Synthesis, electron diffraction, XRD and DSC study of the new elpasolite-related oxyfluoride, Tl₃MoO₃F₃

Tl₃MoO₃F₃, a previously unreported member of the A₂BM^VIO₃F₃ family of elpasolite-related oxyfluorides, has been prepared by the reaction of TlF with MoO₃ at 655°C. DSC shows two major polymorphic phase transitions at 42°C and 130°C, respectively. Electron diffraction and XRD studies of the complex room temperature polymorphic form of this material indicate that the spacegroup symmetry is monoclinic P1a1 with a superstructure unit cell given by a=3a_p+c_p, b=3b_p, c=12(-a_p+3c_p), when expressed with respect to the underlying ideal elpasolite-type parent structure. This superstructure, while related, is not isomorphous to that recently reported for K₃MoO₃F₃. The existence of a shared subset of strong G_p±J/5[204]_p* type satellite reflections suggests the existence of a common intermediate superstructure. A highly structured, three-dimensional continuous diffuse intensity distribution is observed in Tl₃MoO₃F₃ and Rb₂KMoO₃F₃. This suggests that a particular pattern of local O/F ordering and associated Mo ion shifts, recently shown to be responsible for the existence of this diffuse distribution in the case of K₃MoO₃F₃, may be common to the entire family of elpasolite-related A₂BM^VIO₃F₃ compounds.