Unraveling the temperature-responsive charge-disproportionation in BaBiO$_3$

Abstract

This study shows that the charge disproportionation at the Bi site in BaBiO$_3$ alters as a function of temperature. Decreasing the temperature from 300K down to 160K leads to a significant modification of the density of states corresponding to the Bi-O hybridized band near the Fermi level (E$_\text{F}$). This modification indicates reduction of Bi 6$sp$ - O 2$p$ hybridization and O 2$p$ spectral weight near E$_\text{F}$. The strong decrement of covalency at lower temperatures is accompanied by a decrement in O 2$p$ hole density due to possible charge transfer from Bi 6$s$ to the O 2$p$ band. Bi-charge state analysis from Bi-4$f$ core-level spectra showed that at 300K, $δ$ (charge difference between alternate Bi sites) value in 4$\pmδ$ is much less than at 160K, which reveals the transition towards the ionic nature of CD or static CD in BBO at low temperature. On the other hand, O 1$s$ core-level spectra displayed an asymmetric shape, and temperature-dependent modifications of the asymmetric shape and intensity have been observed. This highlights the significant influence of the O 2$p$ band hole on the dynamical CD at the Bi site.

Figures (4)

• Figure 1: (a) RHEED oscillations and reciprocal space image with line profile (inset). (b) Out-of-plane $\theta$--2$\theta$ fitted scan of (002) reflection of BBON thin film.
• Figure 2: Comparison of valence band spectra at O 2$s$-2$p$ resonance energy region of BBON thin film at 300K and 160K at (a) 16 eV, (b) 17 eV, (c) 18 eV, and (d) 19 eV photon energy.
• Figure 3: X-ray photoemission spectra of Bi 4$f$ core-level of BBON thin film at (a) 300K and (b) 160K.
• Figure 4: X-ray photoemission of O 1$s$ core-level spectra of BBON thin film at (left) 300K and (right) 160K.