The formation of a spin glass generally requires that magnetic exchange interactions are both frustrated and disordered. Consequently, the origin of spin-glass behavior in ${\mathrm{Y}}_2{\mathrm{Mo}}_2{\mathrm{O}}_7$—in which magnetic ${\mathrm{Mo}}^{4+}$ ions occupy a frustrated pyrochlore lattice with minimal compositional disorder—has been a longstanding question. Here, we use neutron and x-ray pair-distribution function (PDF) analysis to develop a disorder model that resolves apparent incompatibilities between previously reported PDF, extended x-ray-absorption fine structure spectroscopy, and NMR studies, and provides a new and physical explanation of the exchange disorder responsible for spin-glass formation. We show that ${\mathrm{Mo}}^{4+}$ ions displace according to a local “two-in–two-out” rule on each ${\mathrm{Mo}}_4$ tetrahedron, driven by orbital dimerization of Jahn-Teller active ${\mathrm{Mo}}^{4+}$ ions. Long-range orbital order is prevented by the macroscopic degeneracy of dimer coverings permitted by the pyrochlore lattice. Cooperative ${\mathrm{O}}^{2-}$ displacements yield a distribution of Mo–O–Mo angles, which in turn introduces disorder into magnetic interactions. Our study demonstrates experimentally how frustration of atomic displacements can assume the role of compositional disorder in driving a spin-glass transition.

• Received 9 March 2016

DOI:https://doi.org/10.1103/PhysRevLett.118.067201