Structural elucidation of Β-(Y,Sc) 2Si 2O 7: Combined use of 89Y MAS NMR and powder diffraction

Description

Although the structures of pure Sc2Si2O7 and Β-Y 2Si 2O 7 have been described in the literature using the C2/m space group, 29Si magic angle spinning (MAS) NMR measurements of the intermediate members of the Sc 2Si 2O 7-Β- Y 2Si 2O 7 system indicate a lowering of the symmetry to the C2 space group. Indeed, these compositions exhibit a unique Si crystallographic site and an Si-O-Si angle lower than 180°, incompatible with the C2/m space group. C2 is the only possible alternative. Space group Cm can 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 Sc 2Si 2O 7-Β-Y 2Si 2O 7 system, confirming the lowering of the symmetry to the C2 space group. The viability of the C2 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 2Si 2O 7-Β-Y 2Si 2O 7 system are discussed.

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US Department of Energy DE-AC02-06CH11357

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European Union MRTN-CT-2006-035957

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Dirección General de Investigación Científica y Técnica (DGICYT) CTQ2010-14874/BQU

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Junta de Andalucía FQM 06090

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