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 Pb10–xCux(PO4)6O that display two distinct
compositional ranges differentiated by structural ordering. For x > 0.5, we observe substitution in the apatite archetype
structure, whereas for x < 0.5, we find an apatite
superstructure with coupled anion and cation ordering. The 1 ×
1 × 2 superstructure in the noncentrosymmetric space group P6̅ (no. 174) for Pb10–xCux(PO4)6O with x < 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 x > 0.5 in Pb10–xCux(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.