Crystal Structure and Polarized Raman Spectra of Ca6Sm2Na2(PO4)6F2

The crystal structure of a new apatite Ca6Sm2Na2(PO4)6F2 has been determined using 842 independent reflections (R=0.059, Rw=0.076). The unit cell constants are a=9.3895(3) ?, c=6.8950(4) ? with P63/m space group. The structure contains disordered cations in both the Ca(1) and the Ca(2) sites. The formula assigned to the compound is [Ca1.57Sm0.43Na2](1) [Ca4.43Sm1.57](2)(PO4)6F2. The Raman spectra have been studied and they are consistent with the proposed structure. A comparison with fluorapatite shows that most of the Raman lines of the new compound are significantly broad, which is attributed to the random distribution of three kinds of atoms on the cation sites.