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The geometrically frustrated spin-glass Y 2 Mo 2 O 7 has been widely considered to be crystallographically ordered with a unique nearest-neighbor magnetic exchange interaction, J. To test this assertion, we present x-ray-absorption fine-structure results for the Mo and Y K edges as a function of temperature and compare them to results from a well-ordered pyrochlore, Tl 2 Mn 2 O 7 . We find that the Mo-Mo pair distances are significantly disordered at approximately right angles to the Y-Mo pairs. These results strongly suggest that lattice disorder nucleates the spin-glass phase in this material. RAPID COMMUNICATIONS R756PRB 62 C. H. BOOTH et al. RAPID COMMUNICATIONS R758PRB 62 C. H. BOOTH et al.