Characterisation of the (Y_1-xLa_x)₂Ti₂O₇ system by powder diffraction and nuclear magnetic resonance methods

Structural characteristics of the ( Y1-xLax)(2)Ti2O7 system have been determined using time-of-flight (TOF) neutron and constant wavelength powder X-ray diffraction ( XRD), scanning electron microscopy ( SEM), electron probe micro- analysis ( EPMA) and Y-89 ( I = 1/2) magic-angle spinning nuclear magnetic resonance spectroscopy ( MAS NMR). Lattice parameters obtained from neutron data suggest that a pyrochlore ( Fd ($) over bar 3m, Z = 8) solid solution exists from x = 0 to x = 0.134, whereupon a monoclinic La2Ti2O7- type phase ( P2(1), Z = 4) exsolves. Both phases coexist for 0.134 <= x <= 0.807 and a single-phase solid solution with the monoclinic structure is stable for x. 0.807. Unit cell volumes from X-ray diffraction of samples synthesised within the pyrochlore regime show excellent agreement with the expected linear increase in unit cell volume across a solid solution, increasing from 1028.46 angstrom(3) for Y2Ti2O7 to 1037.49 angstrom(3) for ( Y0.90825La0.(09175)) 2Ti2O7. The limit of the monoclinic solid solution as observed by XRD is higher than that found from neutron diffraction ( 0.88 compared to 0.807), but this difference is attributed to variation in the sintering temperatures employed. Spectral analysis of NMR spectra for Y-rich compositions suggests that increasing proportions of La are incorporated onto the pyrochlore A-site randomly, up to the limit of solid solution. The NMR spectra of the monoclinic phase show four resonances - two broad and two sharp. These have been attributed to the four crystallographically distinct 'A'-type sites within the structure. At lower Y concentrations, the sites corresponding to the broad NMR resonances are preferentially occupied. We have tentatively assigned these broad resonances to the 'slab-edge' sites in the monoclinic perovskite structure, based on their relatively small size and lower average coordination numbers compared to the perovskite-like sites within the centre of the slabs.