«Ellestadite»-type anionic [PO4]3– → [SO4]2– substitutions in β-Ca3(PO4)2 type compounds: A new route to design the inorganic phosphors

Abstract

Sulfate anion plays an important role in the development of inorganic phosphors and is considered as the path for for the enhancement of the luminescent properties. The heterovalent [PO4]3– → [SO4]2– anion substitution in the β-Ca3(PO4)2-type compounds was realized using the “ellestadite”-type strategy called by the analogy with the apatite-type structures. The novel compounds belonging to the Ca10.5–0.5x–yMy(PO4)7–x(SO4)x, M = Ca2+, Mg2+ and Zn2+ series have been synthesized using high-temperature solid state reaction. The used isomorphous scheme was [PO4]3– + ½ Ca2+ = [SO4]2– + ½ □ with the formation of the vacancy for the charge compensation. The successful incorporation of [SO4]2– groups was approved by Fourier-transform infrared spectroscopy, EDX method, and powder X-ray diffraction analysis. It was found that the sulfur atoms predominantly occupied T2 sites in the anionic sublattice according to crystal structure refinement based on the analysis of interatomic distances. The photoluminescent properties were studied for the Eu3+-doped samples and it was shown that Zn2+-containing samples exhibit a 1.5 times higher intensity in comparison with Mg2+-containing substituted phosphates. Thus, a new “ellestadite”-type substitution in β-Ca3(PO4)2-type structure significantly expands the crystal chemistry and give more insights into the deeper understanding of the dependence of luminescent properties on the cation distribution. The pathways of the [SO4]2– incorporation into the crystal structures are discussed in details.