Substitution in barium-fluoride apatite: The crystal structures of Ba 10(PO 4) 6F 2, Ba 6La 2Na 2(PO 4) 6F 2 and Ba 4Nd 3Na 3(PO 4) 6F 2

Abstract

The crystal structures of the apatites Ba 10(PO 4) 6F 2(I), Ba 6La 2Na 2(PO 4) 6F 2(II) and Ba 4Nd 3Na 3(PO 4) 6F 2 (III) have been determined by single-crystal X-ray diffraction. All three compounds crystallize in a hexagonal apatite-like structure. The unit cells and space groups are: I, a = 10.153(2), c = 7.733(1)Å, P6 3 m ; a = 9.9392(4), c = 7.4419(5) Å, P 6 ; III, a = 9.786(2), c = 7.281(1)Å, P 3 . The structures were refined by normal full-matrix crystallographic least squares techniques. The final values of the refinement indicators R w and R are: I, R w = 0.026, R = 0.027, 613 observed reflections; II, R w = 0.081, R = 0.074, 579 observed reflections; III, R w = 0.062, R = 0.044, 1262 observed reflections. In I, the Ba(1) atoms located in columns on threefold axes, are coordinated to nine oxygen atoms; the Ba(2) sites form triangles about the F site and are coordinated to six oxygen atoms and one fluoride ion. The fluoride ions are statistically displaced ∼0.25 Å from the Ba(2) triangles. This displacement of the F ions is analogous to the displacement of OH ion in Ca 10(PO 4) 6(OH) 2. The structures of II and III contain disordered cations. In II there is disorder between La and Na in the column cation sites as well as triangle sites. In III, Nd and Na ions are ordered in the column sites, but there is disorder among Ba and the remaining Nd and Na ions in the triangle sites to give an average site population of 2 3 Ba, 1 6 Nd, 1 6 Na . The coordination of the rare earth ions and Na ions in the ordered column sites are nine and six oxygens, respectively, in accord with the greater charge of the rare earth ions as compared with Na. The F ions in both II and III suffer from considerable disorder in position, and their locations are not precisely known.