Study of the electronic structure of CaFeO3

Abstract

We have studied the charge disproportionation phenomenon in CaFeO3 using the local-spin density approximation with the on-site Coulomb interaction parameter U and exchange parameter J. The calculation reveals that the total number of the 3d electrons is about 5.1 for both Fe(1)(Fe5+) and Fe(2)(Fe3+) atoms, and that there are about 0.25 electron holes in the O-2p band. Therefore, the charge disproportionation can be more accurately described as 2d5L(Fe4+)=d5L2(Fe5+)+d5(Fe3+), where L denotes a hole in the oxygen 2p band, instead of 2d4(Fe4+)=d3(Fe5+)+d5(Fe3+). The hybridization between the Fe-3d and O-2p orbitals is stronger for Fe(1) than for Fe(2) due to the shorter Fe(1)–O bond. The hyperfine magnetic field contributed from conduction electron polarization is larger for Fe(2), resulting from a stronger s-d hybridization between the s orbital of Fe(2) and the d orbitals of its neighboring Fe(1) atoms. The on-site Coulomb repulsion and the exchange interaction increase the splitting between the occupied spin up and unoccupied spin down bands of Fe atoms. Fe-3d electrons become localized and the occupied d-band shifts to a lower energy range, even below the O-2p level. The calculated magnetic moments, hyperfine fields, and electron charge density agree well with the experimental data.

Enhanced charge ordering transition in dopedCaFeO3through steric templating

“…2a shows that although there are separate gaps in each spin channel, the valence band edge in the majority spin touches the conduction band edge in the minority spin, resulting in zero total gap. The absence of band gap and the weak charge ordering is a result of the underestimation of band gaps in DFT [31] due to its unphysical electron delocalization, This result is common in Mott insulators with partially filled d orbitals and is in agreement with another DFT study of CaFeO 3 [12]. Even though DFT does not predict the correct electronic ground state of For comparison, we calculated the PDOS of CaZrO 3 relaxed from experimental P bmn structure [32], shown also in Fig.…”

“…The origin of the charge ordering transition is usually attributed to Mott insulator physics, where the carriers are localized by strong electron-lattice interactions [7,[9][10][11]. More recently, it has been debated whether the difference in charge state resides on the B cations or as holes in the oxygen 2p orbitals [12][13][14], and several computational studies showed that the magnetic configuration, in addition to structural changes, plays a vital role in stabilizing the charge ordered state in CaFeO 3 [15][16][17]. Nevertheless, the amplitude of the cooperative breathing mode is a key indicator of the magnitude of electron trapping and band gap opening in MIT.…”

Formal Valence,3d-Electron Occupation, and Charge-Order Transitions

“…Analogously to YNiO 3 , we obtain identical 3d occupations for Fe1 and Fe2 ions. Quantum chemical embedded cluster calculations [23] and LDA+U studies [24][25][26] had noted that the Fe charge in both "disproportionated" sites differed little, but neither quantified the occupation as we have for YNiO 3 and CaFeO 3 . The pentavalent state of Fe has most often been identified from Mössbauer isomer shift data, but Sadoc et al [23] concluded the difference in isomer shift is primarily a measure of the covalency (Fe-O distance) rather than any real charge on Fe.…”

In quest of cathode materials for Ca ion batteries: the CaMO3 perovskites (M = Mo, Cr, Mn, Fe, Co, and Ni)

“…From those investigations one can infer that the appropriate methodologies are: CaCrO 3 GGA or GGA+U with U below 1.5 eV, 35 CaMnO 3 GGA+U with U between 3-4 eV, 30 and CaFeO 3 GGA+U with large U values of 8 eV. 36 This evidences that a unique methodology cannot be applied to systematically study the whole CaMO 3 family. Therefore, we have also calculated the total energies of the Ca x MO 3 compounds using the GGA+U method, following its simplified rotationally invariant form by Dudarev, et al 37 Effective U values ( J = 1 eV) for the d orbitals of M ions were set to 3 eV.…”