Structural consequences of the coupled substitution of Eu,S in calcium sulfoapatite

Abstract

Substitution of (Eu,S) for calcium in calcium sulfoapatite in a high-temperature solid state reaction produced brown powders in the continuous series Ca 10−x Eu x( PO 4) 6 S 1+ x 2 with space group P6 3 for x from zero to at least 1.3. Rietveld refinements (RR) with powder X-ray diffraction data showed that the Eu 3+ substituted only at the Ca(2) sites and that the S 2− occurs equally at 0,0,0.47 and 0,0,0.97, irrespective of the quantity present. RR in P3 showed no further ordering for either the S 2− or Eu 3+ in the apatite structure. The progressive incorporation of Eu 3+ produced changes in the CaO distances around both Ca sites which can be visualized as a combined rotation and scissoring of the PO 3− 4 group. Substitution in the ratio of (2Eu 3+ + S 2−) for 2 Ca 2+ in the structure was confirmed by the site occupancies obtained in RR's and by the applicability of Vegard's law to the substitution. The RR's were carried out with the constraints that the PO 3− 4 group was stoichiometric and that each of the Ca(2) sites were filled with either Eu 3+ or Ca 2+. The findal R wp and R B were ∼0.16 and 0.03, respectively. For x = 0, a = 9.4560(4) and c = 6.8409(4) Å.