Intercalate order–disorder transition in stage-2 AgxTiS2

Stage 2 samples of AgxTiS2 have been prepared by the electrointercalation of Ag into TiS2. The intercalated silver atoms are observed to undergo an order–disorder transition with Tc = 265 K. Raman spectra from the ordered phase of the samples contain both the modes expected from the pure crystal and two additional modes. The new modes are attributed to a zone-folding mechanism and a theoretical model is presented to describe this process. The model predicts that the intensity of the zone-folding modes should vary as the order parameter squared (η2). Values η2 as a function of temperature were calculated using a two-dimensional lattice gas model and Monte Carlo techniques. Good agreement is obtained between the calculations and the experimentally observed temperature dependence of the zone-folded mode intensities. The results are discussed and compared with recent theoretical and experimental developments.