The role of anionic heterovalent [PO4]3− → [GeO4]4− substitution on the luminescence properties of inorganic phosphors with the β-Ca3(PO4)2-type structure: new data based on accurate crystal structure refinement

Abstract

A series of solid-solution phosphate germanates Ca_8+0.5xZnEu(PO₄)_7−x(GeO₄)_x (x = 0, 0.2, 0, 4, 0.6, 0.8, 1) with the β-Ca₃(PO₄)₂-type structure were synthesized by solid-state reactions. The limit of existence of a single-phase solid solution was determined by X-ray diffraction patterns and it was found at x = 0.8. The heterovalent tetrahedral [PO₄]³⁻ → [GeO₄]⁴⁻ substitution requires a charge compensation according to the scheme: [PO₄]³⁻ + ½ □ → [GeO₄]⁴⁻ + ½ Ca²⁺. The additional amount of Ca²⁺ ions in the crystal structure was detected at the M4 site during Rietveld refinement. It was shown that in β-Ca₃(PO₄)₂-type compounds, charge balancing is not provided by the randomly distributed oxygen vacancies but only by the partial occupancy of the M4 site. The presence of Ca²⁺ at the M4 site leads to a polar structure with the space group R3c which was confirmed by an SHG test for all single-phase samples. It was shown that the Ge⁴⁺ ions preferably occupy the T3 site in the structure, which is connected through common oxygen with the cationic M1–M5 sites. The analysis of the similarity of the previously reported Ca₉La(GeO₄)_0.75(PO₄)₆ compound reveals an unexpectedly high value. The same structural similarity evaluation of the studied compound Ca_8.1EuZn(PO₄)_6.8(GeO₄)_0.2 in the present work with the initial model gives a very small value, which indicates a good match between the initial and under-consideration structures. The luminescence properties of Eu³⁺ were investigated from the point of view of crystal structures and anionic substitutions. The integral intensity increased linearly with the [PO₄]³⁻ → [GeO₄]⁴⁻ substitution. It can be concluded that the anionic substitution on Ge⁴⁺ can improve the luminescence characteristics. The present study includes new data on the anionic substitution based on accurate crystal structure refinement.