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. Issue 2 (15th December 2021)

• Record Type: Journal Article
• Authors: Deyneko, Dina V.; Nikiforov, Ivan V.; Lazoryak, Bogdan I.; Aksenov, Sergey M.
• Is Part Of: Dalton transactions. Volume 51:Issue 2(2021)
• Page Start: 655
• Page End: 663
• Publication Date: 2021-12-15
• Subjects: Chemistry, Inorganic -- Periodicals; Chemistry, Physical and theoretical -- Periodicals; Chemistry, Inorganic -- Periodicals; 546.05
• DOI: 10.1039/d1dt03534g
• Languages: English
• ISSNs: 1477-9226

Abstract

Abstract : The preferable scheme of heterovalent [PO4 ] 3− → [GeO4 ] 4− substitution have been studied and clarified in the β-Ca3 (PO4 )2 -type compounds. The similarity of the structure was calculated for novel Ca8+0.5 x ZnEu(PO4 )7− x (GeO4 ) x series. Abstract : A series of solid-solution phosphate germanates Ca8+0.5 x ZnEu(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 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-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 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 Ca9 La(GeO4 )0.75 (PO4 )6 compound reveals an unexpectedly high value. The same structural similarity evaluation of the studied compound Ca8.1 EuZn(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 Eu 3+ 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 Ge 4+ can improve the luminescence characteristics. The present study includes new data on the anionic substitution based on accurate crystal structure refinement.