= Solution
An <oxygen vacancy> has effective carrier charge $q_v=+2e$. Combining its density with its own <diffusion coefficient> in the <Nernst-Einstein relation> gives
$$
\sigma_{\rm ion}=\frac{n_v(2e)^2D_v}{k_BT},\qquad
\boxed{\sigma_{\rm ion}=2.00\,\mathrm{S\,m^{-1}}}
$$
at $T=873.15\,\mathrm K$. The oxygen ions move oppositely to the vacancies and carry the same conventional current. This estimate assumes independently migrating vacancies, negligible correlation corrections, and negligible electronic contribution to the reported <ionic conductivity>.
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