Orthorombic to monoclinic phase transition in high-Ti-loaded TS-1: an attempt to locate Ti in the MFI framwework by low temperature XRD

The orthorhombic to monoclinic phase transition in an high-titanium-loaded Ti-silicalite sample (2.6 wt.% in TiO2) has been studied by means of temperature-dependent high resolution powder X-ray diffraction data collected at the ESRF BM16 beam line. We observed that the transition temperature between the orthorhombic phase (Pnma space group), and the monoclinic phase (P2(1)/n space group) is at 160 K, as monitored by the splitting of the 1 3 3 peak into (3) over bar 13 and 3 1 3 reflections. The structural data obtained after Rietveld refinement in both the orthorhombic (170 K) and monoclinic (80 K) phases are also reported. Refinement of the former data gives weak evidence that about half of the total Ti content is located in T(10) and T(11), that Ti does not show any tendency to be hosted in T(4) and T(12) and that the remaining Ti atoms seem to be spread among the remaining eight T sites.