Correlated electron metal properties of the honeycomb ruthenate Na2RuO3

Abstract

We report the synthesis and characterization of polycrystalline Na 2 RuO 3 , a layered material in which the Ru 4+ (4d 4 configuration) form a honeycomb lattice. The optimal synthesis condition was found to produce a nearly ordered Na 2 RuO 3 (C2/c phase), as assessed from the refinement of the time-of-flight neutron powder diffraction. Magnetic susceptibility measurements reveal a large temperature-independent Pauli paramagnetism [χ 0 ∼ 1.42(2) × 10 −3 emu/mol Oe] with no evidence of magnetic ordering down to 1.5 K, and with an absence of dynamic magnetic correlations, as evidenced by neutron scattering spectroscopy. The intrinsic susceptibility (χ 0 ) together with the Sommerfeld coefficient of γ = 11.7(2) mJ/Ru mol K 2 estimated from heat capacity measurements gives an enhanced Wilson ratio of R W ≈ 8.9(1), suggesting that magnetic correlations may be present in this material. While transport measurements on pressed pellets show nonmetallic behavior, photoemission spectroscopy indicates a small but finite density of states at the Fermi energy, suggesting that the bulk material is metallic. Except for resistivity measurements, which may have been compromised by near-surface and interface effects, all other probes indicate that Na 2 RuO 3 is a moderately correlated electron metal. Our results thus stand in contrast to earlier reports that Na 2 RuO 3 is an antiferromagnetic insulator at low temperatures.

Quantum materials with strong spin–orbit coupling: challenges and opportunities for materials chemists

“…The oft-made assumption that single crystals are 'better' than powders is not a reliable one. 133,134 One other major challenge is polymorphism. Syntheses of Li 2 IrO 3 , for example, often do not reliably produce the target polymorph, even when a previously-successful method is followed.…”

“…In 2014, Wang et al 138 reported the susceptibility of a single-crystal sample oriented with the plane of the RuO 6 layers perpendicular to (blue) or parallel to (red) a 3 T applied field. In 2020, Veiga et al 134 reported the susceptibility of a polycrystalline sample in a 1 T applied field (green; the dotted line shows an estimated intrinsic susceptibility obtained by subtracting a Curie-like impurity contribution). Recent studies of polycrystalline samples have shown that Na 2 RuO 3 is not stable in air at ambient conditions and that carefully handled samples do not display long-range magnetic order.…”

“…Recent studies of polycrystalline samples have shown that Na 2 RuO 3 is not stable in air at ambient conditions and that carefully handled samples do not display long-range magnetic order. 134,139 ment (Figure 7). 134,139 Na 4 Ir 3 O 8 also degrades on storage under ambient conditions, with sodium gradually being lost and the physical properties changing substantially.…”

“…134,139 ment (Figure 7). 134,139 Na 4 Ir 3 O 8 also degrades on storage under ambient conditions, with sodium gradually being lost and the physical properties changing substantially. 119,141,142 Such problems are not limited to materials containing volatile elements such as Na -in 2019 two groups reported property measurements of the KSbO 3 -type iridate La 3 Ir 3 O 11 , though as one found an anomaly at 3.5 K whilst the other did not, they cannot truly have been measuring the same material.…”

Defect control in the Heisenberg-Kitaev candidate material NaRuO$_2$

“…Even within studies focused predominantly on disordered Na 2 RuO 3 or mixtures of the ordered/disordered phase, there are conflicting reports. Some works suggest insulating behavior with long range antiferromagnetic order [21,22], while others report a paramagnetic, moderately correlated electron metal with no observable magnetic excitations [23].…”