Probing the Electronic Structures of Ternary Perovskite and Pyrochlore Oxides Containing Sn4+ or Sb5+
journal contributionposted on 08.03.2004, 00:00 by Hiroshi Mizoguchi, Hank W. Eng, Patrick M. Woodward
Experimental and computational studies were performed to understand the electronic structure of ternary perovskites (ASnO3, A = Ca, Sr, Ba, Cd), pyrochlores (RE2Sn2O7, RE = Y, La, Lu; Cd2Sb2O7), and defect pyrochlore oxides (Ag2Sb2O6) containing the main group ions Sn4+ and Sb5+. In all compounds, the lowest energy states in the conduction band arise primarily from the antibonding Sn/Sb 5s−O 2p interaction. In the alkaline-earth stannate perovskites (BaSnO3, SrSnO3, and CaSnO3) the conduction bandwidth decreases strongly in response to the octahedral tilting distortion triggered by the decreasing size of the alkaline-earth cation. This in turn leads to a corresponding increase in the band gap from 3.1 eV in BaSnO3 to 4.4 eV in CaSnO3. The band gap of CdSnO3 is relatively small (3.0 eV) considering the large octahedral tilting distortion. The origin of this apparent anomaly is the mixing between the empty Cd 5s orbitals and the antibonding Sn 5s−O 2p states. This mixing leads to a widening of the conduction band and a corresponding decrease in the band gap. The participation of the normally inert A-site cation in the electronic structure near the Fermi level can be considered an inductive effect, as it utilizes substitution on the A-site to directly modify the electronic structure of the SnO32- framework. While the pyrochlore structure is more complicated, the energy level and width of the lowest energy conduction band can be analyzed in a manner similar to that utilized on the perovskite structure. The Sn−O−Sn and Sb−O−Sb bonds are highly distorted from linear geometry in pyrochlore, leading to a relatively narrow conduction band and a wide band gap. In Cd2Sb2O7 and Ag2Sb2O6 the Cd2+ and Ag+ ions exhibit a strong inductive effect that widens the conduction band and lowers the band gap significantly, very similar to the effect observed in the perovskite form of CdSnO3.