Order and disorder in the local and long-range structure of the spin-glass pyrochlore, Tb₂Mo₂O₇

To understand the origin of the spin-glass state in molybdate pyrochlores, the structure of Tb₂Mo₂O₇ is investigated using two techniques: the long-range lattice structure was measured using neutron powder diffraction, and local structure information was obtained from the extended x-ray absorption fine structure technique. While the long-range structure appears generally well ordered, enhanced mean-squared site displacements on the O(1) site and the lack of temperature dependence of the strongly anisotropic displacement parameters for both the Mo and O(1) sites indicate some disorder exists. Likewise, the local structure measurements indicate some Mo-Mo and Tb-O(1) nearest-neighbor disorder exists, similar to that found in the related spin-glass pyrochlore, Y₂Mo ₂O₇. Although the freezing temperature in Tb ₂Mo₂O₇, 25K, is slightly higher than in Y ₂Mo₂O₇, 22K, the degree of local pair distance disorder is actually less in Tb₂Mo₂O₇. This apparent contradiction is considered in light of the interactions involved in the freezing process.