Fig. 1: Inverted charge transfer energy in between BKSO and BKBO.
a, A schematic diagram of different regimes of metallicity in BKBO and BKSO. When charge transfer energy ΔCT is positive (negative), Bi or Sb s electrons (oxygen holes) are predominant. b, The fat-band representation of the electronic band structure of BKBO at x = 0.65 calculated via hybrid DFT. The thickness is proportional to the sum of O 2pσ and Bi 6s contributions, and the colours represent their ratio, with predominant O 2pσ character shown by blue and predominant Bi 6s character by red. The plot shows the predominant O 2pσ (Bi 6s) character in the spσ* (spσ) band. A band with predominant Bi 6s character at Γ at −5.4 eV is formed by non-bonding Bi 6s states (Extended Data Fig. 2 for the O 2pπ contribution). EF, Fermi energy. c, The molecular-orbital diagram of BKBO derived from b. The Bi 6s orbital energy is markedly lower than the O 2p energy, consistent with negative ΔCT. Therefore, BKBO is located in the scheme of the oxygen-hole metal20,21, as illustrated in a. d, The fat-band representation of the electronic band structure of BKSO at x = 0.65 calculated via hybrid DFT. Sb 5s and oxygen 2p are found to be highly mixed in both spσ and spσ* bands. The much enhanced Sb 5s character in the spσ* band is clear compared to that of Bi 6s in BKBO. The non-bonding Sb 5s states at Γ are at −4.0 eV (Extended Data Fig. 2 for the O 2pπ contribution). e, The molecular-orbital diagram for BKSO derived from d. The Sb 5s orbital energy is marginally higher than the O 2p energy, indicating that ΔCT is slightly positive while being close to zero (ΔCT ≳ 0). Thus, BKSO is located in the region of the Bi/Sb s-orbital metal while critically close to the covalency limit in a.
