Revealing Structural Detail in the High Temperature La2Si2O7–Y2Si2O7 Phase Diagram by Synchrotron Powder Diffraction and Nuclear Magnetic Resonance Spectroscopy

High resolution synchrotron powder XRD, 89 Y CPMG NMR, and 139 La MAS NMR spectroscopy have been applied to eventually draw the phase diagram of the La 2 Si 2 O 7 –Y 2 Si 2 O 7 system. The diagram presents a solid solubility region of G-(La,Y) 2 Si 2 O 7, which extends to the La 0.9 Y 1.1 Si 2 O 7 composition at any temperature of this study. Compositions richer in Y show two-phase domains, with G + α at T < 1450 °C and G + δ at T > 1450 °C. The Y-rich extreme is more complex, showing two solid solution regions of δ- and γ-(La,Y) 2 Si 2 O 7 polymorphs which appear with increasing Y content, respectively. It is interesting to note that the La for Y substitution mechanism in the G-(La,Y) 2 Si 2 O 7 polymorph is not homogeneous, but a preferential occupation of Y for the RE 2 site is observed. Finally, the 89 Y and 139 La isotropic chemical shift values in G-(La,Y) 2 Si 2 O 7 have been described here for the first time and assigned to the different RE crystallographic sites of the unit cell.