Quantitative structural parameters of pyrochlore Nd2Ru2O7, with temperature dependence, have been derived upon fitting XRD and EXAFS data. An anomalous expansion of the lattice parameter and the Ru-O bond length indicates a structural instability at low temperatures; in particular, an increase in the non-thermal term of the mean square fluctuation in the bond length is the evidence for a static disorder of Ru atoms. This static disorder is closely correlated with a decrease in the average Ru-O-Ru bond angle with decreasing temperature, favoring the short-range ferromagnetic coupling in the material. This ferromagnetic coupling formed thus triggered the spin frustration at low temperature when the contradictory constraints of antiferromagnetic interaction act upon the same Ru site in the corner-sharing tetrahedrons of pyrochlore Nd2Ru2O7. This study demonstrates that the spin frustration arising from the competition of ferromagnetic/antiferromagnetic interactions in pyrochlore Nd2Ru2O7 will cause structural instability especially on the atomic scale, which provides a new point of view to help understand its particular magnetic state.

We present magnetic susceptibility, heat capacity, and neutron diffraction measurements of polycrystalline NdRuO down to 0.4 K. Three anomalies in the magnetic susceptibility measurements at 146, 21 and 1.8 K are associated with an antiferromagnetic ordering of the Ru moments, a weak ferromagnetic signal attributed to a canting of the Ru and Nd moments, and a long-range-ordering of the Nd moments, respectively. The long-range order of the Nd moments was observed in all the measurements, indicating that the ground state of the compound is not a spin glass. The magnetic entropy of Rln2 accumulated up to 5 K, suggests the Nd has a doublet ground state. Lattice distortions accompany the transitions, as revealed by neutron diffraction measurements, and in agreement with earlier synchrotron x-ray studies. The magnetic moment of the Nd ion at 0.4 K is estimated to be 1.54(2)µ and the magnetic structure is all-in all-out as determined by our neutron diffraction measurements.