title: Orbital Dimer Model for the Spin-Glass State in Y_{2}Mo_{2}O_{7}.
abstract: 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 Y_{2}Mo_{2}O_{7}-in which magnetic 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 Mo^{4+} ions displace according to a local "two-in-two-out" rule on each Mo_{4} tetrahedron, driven by orbital dimerization of Jahn-Teller active Mo^{4+} ions. Long-range orbital order is prevented by the macroscopic degeneracy of dimer coverings permitted by the pyrochlore lattice. Cooperative 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.
authors.affiliation: Department of Chemistry, University of Oxford, South Parks Road, Oxford OX1 3QR, United Kingdom.
authors.forename: Peter M M
authors.initials: PM
authors.lastname: Thygesen
authors.affiliation: Department of Chemistry, University of Oxford, South Parks Road, Oxford OX1 3QR, United Kingdom.|ISIS Facility, Rutherford Appleton Laboratory, Harwell Campus, Didcot, Oxfordshire OX11 0QX, United Kingdom.|School of Physics, Georgia Institute of Technology, 837 State Street, Atlanta, Georgia 30332-0430, USA.
authors.forename: Joseph A M
authors.initials: JA
authors.lastname: Paddison
authors.affiliation: Department of Chemistry, University of Oxford, South Parks Road, Oxford OX1 3QR, United Kingdom.
authors.forename: Ronghuan
authors.initials: R
authors.lastname: Zhang
authors.affiliation: Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA.
authors.forename: Kevin A
authors.initials: KA
authors.lastname: Beyer
authors.affiliation: Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA.
authors.forename: Karena W
authors.initials: KW
authors.lastname: Chapman
authors.affiliation: ISIS Facility, Rutherford Appleton Laboratory, Harwell Campus, Didcot, Oxfordshire OX11 0QX, United Kingdom.
authors.forename: Helen Y
authors.initials: HY
authors.lastname: Playford
authors.affiliation: ISIS Facility, Rutherford Appleton Laboratory, Harwell Campus, Didcot, Oxfordshire OX11 0QX, United Kingdom.|Diamond Light Source, Chilton, Oxfordshire OX11 0DE, United Kingdom.|Spallation Neutron Source, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA.
authors.forename: Matthew G
authors.initials: MG
authors.lastname: Tucker
authors.affiliation: ISIS Facility, Rutherford Appleton Laboratory, Harwell Campus, Didcot, Oxfordshire OX11 0QX, United Kingdom.
authors.forename: David A
authors.initials: DA
authors.lastname: Keen
authors.affiliation: Department of Chemistry, University of Oxford, South Parks Road, Oxford OX1 3QR, United Kingdom.
authors.forename: Michael A
authors.initials: MA
authors.lastname: Hayward
authors.affiliation: Department of Chemistry, University of Oxford, South Parks Road, Oxford OX1 3QR, United Kingdom.
authors.forename: Andrew L
authors.initials: AL
authors.lastname: Goodwin