Ionics 1 (1995) 153 Modelling the Structure and Ionic Conduction of Amorphous (AgI)x(AgPO3)l.x with the RMC Method J. D. Wicks and R. L. McGreevy* Dept of Physics and Astronomy, University College London, Gower St, London WC1E 6BT, UK. *Studsvik Neutron Research Laboratory, S-611 82 Nyktping, Swe.den Abstract. For the Fast time neutron diffraction, X-ray diffraction and EXAFS data have been combined simultaneously using the RMC method to model the fast-ion conducting glass, (AgI)x(AgPO3)l_x. This material is of considerable technological and scientific interest due to its high ionic conductivity at ambient temperature. We present some details of the RMC technique and highlight some of the structural information obtained from our models. The origin of the "first sharp diffraction peak" in the neutron diffraction data is explained, about which there has been considerable speculation. Diffusion pathways for ionic conduction are observed. A simple analysis of available free volume shows that a percolation transition in the ionic conductivity occurs between x = 0.2 and 0.3, in agreement with a prediction based on conductivity measurements. This study highlights the considerable power that these developments of the RMC method have for the structural modelling of complex amorphous materials. 1. Introduction The Reverse Monte Carlo (RMC) method has been used to study the structure of a wide range of disordered mate- rials [e.g. 1]. Neutron diffraction data have been modelled on a number of occasions [e.g. 2-4] and a recent study of vitreous silica combined neutron and X-ray diffraction data for the first time [5]. EXAFS on its own has been mo- delled with RMC [6] and methods of "coordination con- straints" developed to introduce simple chemical informa- tion within an RMC model [7-9]. This work presents a combination of all of these applications within the RMC method to build a series of structural models of the fast- ion conducting glass, (AgI)x(AgPO3)l_ x. This study serves to illustrate the potential power of this modelling technique for structural studies of complex disordered ma- terials. AgI-doped phosphate glasses have served as model examples of amorphous fast-ion conductors. These mate- rials have considerable technological appeal as sensors, micro-power supplies and solid state displays. As we in- crease I content the ionic conductivity increases drama- tically [10] up to the glass-forming limit at x - 0.55. At the same time, a feature at low Q in the neutron diffrac- tion data, usually termed the "first sharp diffraction peak" or FSDP, increases in height and shifts from Q = 1.0,~-1 to Q = 0.7A -1. This feature is generally attributed to an increase in intermediate range order (IRO) [11] and this study was undertaken to investigate the origin of this feature and its relation to the materials ionic conductivity. 2. Experiment Neutron diffraction (on the Liquid and Amorphous ma- terials Diffractometer (LAD) at the pulsed source, ISIS, Rutherford Appleton Laboratory, UK), X-ray diffraction (on Station 9.1 at the synchrotron radiation source (SRS) Daresbury Laboratory, UK) and EXAFS experiments at both the Ag K-edge and I Lm-edges (Stations 9.2 and 7.1 at the SRS) have all been completed on the same samples of (AgI)x(AgPO3)l. x glass, x = 0.0, 0.05, 0.1, 0.2, 0.3, 0.4 and 0.5. The experimental details have been described elsewhere and all data have been appropiately corrected and normalised using standard procedures [8]. In terms of the relative contributions to the scattering, the neutron diffraction data provides information primarily on the P