New-Type Phase Transition of Li₂RuO₃ with Honeycomb Structure

Abstract

A new-type structural transition has been found in Li 2 RuO 3 with honeycomb lattice of edge-sharing RuO 6 -octahedra. With decreasing temperature T, the electrical resistivity exhibits an anomalous increase at T=T c~5 40 K, suggesting the (metal to insulator)-like transition and the magnetic susceptibility also shows a sharp decrease. Detailed structure analyses have revealed that the high temperature space group C2/m changes to P2 1 /m at T c . The most striking fact is that a significant reduction of the bond lengths is found between two of six Ru-Ru pairs of the hexagon in the low temperature phase, indicating a new type phase transition by the mechanism of the formation of molecular orbits of these Ru-Ru pairs.KEYWORDS: Li 2 RuO 3 , honeycomb structure, structural transition *Corresponding author: e43247a@nucc.cc.nagoya-u.ac.jpCompounds with the honeycomb lattice often present interesting behavior originating from their characteristic structures. For example, in the course of the studies on the physical properties of localized spin systems of A 3 T 2 SbO 6 (A=Na, Li; T=Cu, Ni, Co) and Na 2 T 2 TeO 6 on the (distorted) honeycomb lattice, spin gap behaviors have been found for T=Cu, [1][2][3] while the magnetic transitions to the spin-ordered state have been observed for T=Co and Ni. 4)As one of possible examples of conductive electrons on honeycomb lattice, we have investigated physical properties of Li 2 RuO 3 . It has the layers of the honeycomb lattice of edge-sharing RuO 6 octahedra with a LiO 6 octahedron at the center of each hexagon of RuO 6 (Fig. 1). The Ru valence is +4 and the four electrons exist in the 4d t 2g orbits. For this system, we have found a phase transition at temperature T=T c~5 40 K, where the crystal symmetry changes from a monoclinic (space group C2/m) to another monoclinic (space group P2 1 /m) one with decreasing T. As described in detail later, the transition is associated with the molecular orbit formation of Ru 4+ -Ru 4+ ions of the edge-sharing RuO 6 pair, presenting a new mechanism of structural transitions.Polycrystalline samples of Li 2 RuO 3 were prepared by sintering pellets of mixtures of RuO 2 and Li 2 CO 3 with proper molar ratio at 1000 ˚C for 24 h in air. 5,6) The powder neutron diffraction patterns of these samples indicate that a small amount of RuO 2 (molar fraction of ∼1.20 %) exists. There also exists an impurity peak of the unidentified phase, which has the integrated intensity of ~4.5 % of the maximum integrated intensity of the main phase as shown later. The magnetic susceptibilities χ were measured using a Quantum Design SQUID magnetometer under a magnetic field H=1 T in the temperature range of 2-700 K. The electrical resistivities ρ were measured by the standard four-terminal method using an ac-resistance bridge from 4.6 K to 695 K. The specific heats C were measured by a thermal relaxation method in the temperature range of 5-60 K using a Physical Property Measurement System (PPMS, Quantum Design). Powder X-ray diffraction measurements were carrie...

Classical Dimers and Dimerized Superstructure in an Orbitally Degenerate Honeycomb Antiferromagnet

“…2D. This dimerization pattern exactly reproduces the one observed in the insulating phase of Li 2 RuO 3 [2]. We note that above described mechanism of the selection of spin-singlet dimer pattern equally applies to the case when singlets are formed by spin-one-half degrees of freedom.…”

“…The magnetically active Ru 4+ -ions are characterized by four electrons in the threefold degenerate t 2g -manifold coupled into a S = 1 state. Li 2 RuO 3 undergoes a metalto-insulator transition on cooling below 540 K [2]. At the transition the magnetic susceptibility shows a steep decrease and its low temperature value can be considered to be due almost entirely to the Van Vleck paramagnetism.…”

“…In the following we focus on a system with threefold-orbitally-degenerate S = 1 magnetic ions on a honeycomb lattice. This model is suitable to describe d 2 and d 4 -type transition-metal compounds with partially filled t 2g levels, like the layered compound Li 2 RuO 3 [2]. Here the layers are formed by edge-sharing network of RuO 6 and LiO 6 octahedra.…”

Calculation of exchange constants of the Heisenberg model in plane-wave-based methods using the Green's function approach

“…E.g. Na 2 IrO 3 is considered as a possible realization of the Kitaev model [50], while Li 2 RuO 3 shows unusual valence bond liquid phase at high temperatures [51] and spin gapped state below 540 K (at least in polycrystalline samples) [52,53]. In contrast to these systems in Li 2 MnO 3 the long range antiferromagnetic state is formed at T N =36 K with all Mn neighbors in the ab plane ordered AFM [54].…”

First-order magnetostructural transition in single crystals of the honeycomb lattice ruthenate Li2RuO3

“…Nearly perfect hexagons of the high temperature C2/m phase undergo strong distortion, leading to a low temperature structure with significant shortening of one of the three inequivalent Ru-Ru bonds on each honeycomb 20 . Based on DFT calculations on the low and high temperature structures it was proposed that Li 2 RuO 3 undergoes a transition from a highly correlated metal to a molecular orbital insulator involving Ru-Ru dimerization and spin-singlet formation 20,21 . An alternative mechanism of spin-singlet formation driven by magnetoelastic coupling has also been proposed 22 .…”