Structural elucidation of β-(Y,Sc)2Si2O7: combined use of89Y MAS NMR and powder diffraction

Abstract

Although the structures of pure Sc2Si2O7and β-Y2Si2O7have been described in the literature using theC2/mspace group,29Si magic angle spinning (MAS) NMR measurements of the intermediate members of the Sc2Si2O7–β-Y2Si2O7system indicate a lowering of the symmetry to theC2 space group. Indeed, these compositions exhibit a unique Si crystallographic site and an Si—O—Si angle lower than 180°, incompatible with theC2/mspace group.C2 is the only possible alternative. Space groupCmcan be discarded with regard to its two different Si sites per unit cell. Moreover,89Y MAS NMR data have revealed the existence of two different Y sites in the structure of the intermediate members of the Sc2Si2O7–β-Y2Si2O7system, confirming the lowering of the symmetry to theC2 space group. The viability of theC2 model has therefore been tested and confirmed by refinement of synchrotron and neutron powder diffraction data for the different members of the system. The structural evolutions across the Sc2Si2O7–β-Y2Si2O7system are discussed.