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

A series of solid-solution phosphate germanates Ca 8+0.5 x ZnEu(PO 4 ) 7- x (GeO 4 ) x ( x = 0, 0.2, 0, 4, 0.6, 0.8, 1) with the β-Ca 3 (PO 4 ) 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 [PO 4 ] 3- → [GeO 4 ] 4- substitution requires a charge compensation according to the scheme: [PO 4 ] 3- + ½ □ → [GeO 4 ] 4- + ½ Ca 2+ . The additional amount of Ca 2+ ions in the crystal structure was detected at the M4 site during Rietveld refinement. It was shown that in β-Ca 3 (PO 4 ) 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 Ca 2+ at the M4 site leads to a polar structure with the space group R 3 c which was confirmed by an SHG test for all single-phase samples. It was shown that the Ge 4+ 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 Ca 9 La(GeO 4 ) 0.75 (PO 4 ) 6 compound reveals an unexpectedly high value. The same structural similarity evaluation of the studied compound Ca 8.1 EuZn(PO 4 ) 6.8 (GeO 4 ) 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 Eu 3+ were investigated from the point of view of crystal structures and anionic substitutions. The integral intensity increased linearly with the [PO 4 ] 3- → [GeO 4 ] 4- substitution. It can be concluded that the anionic substitution on Ge 4+ can improve the luminescence characteristics. The present study includes new data on the anionic substitution based on accurate crystal structure refinement.