X-ray study of superlattice melting in TiS2Ag0.33

We report the results of a detailed x-ray-scattering study of the layered intercalation compound Ti${\mathrm{S}}_2$ ${\mathrm{Ag}}_{0.33}$. Below room temperature, the Ag atoms form a three-dimensional commensurate superlattice with $\sqrt{3}\times \sqrt{3}R30\mathrm{}°$ ordering within Ag layers and a stacking sequence $\alpha \beta \alpha \beta \dots$. This stacking sequence puts the Ag superlattice ordering into the universality class of the three-dimensional $\mathrm{XY}$ model and hence represents a classic example of what has been called "continuous melting in three dimensions."