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

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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.