The crystal structure of the lacunar apatite NaPb4(PO4)3

Single crystals of NaPb4(PO4)3 were successfully grown from slowly cooled high-temperature solution. X-ray diffraction analysis has shown that the title compound crystallizes in the hexagonal system with apatite-like structure, space group P63/m, and lattice parameters a = b = 9.7249(8), c = 7.190(1) Å, Z = 2. The structure was solved by the Patterson method, using 554 independent reflections. The final values of R and Rw are equal to 0.023 and 0.024, respectively. The structure is built up from regular independent [PO4]3− tetrahedra, arranged in a way to delimit cavities and hexagonal channels where sodium and lead cations are located, respectively. Two distinguished cationic positions were identified. The sixfold coordination site (6h position) is totally occupied by lead cation (Pb1), while the ninefold coordination mixed site (4f position) is statistically filled by both Pb2 (50%) and Na (50%). The mean values of the bond lengths of P–O, Pb–O and Na/Pb–O are equal to 1.545, 2.533, and 2.433 Å, respectively.