Polymorphism of Whitlockite-Type Phosphors Ca9–1.5xZnMeEux(PO4)7 and Enhancement of Eu3+ Luminescence with Me = Li, Na

Abstract

Powders and ceramics with whitlockite-type structure are synthesized using the solid-state method in novel Ca_9–1.5xZnMeEu_x(PO₄)₇ (Me = Li, Na) series, 0 ≤ x ≤ 1. The substances are divided to centro- and non-centrosymmetric by optical second harmonic investigation. There are non-centrosymmetry at x ≤ 0.3 and centrosymmetry at x ≥ 0.9, while at 0.3 < x < 0.9 they coexist. With differential scanning calorimetry and dielectric measurements, the reversible transformations in sequence R3c ↔ R$\overline{3}$c ↔ R$\overline{3}$m are observed and new phase transitions were found. At room temperature, photoluminescence spectroscopy and the Mössbauer effect on ¹⁵¹Eu nuclei pointed to reducing of number of Eu³⁺ non-equivalent positions from n = 3 in the phase R3c (x ≤ 0.3) to n = 2 in R$\overline{3}$c at x = 1. The less positions for Eu³⁺ resulted in well-shaped luminescence spectra of increased intensity. Rietveld refinement evidenced that alkali atoms are settled in some extraordinal positions impeding Ca²⁺ ions hopping. This is consistent with the observed drop in ionic conductivity and better luminescence, probably, due to weakening of Eu³⁺ nonradiative phonon deactivation.