Sr, Ba and Cd arsenates with the apatite-type structure

X-ray diffraction analysis of single crystals of three new arsenates adopting apatite-type structures yielded formula Sr5(AsO4)3F for strontium arsenate fluoride, (I), (Sr1.66Ba0.34)(Ba2.61Sr0.39)(AsO4)3Cl for strontium barium arsenate chloride, (II), and Cd5(AsO4)3Cl0.58(OH)0.42 for cadmium arsenate hydroxide chloride, (III). All three structures are built up of isolated slightly distorted AsO4 tetrahedra that are bridged by Sr2+ in (I), by Sr2+/Ba2+ in (II) and by Cd2+ in (III). Compounds (I) and (II) represent typical fluorapatites and chlorapatites, respectively, with F− at the 2a (0, 0, ) site and Cl− at the 2b (0, 0, 0) site of P63/m. In contrast, in (III), due to the requirement that the smaller Cd2+ cation is positioned closer to the channel Cl− anion (partially substituted by OH−), the anion occupies the unusual 2a (0, 0, ) site. Therefore, Cl− is similar to F− in (I), coordinated by three A2 cations, unlike the octahedrally coordinated Cl− in (II) and other ordinary chlorapatites. Furthermore, in (III), using FT–IR studies, we have inferred the existence of H+ outside the channel in oxyhydroxyapatites and provided possible atomic coordinates for a H atom in HAsO42−, leading to a proposed formulation of the compound as Cd5(AsO4)3−x(HAsO4)xCl0.58(OH)0.42−x−(y/2)Ox+(y/2)□y/2.