Electronic structure of CaFe₂O₄ with antiferromagnetic spin ordering

Abstract

The electronic structure of antiferromagnetic CaFe 2 O 4 is calculated by using a generalized gradient approximation considering on-site Coulomb interaction between d-electrons (GGA+U). We found that the antiferromagnetic phase is the most stable among non-magnetic, ferromagnetic, and antiferromagnetic phases. With GGA+U, the band gap energy of CaFe 2 O 4 is calculated to be ca. 1.9 eV. The lower conduction band consists of the Fe 3d states split into t 2g and e g states by the octahedral FeO 6 environment. The Ca 3d states distribute upper conduction bands. On the other hand, the valence band is mainly composed of the interaction between Fe 3d and O 2p states. The valence band maximum is approximately located at the Z-point and the conduction band minimum at the X-point. This means that antiferromagnetic CaFe 2 O 4 is an indirect energy gap material. The absolute value of the magnetic moment at Fe sites is calculated to be 4.16¯B, which is very close to experimental values.©2013 The Ceramic Society of Japan. All rights reserved.Key-words : Antiferromagnetic CaFe 2 O 4 , Ab initio calculation, FLAPW, GGA+U, Spin ordering [Received April 8, 2013; Accepted June 23, 2013] There are many complex oxides with CaFe 2 O 4 -type structure, for instance MFe 2 O 4 (M = alkali metals, alkaline-earth metals and rare-earth elements), because the Ca and Fe atoms can be individually substituted with different atoms.1) The prototype CaFe 2 O 4 has been investigated with respect to its magnetic properties until now.2)4) Magnetic and Mössbauer measurements revealed that Fe atoms in CaFe 2 O 4 are antiferromagnetic (AFM) ordered below the Neel temperature. 5),6) In recent research, CaFe 2 O 4 has been applied to gas absorbers, electrochemical devices operating at a high-temperature, oxidation catalysts and photocatalysts. 7)9) To understand those behaviors fundamentally, it is indispensable to reveal the precise electronic structure of CaFe 2 O 4 from the valence band (VB) to conduction band (CB). Nevertheless, there have been no useful theoretical researches on the electronic structures of CaFe 2 O 4 , due to the exclusion of spin configuration. 10),11)In the present study, we calculate the theoretical total energies for the non-magnetic (NM), ferromagnetic (FM), and AFM phases, and discuss the stability of three phases in CaFe 2 O 4 . As for the most stable phase (AFM), its electronic and magnetic structures are calculated with a full potential linearized augmented plane wave (FLAPW) method based on the density-functional theory (DFT).For CaFe 2 O 4 , the lattice constants and atomic positions used in the calculation were taken from the experimental values.4) It is well-known that the energy band calculations based on a local spin-density approximation (LSDA) or a generalized gradient approximation (GGA) give an incorrect electronic structure of strongly correlated materials such as CaFe 2 O 4 . Alternatively, we employed a GGA considering on-site Coulomb interaction between d-electrons (GGA+U). 12),13) The FLAPW calculation w...

Photoelectrochemical and theoretical investigations of spinel type ferrites (MxFe3−xO4) for water splitting: a mini-review

“…GGA+U calculations of the electronic structure of antiferromagnetic CaFe 2 O 4 yield an indirect bandgap of ∼1.9 eV. 83 The ionicity of Fe 3 O 4 has been determined using DFT calculations. 84 Furthermore, a new developed quantum mechanical estimation method for the ionicity of spinel ferrites has been proposed and tested.…”

Surface Structures of CaFe₂O₄ (001), (100), (110) and (111): A Density Functional Theory Study

“…Ca atom is surrounded by eight O atoms and Fe atoms are surrounded by six O atoms with a distorted octahedron shape. 32,33,48) In addition, CaFe 2 O 4 has a magnetic property and the most stable structure is antiferromagnetic (AFM). 32,34,49) It is a consequence of the antiferromagnetic alignment of the magnetic moments on the two Fe atoms.…”

“…With the U parameter increasing, the band gap for up and down spin states of CaFe 2 O 4 was linearly increased. Compared to the experimental band gap of 1.82 eV 51) or 1.90 eV, 32,52) the band gap for up and down spin states of CaFe 2 O 4 is closed to that when U was 5 eV. Hence, we took U = 4.7 eV, the total electronic density of states (DOS) of CaFe 2 O 4 bulk was shown in Fig.…”

“…Recently, in order to clarify the structure of CaFe 2 O 4 , Obata 32) and Sun 33) calculated the electronic structure of CaFe 2 O 4 based on the density-functional theory (DFT) and employed the generalized gradient approximation (GGA) by including the Hubbard parameter U = 5.2 eV (GGA + U). They found that the band gap energy was 1.9 eV and the valence band mainly consisted of the interaction Fe 3d and O 2p states while the conduction band was composed of Ca 3d states.…”

Characterization of Ti-doped CaFe₂O₄ prepared from a malic acid complex

“…This result is not consistent with the theoretical results based on a GGA+U calculation. 18) This is because that the energy difference between the direct and indirect transition is very small as predicted by the GGA+U approach. In addition, the optical band gap of undoped, 5, and 10 mol % Ti-doped CaFe 2 O 4 powder were approximately estimated to be 1.9, 1.9, and 2.0 eV from the relation of {f(R¨)h¯} 2 vs. h¯plot, respectively.…”