Polymorphism of Pb(PO)OH within the LK-99 mixture.

During the synthetic exploration targeting the polycrystalline compound LK-99, an unexpected phase, Pb(PO)OH, 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 Pb(PO)OH has an orthorhombic crystal structure with space group Pnma, representing a unique structure differing from the hexagonal apatite phases of Pb(PO)O and Pb(PO)OH. Comprehensive temperature- and magnetic-field-dependent magnetization studies unveiled a temperature-independent magnetic characteristic of Pb(PO)OH. Solid-state nuclear magnetic resonance spectroscopy was employed to decipher the origins of the phase stability and confirm the presence of hydrogen atoms in Pb(PO)OH. 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.