  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,
enhanced hole mobility by reducing defect concentrations is required for
practical use. In this work, we investigate the correlation between the
formation of oxygen vacancy 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 Rietveld analysis of x-ray
diffraction revealed the preferential formation of oxygen vacancy at the O site
bonded to the Sn ions in both the tin niobates. Moreover, a large amount of
oxygen vacancy around the Sn ions were found in the p-type Sn2Nb2O7, thereby
indicating the effect of oxygen vacancy to the low hole-generation efficiency.
The dependence of the formation of oxygen vacancy on the crystal structure can
be elucidated from the Sn-O bond strength that is evaluated 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 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 oxygen vacancy in p-type Sn2+ oxides.
