Polymorphism of Pb₅(PO₄)₃OH_δ within the LK-99 mixture

During the synthetic exploration targeting the polycrystalline compound LK-99, an unexpected phase, Pb₅(PO₄)₃OH_δ, was identified as a byproduct. We elucidated the composition of this compound through single-crystal X-ray diffraction analysis. Subsequent synthesis of the target compounds was achieved via high-temperature solid-state pellet reactions. The newly identified Pb₅(PO₄)₃OH_δ has an orthorhombic crystal structure with space group Pnma, representing a unique structure differing from the hexagonal apatite phases of Pb₁₀(PO₄)₆O and Pb₅(PO₄)₃OH. Comprehensive temperature- and magnetic-field-dependent magnetization studies unveiled a temperature-independent magnetic characteristic of Pb₅(PO₄)₃OH_δ. Solid-state nuclear magnetic resonance spectroscopy was employed to decipher the origins of the phase stability and confirm the presence of hydrogen atoms in Pb₅(PO₄)₃OH_δ. These investigations revealed the presence of protonated oxygen sites, in addition to the interstitial water molecules within the structure, which may play critical roles in stabilizing the orthorhombic phase.