= Solution
The conventional cubic cell has <volume>
$$
a^3=(0.52\times10^{-9})^3=1.40608\times10^{-28}\,\mathrm{m^3}.
$$
Consequently the same effective carrier count is
$$
\boxed{N_{\rm eff}=n_{\rm eff}a^3=0.9236\quad\text{carriers per cell}.}
$$
A noninteger mean is normal for a doped crystal: it describes an ensemble average, not a requirement that every cell contain the same number of defects. The <fluorite structure> has four cation sites and eight oxygen sites in this conventional cell.
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