X-ray structural study of in-plane atomic arrangements in the layered compounds CuxTiS2

X-ray diffuse intensity measurements have been performed at room temperatur e on the local arrangements of intercalated Cu atoms in the layered compoun ds CuxTiS2 with compositions of x = 0.13, 0.23, 0.32 and 0.37. Two types of diffuse maximum due to the different in-plane correlations of Cu atoms app eared, depending on the composition. For the lowest content of Cu atoms(x = 0.13), diffuse maxima appeared at 1/2,0,1/2, 0,1/2,1/2, 1/2,1/2,1/2 and th eir equivalent positions. For the highest content of Cu atoms (x = 0.37), d iffuse maxima appeared at 1/3,1/3,1/2, 2/3,2/3,1/2 and their equivalent pos itions. In the cases of x = 0.23 and 0.32, the two types of diffuse scatter ing coexist, with intensities depending on the composition. From the experi mental results, we expect the following in-plane ordered structures for the low- temperature phases in the compounds CuxTiS2:2 x 2 and (root 3 x root 3)R30 degrees structures in Wood notation, whose stoichiometric composition s x correspond to 1/4 and 1/3, respectively. The diffuse rods are modulated along the scan parallel to the [001]* direction centred on 1/2,1/2,0 for x = 0.13, 0.23 and 0.32 and on 1/3,1/3,0 for x = 0.23, 0.32 and 0.37, with m axima appearing at every half-integer. This reveals an enhancement of the t hree-dimensional nature and a stacking sequence alpha beta alpha beta...is deduced. Finally, the observed in-plane diffuse intensities are compared wi th the calculated ones, using a linearized mean-field approximation for the correlations of a binary Ising system well above T-c developed by Clapp an d Moss.