Ferrodistorsive orbital ordering in the layered nickelate NaNiO2: A density-functional study

Abstract

The electronic structure and magnetism in the sodium nickelate NaNiO 2 in the low-temperature phase is studied from density-functional calculations using the linear muffin-tin orbitals method. An antiferromagnetic solution with a magnetic moment of 0.7 B per Ni ion is found. A ferrodistorsive orbital ordering is shown to occur due to the Jahn-Teller distortion around the Ni +3 ion in agreement with the orbital ordering inferred from neutron diffraction. While the intralayer exchange is ferromagnetic, the interlayer exchange is weakly antiferromagnetic, mediated by a long Ni-O-Na-O-Ni superexchange path. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.1854414͔Since their discovery in the early 1950s, 1 the nickelates have been the subject of attention due to their many applications such as the base materials in batteries. More recently, the family of compounds Li x Me 1−x NiO 2 ͑Me being a metal͒ have attracted special interest due to their triangular lattice structure which makes them good candidates as frustrated magnetic systems. In fact, magnetic or orbital frustration mechanisms are usually invoked in explaining the unusual behavior of LiNiO 2 . Many different scenarios have been proposed to explain the absence of orbital and magnetic orderings in LiNiO 2 , such as spin glass, 2 quantum disordered state, 3,4 spin-orbital liquid, 5 impurity effect, 6 or frustrated antiferromagnet. 7 In comparison, NaNiO 2 seems to show none of the unusual behaviors of its sister compound including the magnetic behavior. 8,9 For example, while in LiNiO 2 , no long-range magnetic order has been observed, NaNiO 2 in contrast is a type A antiferromagnet ͑ferromagnetic layers stacked on top of one another and coupled antiferromagnetically͒. It has been a long puzzle as to why their magnetic properties are so different, in spite of the fact that the two compounds are very similar.In this paper, we study the electronic structure of NaNiO 2 using density-functional methods, with the goal of gaining insight into the physics of this family of materials. The nature of orbital ordering as well as the origin of the magnetic exchange are also discussed.The high-temperature hexagonal crystal structure of NaNiO 2 ͑space group R3m, no. 166͒ is shown in Fig. 1. The compound undergoes a structural transition to a lowersymmetry monoclinic structure with the paramagnetic space group C2/m ͑no. 12͒ at about 450 K, below which the oxygen octahedra become elongated. The magnetic transition is at a much lower temperature T N =20 K, 9 below which the A-type antiferromagnetic structure occurs. The layered structure may be viewed as an arrangement of slightly elongated NiO 6 octahedra separated by Na sheets. In the lowtemperature structure, the Jahn-Teller ͑JT͒ distortion leads to different Ni-O bond lengths: four short bonds of 1.91 Å and two long ones of 2.14 Å. The lattice parameters are listed in Table I. In the low-temperature phase, NaNiO 2 shows the ferro-distorsive orbital ordering caused by the Jahn-Teller distortions of the NiO 6 octahedr...

Charge disproportionation and Jahn-Teller distortion in LiNiO2and NaNiO2: A density functional theory study

“…NaNiO 2 , unlike LiNiO 2 , is found exclusively in the Jahn--Teller relaxed state. Previous calculations 41 on the m 3 R and C2/m cells of NaNiO 2 were performed using a U eff value of 4.5eV but from Figure 1 it is clear that similar results are expected for our value of 6.5eV except for the P2/c disproportionation cell which the previous work did not consider. Our calculations find the lowest energy configuration to be the cooperative C2/m Jahn--Teller cell by approximately 32meV (and 58meV below the charge--disproportionation cell P2/c).…”

Thermodynamic and magnetic properties of the layered triangular magnet NaNiO2

“…The fit gives ν µ (0) = 64.2(2) MHz corresponding to a field at the muon site of ∼ 0.5 T. Dipole field calculations show that this field will be experienced by muons near any of the oxygen atoms in the octahedron surrounding a nickel atom, in regions of high electron density 26 , and show that our results are consistent with the magnetic structure determined by Darie et al 11 . Our calculations also suggest that the muon precession frequency is insensitive to small deviations from this magnetic structure.…”

Thermodynamic and magnetic properties of the layered triangular magnetNaNiO2

“…The fit gives ͑0͒ = 64.2͑2͒MHz corresponding to a field at the muon site of ϳ0.5 T. Dipole field calculations show that this field will be experienced by muons near any of the oxygen atoms in the octahedron surrounding a nickel atom, in regions of high electron density, 22 and show that our results are consistent with the magnetic structure determined by Darie et al 10 Our calculations also suggest that the muon precession frequency is insensitive to small deviations from this magnetic structure. Fitting Eq.…”