Site-Selective Oxygen Vacancy Formation Derived From The Characteristic Crystal Structures Of Sn-Nb Complex Oxides

Divalent tin oxides have attracted considerable attention as novel p-type oxide semiconductors, which are essential for realizing future oxide electronic devices. Recently, p-type Sn2Nb2O7 and SnNb2O6 were developed; however, an enhanced hole mobility by reducing defect concentrations is required for practical use. In this work, we investigated the correlation between the formation of oxygen vacancy (V-O(center dot center dot)), which may reduce the hole-generation efficiency and hole mobility, and the crystal structure in Sn-Nb complex oxides. Extended X-ray absorption fine structure spectroscopy and a Rietveld analysis of X-ray diffraction data revealed the preferential formation of V-O(center dot center dot) at the O site bonded to the Sn ions in both the tin niobates. Moreover, a larger amount of V-O(center dot center dot) around the Sn ions was found in the p-type Sn2Nb2O7 than in the p-type SnNb2O6, indicating the effect of V-O(center dot center dot) on the low hole-generation efficiency. To elucidate the dependence of the formation of V-O(center dot center dot) on the crystal structure, we evaluated the Sn-O bond strength based on the bond valence sum and Debye temperature. The differences in the bond strengths of the two Sn-Nb complex oxides are correlated through the steric hindrance of Sn2+ with an asymmetric electron density distribution. This suggests the importance of the material design with a focus on the local structure around the Sn ions to prevent the formation of V-O(center dot center dot) in p-type Sn2+ oxides.