Structural disorder, magnetism, and electrical and thermoelectric properties of pyrochlore Nd2Ru2O7

TL;DR

This study investigates the structural, magnetic, electrical, and thermoelectric properties of Nd2Ru2O7, revealing site disorder, magnetic transitions, and high electrical resistivity that limit thermoelectric applications.

Contribution

It provides detailed analysis of disorder and magnetic transitions in Nd2Ru2O7 and discusses its limited thermoelectric potential due to high resistivity.

Findings

Ru 4d spins order antiferromagnetically at 143 K

Nd 4f spins order below 10 K

Nd2Ru2O7 is an electrical insulator

Abstract

Polycrystalline Nd2Ru2O7 samples have been prepared and examined using a combination of structural, magnetic, and electrical and thermal transport studies. Analysis of synchrotron X-ray and neutron diffraction patterns suggests some site disorder on the A-site in the pyrochlore sublattice: Ru substitutes on the Nd-site up to 7.0(3)%, regardless of the different preparative conditions explored. Intrinsic magnetic and electrical transport properties have been measured. Ru 4d spins order antiferromagnetically at 143 K as seen both in susceptibility and specific heat, and there is a corresponding change in the electrical resistivity behaviour. A second antiferromagnetic ordering transition seen below 10 K is attributed to ordering of Nd 4f spins. Nd2Ru2O7 is an electrical insulator, and this behaviour is believed to be independent of the Ru-antisite disorder on the Nd site. The electrical properties of Nd2Ru2O7 are presented in the light of data published on all A2Ru2O7 pyrochlores, and we emphasize the special structural role that Bi3+ ions on the A-site play in driving metallic behaviour. High-temperature thermoelectric properties have also been measured. When considered in the context of known thermoelectric materials with useful figures-of-merit, it is clear that Nd2Ru2O7 has excessively high electrical resistivity which prevents it from being an effective thermoelectric. A method for screening candidate thermoelectrics is suggested.