Disappearance of Localized Valence Band Maximum of Ternary Tin Oxide with Pyrochlore Structure, Sn₂Nb₂O₇

Abstract

The electronic structure of ternary tin oxide with pyrochlore structure, Sn 2 Nb 2 O 7 , which has the contribution of the Sn 5s orbitals in the valence band, is examined by synchrotron-radiation-excited photoelectron spectroscopy and X-ray absorption spectroscopy together with ab initio calculations. The empirical spectra are qualitatively consistent with the calculated density of states with the exception of a striking discrepancy in the electronic structure around the valence band maximum (VBM). The band calculation suggests the presence of a sharp peak around the VBM, mainly due to dispersionless bands that are derived from hybridization of Sn 5s orbitals and O 2p orbitals close to the Sn atom. However, the photoelectron spectra show no such characteristic spectral feature. From the theoretical prediction about the orbital origin of the localized VBM and the experimental estimation of the elemental composition by chemical analysis, it seems reasonable to conclude that the discrepancy of the electronic structure around the VBM is caused by the off-stoichiometric effect due to the point defects such as oxygen vacancies close to Sn 2+ , Sn 2+ vacancies, and Sn 4+ -on-Nb 5+ substitutional defects. Oxides with such localized VBM are not considered to be suitable for practical p-type oxide semiconductors. Therefore, the disappearance of the theoretically predicted localized VBM suggests the possibility to improve the mobility of hole carriers, and is considered to be closely related to our success in the development of the p-type oxide semiconductor Sn 2 Nb 2 O 7 .

Site-Selective Oxygen Vacancy Formation Derived from the Characteristic Crystal Structures of Sn–Nb Complex Oxides

“…•• formation, occurs preferentially around the Sn ions. These findings are supported by our preliminary calculations on the defect formation energy [16].…”

“…Recently, we developed novel p-type Sn 2+ oxides, namely, SnNb 2 O 6 and Sn 2 Nb 2 O 7 , with a band gap of 2.4 eV (Figure ). − Although their valence band is composed of a Sn 5s orbital, − they showed a low hole mobility of 0.03–0.4 cm 2 V –1 s –1 . , In general, because structural defects act as the scattering centers for electrons (holes), the mobility decreases with the increase in the defect density; hence, the reduction of these defects is a common strategy for increasing the hole mobility . At the same time, the carriers are generated by the defect formations, and therefore, a high carrier generation efficiency of the donor (acceptor) is required to improve the hole mobility for practical applications.…”

Flat-Band in Pyrochlore Oxides: A First-Principles Study

“…Photoemission spectra of this sample did not show the sharp peak predicted by the expected FB. Møssbauer spectroscopy revealed that this sample had a large amount of Sn deficiency [51]. This deficiency may greatly affect FB, and it is expected that a less Sn-deficient sample may cause unusual magnetic properties.…”

Computational Design of Flat-Band Material

“…The compounds (b) Sn 2 Nb 2 O 7 and (c) Bi 2 Ti 2 O 7 have already been synthesized [19][20][21][22][23], while (a) Tl 2 Mo 2 O 7 has not been reported yet. However, an analogous pyrochlore oxide Tl 2 Ru 2 O 7 has been already synthesized and shows unique metal-insulator transition [24].…”