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

The Haven ratio H-R 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-1) is uncorrelated (H-R approximate to1), but H-R decreases rapidly with increasing x and remains nearly constant (H-R = 0.2-0.4) at x greater than or equal to 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 x 10(3) to 40 Angstrom (-1), for a number of silver chalcogenide glassy systems suggest a structural origin for this universal trend. Glasses from the critical percolation domain (x(c) less than or equal to x less than or equal to 1-3 at.%Ag, where x(e) approximate to 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 H-R. In contrast, glasses from the modifier-controlled domain (x greater than or equal to 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 approximate to3 Angstrom, and so the Haven ratio remains essentially constant. (C) 2001 Elsevier Science B.V. All rights reserved.