Dilute stuffing in the pyrochlore

The pyrochlore Eu2Ir2O7 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 (AFM) long-range ordering of the Ir-moments below TN ∼120 K. However, there are contradictory reports concerning the slope dρ/dT of the resistivity plots (ρ) in the ”metallic” state above the metal-insulator (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 Eu2Ir2O7 samples with slightly varying Eu:Ir ratio. High-resolution synchrotron powder diffraction are done to probe minor variations in the cell parameters of the various Eu2Ir2O7 samples investigated here. Specific heat (Cp) and magnetic susceptibility of all the samples showed long-range antiferromagnetic ordering upon cooling below TN ∼120 K. The transitions are, however, found to be smeared out for the off-stoichiometric samples. We show that the sign of dρ/dT above the metal-insulator (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 dρ/dT from negative to positive upon cooling below a certain temperature T ∗ > TMI. With increasing Eu-stuffing T∗ decreased until a negative dρ/dT persisted without any sign change down to TMI.