Table 1.

Relevant interatomic distances

	Cu2–O2 Cu1–O5/6	Cu1–Cu1 Cu2–Cu2	Cu1–Oap	Cu2–Oap	Cu2–Cu2: E||c	Cu1–Cu2	Cu2–Y	Cu2–Sr	Cu1–Sr
xtal (14)	1.92	3.83	1.88	2.16	3.46	4.05	3.21	3.31	3.46
Cu EXAFS	1.91	3.81	1.88	2.14	3.45	4.04	3.19	3.35/3.31
Mo EXAFS	1.84/2.04/2.33/2.54	3.79 (Cu1)/3.95 (Mo)	1.88/2.39/2.69			4.05			3.48

Errors are 0.01 to 0.02 Å. The resolution limit for the Cu EXAFS is 0.11 Å and for the Mo EXAFS 0.12 Å. Because of the Cu/Mo and additional disorder, the referenced crystal structure found multiple Cu1–O positions labeled O5 and O6, none of which were along the Cu1–Cu1 vectors. In contrast, as stated in the description of the results of the curve fits, the EXAFS finds O positions very similar to YBa₂Cu₃O₇ for both the Cu and the Mo and, because there is apparently only a single O site, we labeled it O4. These differences are not unexpected; the diffraction analysis must describe the disorder under the constraints of the space-group symmetry derived from the average Cu and Mo positions that, as described in the text, are not equivalent. As discussed in the text and shown in Fig. 3A, the single Cu2–Oap distance listed here is the average position of its actual two-site distribution. The two-site Cu–Oap distribution was analyzed separately. The two Cu2–Sr distances are from the E||aa and E||c spectra. Cu1–Sr did not make an identifiable contribution to the EXAFS in both orientations. In the Cu EXAFS, the Cu1–Cu1 and Cu2–Cu2 contributions are combined; the Mo EXAFS will contain Mo–Cu1 and Mo–Mo. The analysis of the Mo E||aa EXAFS spectrum found four Mo–O4 distances separated by more than the resolution limit; the Mo E⊥||c found 3. The Mo–O pairs with Mo–O distances 2.33 and 2.39 Å Mo–O found in the analysis of the spectra from the two orientations most likely represent the same O neighbor that would be at an interstitial position off of the vectors through the Cu and Mo sites parallel to the crystallographic axes because it makes contributions to both orientations. The Mo–Mo, Mo–Cu2, and Cu1–Cu1/Cu2–Cu2 distances were obtained by using amplitudes and phases that are the sum of the two-, three-, and four-leg paths calculated by FEFF9 (69) from the crystal structure.