Luminescence of Eu³⁺ as a probe for the determination of the local site symmetry in β-Ca₃(PO₄)₂-related structures

Ca9−xMgxEu(PO4)7 solid solution with a β-Ca3(PO4)2-type (β-TCP) structure was obtained using a solid-state technique and studied by X-ray diffraction, and luminescence and dielectric spectroscopy. The luminescent properties were measured, and the optical properties were related to the structural properties of the material. The number of bands for the 5D0 → 7F0 transition in the emission spectra depended on the cationic composition. Detailed investigation of the 5D0 → 7F0 transition of Eu3+ in the host lattice revealed changes in the number of Shtark components and, thus, europium-occupied sites in the structure. This data, in conjunction with the dielectric loss tangent (tan δ), indicate the appearance of the center of symmetry with increasing magnesium concentration up to 1. The analysis of different approaches allowed the establishment of three compositional areas in Ca9−xMgxEu(PO4)7 solid solution: 0 ≤ x ≤ 0.5 (R3c, ferroelectric phase), 0.5 < x < 0.8 (R3c + Rc), and 0.8 ≤ x ≤ 1.0 (Rc, antiferroelectric phase). The X-ray diffraction method does not allow these phases to be distinguished. It was found that magnesium cations have a significant effect on the luminescent properties of Ca9−xMgxEu(PO4)7 solid solution. The present research reveals that Eu3+ emission luminescence spectroscopy can be used as a method for the determination of the symmetry of crystalline phases that are very similar in structure.