Band Jahn-Teller Instability and Formation of Valence Bond Solid in a Mixed-Valent Spinel OxideLiRh2O4

Abstract

We have synthesized a new spinel oxide LiRh2O4 with a mixed-valent configuration of Rh 3+ and Rh 4+ . At room temperature it is a paramagnetic metal, but on cooling, a metal-insulator transition occurs and a valence bond solid state is formed below 170 K. We argue that the formation of valence bond solid is promoted by a band Jahn-Teller transition at 230 K and the resultant confinement of t2g holes within the xy band. The band Jahn-Teller instability is also responsible for the observed enhanced thermoelectric power in the orbital disordered phase above 230 K.Among the wide variety of structural categories of complex transition metal oxides, spinel, with chemical formula AB 2 O 4 , is one of the most common structures and provides a unique playground for the physics of geometrical frustration. The B-sublattice of the spinel structure consists of a three-dimensional network of tetrahedra, known as the pyrochlore lattice and can underpin strong geometrical frustration effects. When antiferromagnetically coupled spins are placed on the pyrochlore lattice, long range magnetic ordering is suppressed substantially leading to, amongst other things, quantum spin liquid behavior [1]. In many cases, however, a nontrivial self-organized state of spins marginally emerges by means of coupling with lattice distortion and/or orbital ordering [2]. In analogy with spins, when a spinel B-sublattice is occupied by ions with a formally half-integer valence, suppression of charge ordering and the eventual formation of a nontrivial state of charges results. Such charge frustration has attracted much interest since the discovery of Verwey ordering in the spinel

Structural instability of the rutile compounds and its relevance to the metal–insulator transition of VO2

“…For example, ZnV 2 O 4 (V 3+ ; 3d 2 ) undergoes an orbital ordering transition from a cubic to a tetragonal structure at 50 K, which results in two sets of orthogonal spin chains, and then an antiferromagnetic order below 40 K [155,156]: no dimers or local clusters are formed at the lowest temperature. This is in sharp contrast to LiRh 2 O 4 , in which a similar orbitalordering transition is followed by the formation of a dimer crystal [145]. Probably, in this typical Mott-Hubbard insulator, electron correlations are too strong to allow a metal-metal bonding with a reduced interatomic distance.…”

“…Concerning the relation between the orbital order and the MOC, it is meaningful to focus on the successive transitions found by Okamoto and coworkers in LiRh 2 O 4 in more detail [145]. The compound first exhibits an orbital ordering transition or a band Jahn-Teller transition at 230 K upon cooling by selecting the d xy orbital out of the triply degenerate t 2g orbitals, so that the structure changes from cubic to tetragonal there.…”

Is there a common theme behind the correlated‐electron superconductivity in organic charge‐transfer solids, cobaltates, spinels, and fullerides?

“…The [20] and LiRh 2 O 4 [21]. We propose that the superconducting state is a 3D pairedelectron liquid, with dynamical (as opposed to static) orbital ordering.…”

Synthesis and thermoelectric properties of the novel A-site deficient Zn0.5Rh2O4 compound

“…Based on these pieces of knowledge, Rh oxides with Rh 4 þ and Rh 3 þ ions (Rh 4 þ /Rh 3 þ ¼1/1) may be promising candidates for high-ZT materials, and might even be superior to Co oxides. In fact, several Rh oxides, including LiRh 2 O 4 [5], SrRh 2 O 4 [6,7], [9,10] and layered Rh-oxides [11,12], exhibit high S by analogy to Co oxides with Co 4 þ /Co 3 þ ¼1/1.…”