Hybridization effects and bond disproportionation in the bismuth perovskites

Abstract

We propose a microscopic description of the bond-disproportionated insulating state in the bismuth perovskites XBiO3 (X=Ba, Sr) that recognizes the bismuth-oxygen hybridization as a dominant energy scale. It is demonstrated using electronic structure methods that the breathing distortion is accompanied by spatial condensation of hole pairs into local, molecular-like orbitals of the A1g symmetry composed of O-2pσ and Bi-6s atomic orbitals of collapsed BiO6 octahedra. Primary importance of oxygen p-states is thus revealed, in contrast to a popular picture of a purely ionic Bi 3+ /Bi 5+ charge-disproportionation. Octahedra tilting is shown to enhance the breathing instability by means of a non-uniform band-narrowing. We argue that formation of localized states upon breathing distortion is, to a large extent, a property of the oxygen sublattice and expect similar hybridization effects in other perovskites involving formally high oxidation state cations.PACS numbers: 74.20. Pq,71.30.+h,71.45.Lr The physics of perovskite compounds, featuring BO 6 octahedra (B=cation) as building blocks, is exceptionally rich. The perovskites can be driven through a variety of structural, electronic, and magnetic phase transitions and are hosts to such intriguing states of matter as high-transition-temperature (T c ) superconductivity [cuprates [1], bismuth perovskites XBiO 3 (X=Ba, Sr) [2-4]], a pseudo-gap state with strongly violated Fermiliquid properties (cuprates[5]), and a spin/charge density wave [rare-earth nickelates RNiO 3 (R=rare-earth atom) [6]], to name a few. It is appealing to relate the diverse physical properties observed across the perovskite family of materials with the individual characteristics of the cation B. The latter can be magnetic (Cu in the cuprates) or not (Bi in the bismuth perovskites), orbitally active (Mn in the manganites [7]) or not (Cu, Bi), prone to strong electronic correlations (transition-metal elements Cu and Ni) or not (Bi). With the due appreciation of the cation factor, there are, however, many striking similarities among different perovskite families suggesting an equally important role of their common structural framework. A vivid example is the transition into a bond-disproportionated insulating phase found in both the bismuthates and the rare-earth nickelates, in which oxygen plays an extremely important role [8][9][10][11]. Observations of this kind have given rise to theories aimed at a unified description of the perovskites, where the various competing phases emerge from polaronic and bipolaronic excitations of the polarizable oxygen sublattice.While it is a common practice within this approach to assume that the only effect of electron-phonon coupling is variation of on-site energies, in this Letter we demonstrate that the effects due to hybridization between the oxygen-p orbitals and cation orbitals can be even more important. For this purpose, we focus on the bismuth perovskites XBiO 3 where the analysis is greatly facilitated by the fact that the Bi ion valence states are non-m...

Epitaxial growth of perovskite SrBiO3 film on SrTiO3 by oxide molecular beam epitaxy

“…The antibonding state in the expanded BiO 6 octahedron just below the Fermi level shown in Fig. 4(b) [8] is seen as a small shoulder at ∼2 eV binding energy in Fig. 4(a).…”

“…The Bi 4f 7/2 and O1s spectra show a shift of ∼1.2 eV to higher binding energy as compared to that in BBO [20]. This could be explained in part by the larger indirect band gap in SBO, suggesting that the chemical potential is located below the bottom of conduction band [8]. Transport measurement shows the film is insulating, indicating the Fermi level located in the band gap, close to the conduction band.…”

“…For comparison with the XPS result, as shown in Fig. 4(b), total and partial DOS are calculated using a P 2 1 /n unit cell, a 6 × 6 × 5 grid of k points and the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional in Vienna ab initio simulation package [8,34,35]. The spectral weight in Fig.…”

“…The spectral weight in Fig. 4(a) starts to increase at ∼1.4 eV, suggesting its ∼1.4 eV band gap and the film Fermi level is pinned closer to the conduction band in band gap [8]. The peak with ∼12 eV binding energy in XPS [ Fig.…”

Charge density wave modulation in superconducting BaPbO3/BaBiO3 superlattices

“…Doped BaBiO3 (BBO) superconductors possess a commensurate CDW in the parent compound, stable to high temperatures, in an isotropic, nonmagnetic complex oxide. The CDW manifests as bond disproportionation through a frozen breathing mode of the BiO6 octahedra, as pictured in Figure 1(a), with recent experimental and first principle work suggesting hybridization of the BiO6 octahedra plays a dominate role [3][4][5][6]. Doping BBO with Pb in BaPb1-xBiXO3 (BPBO, Tc=11 K) or K in Ba1-xKxBiO3 (BKBO, Tc=30 K) destroys the long range CDW [7][8][9].…”