X-ray diffraction study of basal-(ab)-plane structure and diffuse scattering from silver atoms in disordered stage-2 Ag₀.₁₈TiS₂

X-ray results are presented on the crystallographic parameters in the basal (ab) plane and the short-range-order diffuse scattering from intercalated silver atoms in disordered stage-2 Ag_0.18TiS₂ at room temperature. The silver atoms in the intercalated plane occupy the octahedral sites and the in-plane temperature parameter of the silver atoms has a value of B = 3.0 ± 0.1 Ų, suggesting both weak bonding and correspondingly rapid diffusion. Rodlike diffuse scattering parallel to c* at ⅓⅓.0, ⅔⅔.0 and their equivalent positions is observed and reveals the two-dimensional (2D) feature of the disordered state. From analyzing this 2D diffuse intensity by the method of Borie & Sparks [Acta Cryst. (1971), A27, 198-201], the planar short-range-order parameters were determined. A comparison of this planar short-range order with the 2D Ornstein-Zernike correlation function demonstrates the long-range nature of the 2D short-range order with a correlation range κ^-1 of 4.88 ± 0.4 Å. Using a linearized mean-field approximation for the correlation functions of a binary Ising system well above T_c developed by Clapp & Moss [Phys. Rev. (1966), 142, 418-427], a set of oscillatory atomic-pair interaction potential ratios was obtained where the direct interactions were significant out to at least four to five neighbors.