Dilute stuffing in the pyrochlore iridate Eu2 Ir2 O7

Abstract

The pyrochlore Eu 2 Ir 2 O 7 has recently attracted significant attention as a candidate Weyl semimetal. The previous reports on this compound unanimously show a thermally induced metal-to-insulator (MI) transition, concomitant with antiferromagnetic long-range ordering of the Ir moments below T N ∼ 120 K. However, there are contradictory reports concerning the slope d ρ / d T of the resistivity plots ( ρ ) in the “metallic” state above the MI transition, and the value of ρ in the insulating state, both of which show significant sample dependence. Here, we explore this issue by investigating six different Eu 2 Ir 2 O 7 samples with slightly varying Eu:Ir ratio. High-resolution synchrotron powder diffraction is done to probe minor variations in the cell parameters of the various Eu 2 Ir 2 O 7 samples investigated here. Specific heat ( C p ) and magnetic susceptibility of all the samples showed long-range antiferromagnetic ordering upon cooling below T N ∼ 120 K. The transitions are, however, found to be smeared out for the off-stoichiometric samples. We show that the sign of d ρ / d T above the MI transition is highly sensitive to the unit-cell length, which, in turn, depends on the level of Eu stuffing at the Ir site. Samples with composition close to the ideal stoichiometry (Eu: Ir = 1 ) showed a change of sign of d ρ / d T from negative to positive upon cooling below a certain temperature T ∗ > T MI . With increasing Eu stuffing, T ∗ decreased until a negative d ρ / d T persisted without any sign change down to T MI .