Low temperature structural anomalies arising from competing exchange interactions in pyrochlore Nd₂Ru₂O₇ probed by XRD and EXAFS

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.