The XRD patterns of the
XANES spectroscopy probes the local chemical environment including the valence state, coordination and site geometry. Fig. 6 shows the normalized Ti K-edge XANES spectra obtained from Y2HfxTi2−xO7 with x=0, 0.5, 1.0, 1.5 and 1.7; our results are similar to those reported for Gd2Ti2−xSnxO7 and Gd2Ti2−xZrxO7  and . The main-edge feature (labeled B) corresponds to a dipole transition from the Ti 1s core level to the unoccupied Ti 4p states. In addition, the pre-edge region is particularly sensitive to the local coordination environment around the Ti DAPTinhibitor . Three features (A1, A2 and A3) are observed in the pre-edge region, due to both local and non-local transitions . For Ti in an octahedral environment (e.g., B-site cations in the ordered pyrochlore structure), feature A1 is attributed to the 1s→3d-t2g quadrupolar (local) transitions, A3 originates from the non-local 1s→p transitions (where the empty p states of the absorbing Ti atom are hybridized with the empty d states of the Ti second neighbors via O 2p states), and A2 is comprised of both local and non-local contributions (lower energy region, A2a, resulting from 1s→3d-eg transitions and higher energy region, A2b, from non-local inter-site hybridization) . As shown in Fig. 6 the main edge does not shift with the substitution of Hf for Ti, confirming that Ti remains tetravalent in Y2HfxTi2−xO7. There are, however, obvious changes in the pre-edge region (inset of Fig. 6). The spectrum of the Ti end member, Y2Ti2O7, is similar to that of Gd2Ti2O7  and . When Hf content (x) is increased to 0.5, the main change in the pre-edge region is the decrease in intensity of A2 (particularly A2b) and A3, consistent with Ti remaining in a 6-coordinated site but having fewer unoccupied next-nearest neighbor d states (i.e., decreased inter-site hybridization) . For Y2HfTiO7 (x=1.0, when the long-range crystal structure is still ordered pyrochlore based on XRD), feature A1 shifts to higher energy and becomes broader and more intense, while A2a shifts to slightly lower energy. This trend continues with the increase of x to 1.5 and 1.7. The smaller energy gap between A1 and A2a and increased intensity ratio of A1/A2a are consistent with the presence of Ti anti-site (i.e., Ti occupying the A-site in pyrochlore, and eventually the average 7-coordinated cation site in disordered defect-fluorite structure), as the crystal field splitting and intensity ratio are both expected to decrease with increasing coordination number (CN) . It is interesting to note that the Ti K-edge XANES spectrum for the pyrochlore structured Y2Hf1.5Ti0.5O7 is essentially the same as that for the defect-fluorite structured Y2Hf1.7Ti0.3O7 (although the latter is noisier due to lower Ti concentration), suggesting that the local structure is very similar in these two samples, in excellent agreement with Raman analysis ( Fig. 4D and E). In summary, our XANES results show the gradual nature of the disordering process in the local structure, which starts at much lower Hf content than that corresponding to the pyrochlore to defect-fluorite phase transition (x>1.5) as determined by XRD. Similar conclusions were made in a recent study of the Y2Sn2−xZrxO7 series .