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

Abstract

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.

Unveiling the Catalytic Origin of Nanocrystalline Yttrium Ruthenate Pyrochlore as a Bifunctional Electrocatalyst for Zn–Air Batteries

“…39 As shown in Figure 4a,b, the corresponding radial distribution function (RDF) of Fourier-transformed k 3 -weighted EXAFS spectra for ruthenium ions demonstrated the highest peak at reduced distances of around 1.6 Å, indicating Ru−O bonds at B-sites of the pyrochlore structure. 40 Notably, the reduced distances of Ru− O bonds remained constant (∼1.6 Å) upon cycling, whereas those of XANES spectra were changed, as discussed previously (insets of Figure 4a,b). This result could be explained by the high structural stability of the rigid RuO 6 structure in the pyrochlore oxide.…”

“…The EXAFS spectra were calculated by the general equation, which could be considered a variety of factors regarding the neighboring atoms . As shown in Figure a,b, the corresponding radial distribution function (RDF) of Fourier-transformed k 3 -weighted EXAFS spectra for ruthenium ions demonstrated the highest peak at reduced distances of around 1.6 Å, indicating Ru–O bonds at B-sites of the pyrochlore structure . Notably, the reduced distances of Ru–O bonds remained constant (∼1.6 Å) upon cycling, whereas those of XANES spectra were changed, as discussed previously (insets of Figure a,b).…”

Low temperature magnetic properties of Nd2Ru2O7

“…In this structure, Nd atoms are surrounded by 8 O-atoms, while each Ru atom is surrounded by 6 O-atoms. We have previously published structural studies of Nd 2 Ru 2 O 7 which combine high resolution powder x-ray diffraction (HR-XRD) and extended x-ray absorption fine structure (EXAFS) techniques [14].…”

“…For ruthenium pyrochlores, both the A (16d) and B (16c) sites are occupied by magnetic ions. The perturbation driven by the moment at the 16c site may cause a symmetry breaking and structural distortions [14]. Early studies on the ruthenium pyrochlores revealed that there is a λ-type jump in the specific heat in temperature range 75-160 K depending on the size of the rare-earth ion [15,16].…”

Evidence of a correlation between magnetic and structural transitions in Y2−xZnxRu2O7 pyrochlore compounds

“…14 Moreover in pyrochlores, cation or anion disorder may give rise to order-disorder transitions inuencing also the conductivity and the presence of metal-insulator transitions. 7,[15][16][17] This phenomenon is coherent with the pyrochlores predisposition towards full or partial disorder, structural distortion and phase transition from pyrochlore to a defect uorite structure. 6,18 In this general framework, doping Y 2 Ru 2 O 7 with Zn 2+ results in mixed charge at both A and B cationic sites and consequently a ruthenium mixed valence 4+/5+ is present.…”