Structural elucidation of β‐(Y,Sc) 2 Si 2 O 7 : combined use of 89 Y MAS NMR and powder diffraction

Although the structures of pure Sc 2 Si 2 O 7 and β‐Y 2 Si 2 O 7 have been described in the literature using the C 2/ m space group, 29 Si magic angle spinning (MAS) NMR measurements of the intermediate members of the Sc 2 Si 2 O 7 –β‐Y 2 Si 2 O 7 system indicate a lowering of the symmetry to the C 2 space group. Indeed, these compositions exhibit a unique Si crystallographic site and an Si—O—Si angle lower than 180°, incompatible with the C 2/ m space group. C 2 is the only possible alternative. Space group Cm can be discarded with regard to its two different Si sites per unit cell. Moreover, 89 Y MAS NMR data have revealed the existence of two different Y sites in the structure of the intermediate members of the Sc 2 Si 2 O 7 –β‐Y 2 Si 2 O 7 system, confirming the lowering of the symmetry to the C 2 space group. The viability of the C 2 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 Sc 2 Si 2 O 7 –β‐Y 2 Si 2 O 7 system are discussed.