  The crystal structure of Pb$_{10}$(PO$_4$)$_6$O, the proposed matrix for the
potential room-temperature superconductor LK-99, Pb$_{10-x}$Cu$_x$(PO$_4$)$_6$O
($x = 0.9-1.0$), has been reinvestigated by single-crystal X-ray diffraction
using crystals prepared by Merker and Wondratschek (Z. Anorg. Allg. Chem. 1960,
306, 25-29). The crystal structure is trigonal, P-3, a = 9.8109(6), c =
14.8403(12) A, V = 1237.06(15) A3, R1 = 0.0413 using 3456 unique observed
reflections. The crystal structure of Pb$_{10}$(PO$_4$)$_6$O is a
superstructure with regard to the standard P63/m apatite structure type. The
doubling of the c parameter is induced by the ordering of additional O atoms
within the structure channels running parallel to the c axis and centered at
(00z). The O atoms form short bonds to the Pb1 atoms, resulting in splitting
the Pb1 site into two, Pb1A and Pb1B. The structural distortions are further
transmitted to the Pb phosphate framework formed by four Pb2 sites and PO$_4$
groups. The structure data previously reported by Krivovichev and Burns (Z.
Kristallogr. 2003, 218, 357-365) may either correspond to the
Pb$_{10}$(PO$_4$)$_6$O$_x$(OH)$_{2-x}$ ($x = 0.4$) member of the
Pb$_{10}$(PO$_4$)$_6$O-Pb$_{10}$(PO$_4$)$_6$(OH)$_2$ solid solution series, or
to the high-temperature polymorph of Pb$_{10}$(PO$_4$)$_6$O (with the phase
with doubled c parameter being the low-temperature polymorph).
