Abstract
The transition from a semiconductor to a fast-ion conductor with increasing silver content along the Ag x (Ge 0.25 Se 0.75) (100−x) tie line (0 ≤ x ≤ 25) was investigated on multiple length scales by employing a combination of electric force microscopy, X-ray diffraction, and neutron diffraction. The microscopy results show separation into silver-rich and silver-poor phases, where the Ag-rich phase percolates at the onset of fast-ion conductivity. The method of neutron diffraction with Ag isotope substitution was applied to the x = 5 and x = 25 compositions, and the results indicate an evolution in structure of the Ag-rich phase with change of composition. The Ag-Se nearest-neighbours are distributed about a distance of 2.64(1) Å, and the Ag-Se coordination number increases from 2.6(3) at x = 5 to 3.3(2) at x = 25. For x = 25, the measured Ag-Ag partial pair-distribution function gives 1.9(2) Ag-Ag nearest-neighbours at a distance of 3.02(2) Å. The results show breakage of Se-Se homopolar bonds as silver is added to the Ge 0.25 Se 0.75 base glass, and the limit of glass-formation at x 28 coincides with an elimination of these bonds. A model is proposed for tracking the breakage of Se-Se homopolar bonds as silver is added to the base glass.