Electron Spin Resonance and Mössbauer Studies of CaS:Eu Synthesized Using the Flux Method

Abstract

CaS:Eu phosphors were synthesized using the flux method. ESR and Mössbauer spectroscopies show the coexistence of Eu3+ and Eu2+. Eu3+ and Eu2+ are mainly observed in phosphors obtained with Na2S z ((Na) phosphors) and K2S z fluxes ((K) phosphors), respectively. The Eu2+ fraction in (K) phosphors varies with total Eu content and temperature of synthesis. Under reducing thermal treatment, part of the Eu3+ is converted into Eu2+ in (Na) phosphors, and also in (K) phosphors with Eu content ≤10-3 mole. For the latter phosphors, addition of chloride ions in the flux enhances the Eu2+ level, and after reducing thermal treatment practically all europium ions are in the Eu2+ state. In similar conditions, (K) phosphors with more than 10-2 mole Eu exhibit nearly no Eu2+ level variation, and (Na) phosphors undergo a separation of phases to obtain CaS:Eu3+ and CaS:Eu3+Eu2+. Interpretation of the various features on the basis of Na (K), Eu codoping and dopant-induced lattice distortions is proposed. It is suggested that (K) phosphors are partially oxygenated, with Eu3+ ion charge balanced by K+ ions and/or Ca2+ vacancies.

Synthesis and characterization of strongly fluorescent europium-doped calcium sulfide nanoparticles

“…In bulk materials, europium ions substitute calcium ions in the cubic symmetry lattice. 18,22 Therefore it follows that the europium ions reside as internal ions substituting calcium ions in the CaS lattice.…”

Valencies of Eu in CaS:Eu, La

“…They suggested that Eu> had two kinds of sites in CaS, forming center A (isolated Eu> center) and center B (paired Eu>-Eu>) (11). M. Pham-Thi et al observed two oxidation states of Eu> and Eu> in CaS prepared in alkaline polysulfide flux, however, they observed Eu> luminescence in CaS only at low temperature as many other investigators (6,12).…”

“…Europium has two isotopes of Eu and Eu with nuclear spin 5/2 (the natural abundance and nuclear g factor of Eu and Eu are: 47.8%, 1.39 and 52.2%, 0.61, respectively) (10,12), therefore, Eu> in cubic symmetry is expected to show two sets of six hyperfine lines with equal intensity. Fig.…”

Characterization of Luminescent Materials with 151Eu Mössbauer Spectroscopy

“…For CaS:Eu luminophors, Danilkin et al [136] observed only one Mössbauer absorption peak, corresponding to Eu${}^{3+}$ ions. Pham-Thi et al [103] synthesized CaS:Eu using the flux method with Na and K. The Mössbauer spectrum of the sample prepared with Na polysulphide flux showed only Eu${}^{3+}$ absorption while with K flux both ions were detected.…”