Tuning the ground state of pyrochlore oxides using chemical pressure

Abstract

A2B2O7 oxides (where A3+ is a rare-earth and B4+ a transition-metal ion) crystallizing with the cubic pyrochlore structure are well-known three-dimensional spin frustrated systems. The source of frustration in these systems originate from the interpenetrating corner-linked tetrahedral frameworks of A3+ and B4+ ions. Recently, some of these pyrochlores with 5d transition metal ions occupying the B sublattice have gained considerable attention due to their novel topological properties emerging from the interplay of three fundamental energy scales: the onsite Coulomb repulsion (U), crystal field energy (∆), and, the spin-orbit coupling (λ). In the pyrochlores with B = Ir, these three energies are of the order of 1 eV, which places them at the crossroads leading to strongly correlated systems in large U limit, and topological insulators in the strong λ limit. The energy scale ∆, on the other hand, splits the spin-orbit coupled lowest J-multiplet in Ir4+ (d5), reducing the moment in the ground state to an effective J = 1/2. Consequently, the pyrochlore iridates exhibit interesting and unexpected properties, including a spin-liquid ground state in Pr2Ir2O7 (PIO), temperature-induced metal-to-insulator transition concomitant with the onset of antiferromagnetic ordering in R = Nd, Sm, Eu and Gd members, and an insulating behaviour in the members consisting of heavier rare-earths. Some of these iridates, namely the ones with R = Sm, Eu, Gd (and Y), are predicted to be in the close proximity of a Weyl semimetallic ground state, which motivated the present research. In order to realise a possible Weyl semimetallic ground state, we synthesized a series of (Eu1−xBix)2Ir2O7 samples. We chose Eu2Ir2O7 (EIO) due to its proximity to the Weyl semimetallic ground state. The other end member Bi2Ir2O7 (BIO) (which also crystallize with the cubic pyrochlore structure) is a strongly correlated metal. The crucial point is that both Eu and Bi form their respective compounds, which implies that a solid-solution between them will not affect the valence of Ir, and hence the J = 1/2 on the pyrochlore lattice is expected to remain unperturbed throughout the (Eu1−xBix)2Ir2O7 series. Additional advantage of doping with Bi is that the A-site remains magnetically inactive. We discovered a rather counter-intuitive result in this solid solution series. We found that even though the ionic radius of Bi3+ is considerably bigger than Eu3+, dilute Bi doping at the Eu site results in a lattice volume collapse without any apparent structural change or the valence of the constituents. The lattice contraction was confirmed using high-resolution synchrotron x-ray diffraction. The ground state of EIO was found to be robust for compositions in the anomalous volume contraction region and is strongly suppressed for higher doping concentration. In the anomalous region, we find that dilute Bi-doping tunes the ground state closer to the Weyl point, inferred from the behaviour of ρ vs. T, which is theoretically shown to follow a 1=T dependence by Hosur et al. At the metal-insulator boundary at x = 0.1, we observed absence of any magnetic ordering confirmed using µSR down to 20 mK. Further, from the transport behaviour, we conjectured a Quadratic Band Touching (QBT), another novel ground state from which other non-trivial topological phases can be derived, for this composition. We, therefore, conclude that the composition (Eu1:8Bi0:2)Ir2O7 is the parent phase hosting a QBT from which the WSM phase is derived through breaking of the time reversal symmetry in a narrow composition range around x = 0.02 where the index ‘n’ in the 1/Tn fit approaches n = 1 over a broad temperature range. We further extended this study to other members of the pyrochlore iridate series A2Ir2O7 (A = Sm, Gd and Dy) series. Though the anomalous lattice contraction was observed at dilute Bi doping in all the members, it was found to be the highest for the Bi substituted Sm2Ir2O7 (SIO). As BIO and SIO have the same lattice constant value, this established the electronic origin of the anomalous lattice contraction phenomena. Bi doping affected the physical properties for all the pyrochlore iridates. As the difference between the A-site ionic radius and Bi decreased, rapid suppression of the insulating and magnetic ground state of the pyrochlore compounds was observed. We carried out photoelectron spectroscopy to understand the mechanism of the metal-insulator transition in the (Eu1−xBix)2Ir2O7 series. The core-level spectra along with the valence band spectra indicated hybridisation between Bi 6p and Ir 5d orbitals as a possible mechanism for the emergence of metallic behaviour with increasing Bi doping. Additionally, we confirmed that the valence state of Eu, Bi and Ir remains unchanged over the whole range of x values. Our study suggests that Bi-doping in pyrochlore iridates is a unique way to realise novel ground states in this system. This is achieved as the isovalent Bi doping preserves the Ir4+ sublattice, which is a key to obtaining various non-trivial topological phases. In particular, we show that dilute Bi doping is a possible route to realizing the Weyl semimetallic ground state in A = Eu, and Sm. Our findings are expected to motivate further research in exploring new quantum phases in the U − λ phase space of 5d transition metal oxides.