Average Structure of Ag₂S_1−xTe_x

Abstract

X-ray diffraction experiments of α-Ag₂S_1−xTe_x (x=0.2∼0.8) have been carried out at room temperature by the use of a step scanning method to study the distribution of cations in the so-called “averaged structure”. CuKα-radiation was used in the small region of s, defined as s=2sinθ⁄λ, and MoKα in the large one. The X-ray profiles show a remarkable oscillatory background together with Debye lines due to the regular arrangement of anions. On the assumption that Ag ions are randomly distributed, this oscillation can be separated into two partial interference functions, a_Ag-Ag(s) and a_Ag-anion(s), in a similar way to that for liquid binary alloys. Two partial radial distribution functions, g_Ag-Ag(r) and g_Ag-anion(r), were also obtained from the partial interference functions. From the above analysis, the following information has been obtained.
(1) Mean distance between Ag and anion is smaller than that between Ag and Ag. This is physically reasonable.
(2) g_Ag-Ag(r) shows a similar curve to that for liquid metals, indicating that Ag ions are randomly distributed. However, the sharp first peak observed in g_Ag-anion(r) shows that localization of Ag ions in a narrow region around the anion. The behavior of g(r) in the region of large r is close to that of liquid metals.
(3) Coordination number n_Ag-anion, estimated from 4πr²ρ₀g_Ag-anion, is 8.0. This value seems to be reasonable in terms of the arrangement of anions occupying bcc sites. On the other hand, n_Ag-Ag shows some scatter by the method of calculation, as in the case of liquid metals.