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B. D. Begg coauthored research on plutonium substitution at zirconium sites in zirconolite. The reported structural changes include transitions between zirconolite variants and a pyrochlore-related structure.

Replacing one ion by another can change preferred coordination and charge compensation. A nominal substitution is consequently a hypothesis about a crystal's atomic sites, not merely a change in its bulk elemental recipe.^[ref]