Evidence for orbital ordering inLaCoO3

Abstract

We present powder and single crystal X-ray diffraction data as evidence for a monoclinic distortion in the low spin (S = 0) and intermediate spin state (S = 1) of LaCoO3. The alternation of short and long bonds in the ab plane indicates the presence of eg orbital ordering induced by a cooperative Jahn-Teller distortion. We observe an increase of the Jahn-Teller distortion with temperature in agreement with a thermally activated behavior of the Co 3+ ions from a low-spin ground state to an intermediate-spin excited state.The study of orbital degrees of freedom in transition metal (TM) oxides has gained prominent interest. Novel techniques such as Resonant X-ray Scattering and X-Ray Absorption Spectroscopy give direct information about orbital occupancy. It is realized that the orbital moments are as important as the spin moments to understand the electronic properties. Prominent examples in which the orbital degrees of freedom determine the electronic properties are the metal-insulator transitions in V 2 O 3 [1] and doped LaM nO 3 [2]. Here, spin-and orbital-induced transitions are intimately related. In the perovkites, metallicity and orbital ordering are mutually exclusive because of the large Jahn-Teller (JT) splitting of the e g orbitals [3]. Furthermore, novel ground states can be obtained such as an orbital liquid [4,5].Considerable interest in the perovskite-type LaCoO 3 originates from the puzzling nature of two transitions in this compound and the vicinity to a metal-insulator transition. The ground state of LaCoO 3 is a nonmagnetic insulator and there is no long range magnetic order at all temperatures. At low temperatures, the magnetic susceptibility increases exponentially with temperature exhibiting a maximum near 100K. At higher temperatures, a second anomaly is observed around 500K which is accompanied by a semiconductor to metal transition. The maximum at 100K was ascribed initially to a change of the spin state in the Co 3+ ions i.e. a transition from a low-spin (LS) nonmagnetic ground state (t 6 2g , S = 0) to a high-spin (HS) state (t 4 2g e 2 g , S = 2) [6,7,8,9]. In the more recent literature [10,11,12,13,14], new scenarios involving an intermediate-spin state (IS) (t 5 2g e 1 g , S = 1) have been proposed. Using LDA+U calculations, Korotin et al. [15] proposed the stabilization of the IS state due to the large hybridization between the Co-e g and O-2p levels. Due to the partially filled e g level, the IS state is JT active. The degeneracy of the e g orbitals of Co 3+ ions in the LS state is expected to be lifted in the IS state by a JT distortion.All structural studies, based on powder X-ray and neutron diffraction experiments are consistently interpreted in rhombohedral R3c symmetry and no structural transitions are reported in the temperature interval 4.2K − 1248K [16,17,18,19]. A cooperative JT distortion is incompatible with this space group. The rhombohedral distortion of the parent cubic perovskite structure consists of a deformation along the body diagonal, and preserves only one Co ...

Influence of Al doping in LaCoO3 on structural, electrical and magnetic properties

“…1. Although most of the structure analysis of LaCoO 3 is based on a rhombohedral space group R-3c [31], there are reports which favour a monoclinic I2/a structure as well [32,33]. The rhombohedral space group R-3c fits the x = 0.0 sample perfectly in our refinements.…”

Spin State Transition inLaCoO3Studied Using Soft X-ray Absorption Spectroscopy and Magnetic Circular Dichroism

“…All this changed with the theoretical work in 1996 by Korotin et al, who proposed on the basis of local density approximation + Hubbard U (LDA+U) band structure calculations, that the excited states are of the intermediate spin (IS, t 5 2g e 1 g , S = 1) type [13]. Since then many more studies have been carried out on LaCoO 3 with the majority of them [14,15,16,17,18,19,20,21,22,23,24,25,26,27] claiming to have proven the presence of this IS mechanism. In fact, this LDA+U work is so influential [28] that it forms the basis of most explanations for the fascinating properties of the recently synthesized layered cobaltate materials, which show giant magneto resistance as well as metal-insulator and ferroferri-antiferro-magnetic transitions with various forms of charge, orbital and spin ordering [29,30].…”

Experimental verification of orbital engineering at the atomic scale: Charge transfer and symmetry breaking in nickelate heterostructures

“…directly correlates to the material's resulting properties. [1][2][3][4][5] For example, systems such as the manganites (colossal magnetoresistance), 6 the cobaltates (spin-state transitions), 7,8 and the cuprates (hightemperature superconductivity) 9,10 owe their behaviors to specific configurations of the electronically active transition-metal cation d orbitals, which, for near-cubic symmetry, are split into the (lower energy) t 2g and (higher energy) e g orbitals. The development of atomically precise growth techniques for oxides has opened up the possibility of controlling orbital configurations via layered heterostructures.…”