We propose to investigate the structure of the new ¿ordered¿ phases within the Bi2Se3-Bi2Te3 phase diagram of the newly discovered Topological Insulators. The bulk properties of the 3D- topological insulators discovered recently can be tuned by making subtle adjustments to their defect stoichiometry. We will investigate the ordering of the various Bi-Se-Te atoms within the tetradymite structure of the two intermediate compounds, Bi2Te2Se and Bi2Se2Te, as well as the end member,Bi2Te3, for comparison. This investigation will involve two parts, the first using powder diffraction studies on HRPD (2 days) and the second using single crystals on SXD (4 days). We propose to obtain a complete set of structural data on these new phases which will throw light on the bulk conduction properties, knowledge of which is essential in understanding their exotic surface transport properties.