angle neutron scattering experiments carried out over a Q-range of four orders of magnitude, from 3×10−3 to 40 Å−1, for a number of silver chalcogenide glassy systems suggest a structural origin for this universal trend. Glasses from the critical percolation domain (xc ⩽ x ⩽ 1–3 at.% Ag, where xc≈30 ppm Ag is the percolation threshold) are characterised by a random silver distribution. The average Ag–Ag separation distance decreases with increasing x in this domain, leading to an increase in the interionic interactions and thus to a monotonic decrease of HR. In contrast, glasses from the modifier-controlled domain (x ⩾ 10 at.% Ag) are characterised by a non-random Ag distribution. Edge-shared AgX3 pyramids (X = S, Se) form chains, cross-linking chains, sheets, tunnels, etc., depending on the system, and provide preferential conduction pathways. The interionic interactions, however, are controlled by an invariant Ag–Ag second neighbour distance of ≈3 Å, and so the Haven ratio remains essentially constant.