Elizabeth J. Harvey

Characterisation of the (Y1−xLax)2Ti2O7 system by powder diffraction and nuclear magnetic resonance methods

Structural characteristics of the (Y1−xLax)2Ti2O7 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 89Y (I = ½) magic-angle spinning nuclear magnetic resonance spectroscopy (MAS NMR). Lattice parameters obtained from neutron data suggest that a pyrochlore (Fdm, Z = 8) solid solution exists from x = 0 to x = 0.134, whereupon a monoclinic La2Ti2O7-type phase (P21, 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 A3 for Y2Ti2O7 to 1037.49 A3 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.

Radiation Damage in Pyrochlore and Related Compounds

The radiation damage properties of synthetic pyrochlore-defect fluorite compounds containing lanthanides on the A-site and Ti, Zr, Sn, and Hf on the B-site have been studied extensively using Kr ion irradiation. Using statistical analysis, we show that the results can be quantified in terms of the critical temperature for amorphization, structural parameters, classical Pauling electronegativity difference, and defect energies. The best current model is able to predict the critical temperature to within about 80 degrees Kelvin. The model indicates that radiation tolerance is correlated with an increase in the X anion coordinate toward the value characteristic of the defect fluorite topology, a smaller unit cell dimension, and lower defect energies. Our analysis also demonstrates that radiation tolerance is promoted by an increase in the Pauling cation-anion electronegativity difference or, in other words, an increase in the ionicity of the chemical bonds. Of the two possible cation sites in ideal pyrochlore, the B-site cation appears to play the major role in bonding. This result is supported, for a subset of pyrochlore compounds, by ab initio calculations, which reveal a correlation between the Mulliken overlap populations of the B-site cation and the critical temperature.

Composition driven structural phase transitions in the (Y1−xLax)2Ti2O7 system

The structural characteristics of samples in the (Y1-xLax)2Ti2O system have been studied as a function of bulk composition using time-of-flight (TOF) powder neutron diffraction, powder X-ray diffraction (XRD), Electron Microscopy and 89Y (I = ½) Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR). Analysis of the neutron diffraction data suggests the following: For compositions where 0 ≤ x ≤ 0.132, a single cubic phase is present (Fd3,¯ m, Z = 8). Between 0.832 ≤ x ≤ 1, a solid solution with the La2Ti2O7 structure (P21, Z = 4) forms. The two phases coexist between x values of 0.132 and 0.832. XRD suggests that the limits of the two regions of solid solution fall within the ranges of x = 0.1-0.125 and x = 0.875-0.9. The variation in the limits found by the two diffraction methods may be due to stoichiometric errors in the samples. Line-shape analysis of NMR spectra for Y rich compositions suggests that increasing proportions of La are incorporated onto the pyrochlore A site in a statistically random manner, up to the limit of solid solution. The NMR spectra of the monoclinic phase suggest that occupation of the four crystallographically distinct ‘A’ type sites within the structure is not random. At low Y concentrations, two of these sites are preferentially occupied by Y. These sites are suggested to be those found at the edge of the perovskite slabs, which exhibit smaller coordination numbers (based on the number of oxygens within bonding distance of the ‘A’ type cation) than the true perovskite A sites found at the slab centres.