Comparative studies of low temperature X-ray diffraction experiments on R2Ir2O7 (R = Nd, Eu, and Pr)

TL;DR

This study uses high-resolution X-ray diffraction to examine the structural symmetry of R2Ir2O7 compounds at low temperatures, finding minimal symmetry breaking but notable magnetovolume effects associated with the metal-insulator transition in Nd2Ir2O7.

Contribution

It provides detailed experimental evidence that cubic symmetry remains intact across the MIT, highlighting the importance of Ir moment arrangements over structural symmetry breaking.

Findings

Negative thermal expansion in Nd2Ir2O7 below T_MIT

No cubic symmetry breaking observed in all compounds

Ir moment arrangement influences the MIT

Abstract

The cubic symmetry of pyrochlore iridium oxides R2Ir2O7 (R = Nd, Eu, and Pr) has been investigated by high resolution X-ray diffraction experiments down to 4 K,in order to clarify the relationship between the metal-insulator transition (MIT) and the small structural phase transition suggested by Raman scattering experiments in these compounds. We have found that a small negative thermal expansion of the order of 10^-3 angstroms appears only in Nd2Ir2O7 below the MIT, T_MIT = 34 K, resulting in the magnetovolume effect of the long-range order of Ir moments. However, any breaking of the cubic symmetry of three iridates has not been observed as appearance of superlattice reflections nor splittings of cubic reflections below T_MIT. These results imply that lowering of the cubic symmetry plays a minor role for the change in the electronic state of these compounds, while a formation of the special arrangement of Ir moments plays a major role for the MIT.