Isomorphous substitution of europium for strontium in the structure of synthetic hydroxovanadate

Polycrystalline strontium–europium hydroxovanadates Sr 10− x Eu x (VO 4) 6(OH) 2− x O x were synthesized and studied by X-ray powder diffraction, infrared absorption and diffuse-reflectance spectroscopies and also thermogravimetry. The single-phase region of isomorphous substitution of Eu for Sr in hydroxovanadate has been found to lie in the compositional interval 0⩽ x⩽0.18 at 800 °C. Refinement of X-ray diffraction (XRD) patterns by the Rietveld method shows that Eu substitutes for Sr preferentially at the Sr(1) sites of the apatite structure. The substitution results in a more uniform distribution of Sr–O bond lengths in coordination polyhedra both around Sr (1) and Sr (2) sites and also in a slight contraction of the vanadate anion VO 4 3−. Synthetic Sr hydroxovanadates absorb a considerable amount of water in two forms: capillary condensed and chemically combined. The latter form may be absorbed at temperatures as high as 500 °C. The results of this paper contribute to a general picture of substitution effects of the rare-earth (RE) elements in the apatite structure. The dependence of the substitution limit x max in Sr 10− x RE x (VO 4) 6(OH) 2− x O x on the atomic number of the RE element is shown here with the present data marked with red color.