Structural data from X-ray powder diffraction for new high-temperature phases (Y1-xLnx)2Si2O7 with Ln=Ce, Pr, Nd

Extending the investigation on possible mixed phases with composition (Y1xLnx)2Si2O7 formed at high temperature under oxidizing conditions in silicon nitride samples doped with Y2O3 and Ln2O3 as sintering aids, the compounds with Ln=Ce, Pr, Nd were successfully synthesized and structurally characterized by Rietveld whole powder pattern refinement. Some fluctuations in the x values were found as well, but all the compounds are monoclinic (space group P21/c), with lattice parameters close to those of the previously studied (Y2/3La1/3)2Si2O7. Further efforts with Sm (and Dy) failed in producing the expected G-form phase, indicating that the family does not extend beyond Nd: in these cases Ln ions partially substitute Y in its already known a- and d-disilicates. Under the adopted experimental conditions (1 atm, ambient air), an explanation of the impossibility to obtain the phase in question containing lanthanides with Z higher than 62 was attempted on the basis of lattice strains introduced by foreign ions larger than Y.