Superstructure Formation through Coupled Anion and Cation Ordering in Cu-Substituted Lead Oxyapatites

Apatites are an important mineral-based material family with huge chemical and structural diversity. They were recently implicated in the claims of high-temperature superconductivity in materials labeled LK-99 that display complex phase mixtures containing Pb, Cu, phosphate, and oxide components. We report Cu-substituted lead apatite solid solutions Pb_10-<i>x</i> Cu _<i>x</i> (PO₄)₆O that display two distinct compositional ranges differentiated by structural ordering. For <i>x</i> > 0.5, we observe substitution in the apatite archetype structure, whereas for <i>x</i> < 0.5, we find an apatite superstructure with coupled anion and cation ordering. The 1 × 1 × 2 superstructure in the noncentrosymmetric space group <i>P</i>6̅ (no. 174) for Pb_10-<i>x</i> Cu _<i>x</i> (PO₄)₆O with <i>x</i> < 0.5 exhibits a unique oxygen ordering motif in the hexagonal channels and selective Cu substitution only on two out of seven Pb sites. At <i>x</i> > 0.5 in Pb_10-<i>x</i> Cu _<i>x</i> (PO₄)₆O, Cu cations are introduced onto all Pb sites, which triggers the transition to the archetypical apatite structure, reflecting the coupling of the core structural components of the apatite framework in the ordering pattern.