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 (Y1-xLnx)2Si2O7 formed at high temperature under oxidising conditions in silicon nitride samples doped with Y2O3 and Ln2O3 as sintering aids, the compounds with Ln=Pr, Ce, Nd were successfully synthesised and structurally characterised 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 alfa and delta-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>=62 was attempted on the basis of lattice strain introduced by foreign ions larger than Y.