= Solution
The host is \b[<zirconolite>], with ideal end-member composition $\mathrm{CaZrTi_2O_7}$. In the proposed formula, the substitution is formally
$$
\mathrm{Zr^{4+}\longleftrightarrow Pu^{4+}}.
$$
It is an <isovalent substitution>: the mean charge of the mixed <zirconium>/<plutonium> site remains $+4$, so
$$
2+\big[0.2(4)+0.8(4)\big]+2(4)=14=7(2).
$$
No compensating oxygen vacancy or aliovalent co-substituent is required by this particular charge account. Suitable <oxidation state> is essential; the same formula interpreted with only trivalent <plutonium> would not be charge balanced.
In <zirconolite-2M> the <zirconium> site has an oxygen <coordination number> of seven. The <ionic radius> of <plutonium>(IV) is larger than that of <zirconium>(IV), so increasing its occupancy changes local strain and the preferred arrangement of the <cation> layers. The observed progression in the investigated substitution series was \b[<zirconolite-2M> → <zirconolite-4M> → <pyrochlore structure>], rather than unlimited substitution in one unchanged monoclinic lattice. The primary study found a 2M <solid-solution limit> near $x=0.15$, a narrow 4M range near $x=0.41$, and a pyrochlore composition at $x=0.60$ in $\mathrm{CaZr}_{1-x}\mathrm{Pu}_x\mathrm{Ti_2O_7}$. These are experimental observations for that series, not universal boundaries independent of preparation and <oxidation state>.<Structural Effect of Pu Substitutions on the Zr-Site in Zirconolite by B. D. Begg and others>[MRS Proceedings 663, 259; abstract, seven-coordinate site and reported polytype transitions]{ref}
Consequently the nominal $x=0.20$ composition identifies a zirconolite-based waste-form design, but \b[does not by itself prove a pure 2M product]. Diffraction and phase-composition measurements are needed. At larger substitution, the fluorite-related pyrochlore arrangement accommodates a different <cation> distribution and coordination environment.
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