Revealing the substitution mechanism in Eu3+:CaMoO4 and Eu3+,Na+:CaMoO4 phosphors

Abstract

Eu³⁺-Doped calcium molybdate is an excellent phosphor for lighting and display devices due to the very intense pure red emission after UV excitation. It has been reported in the literature that the CaMoO₄ unit cell volume expands after Eu³⁺ doping, in spite of the smaller Eu³⁺ ionic radius compared with Ca²⁺. Likewise, several studies found that the emission intensity of the phosphor could be improved by codoping with alkaline ions like Li⁺, Na⁺ or K⁺. None of these studies correlated the apparent volume expansion and luminescence enhancement with the crystal structural details. This paper analyses the aliovalent substitution mechanism and crystal structure of Eu³⁺:CaMoO₄ and Eu³⁺,Na⁺:CaMoO₄ phosphors using complementary techniques like Raman spectroscopy, EXAFS and SPD. We found that the substitution mechanism was different for both systems, with Ca site vacancies forming in the Eu³⁺:CaMoO₄ phosphors and leading to Ca_1−3xEu_2x□_xMoO₄ compositions, while the Eu³⁺,Na⁺:CaMoO₄ phosphors formed Ca_1−2xEu_xNa_xMoO₄. SPD showed that the cell volume expansion observed with increasing Eu³⁺ content is related to the increase of the Mo–O bond distance due to the higher electronegativity of Eu³⁺ compared with Ca²⁺. Finally, it was shown that the luminescence properties, i.e. lifetime values and quantum yields (the latter reported here for the first time), do not depend on the presence of monovalent ions in the crystal structure but, exclusively, on the Eu³⁺ content of the phosphor. The integral and detailed analysis of the materials presented in this paper, ranging from crystal structure to luminescent properties including elemental composition, allows a full picture of the structure–property relationships that had never been addressed before for CaMoO₄-based phopshors.