Energy transfer dynamics and luminescence properties of Eu3+ in CaMoO4 and SrMoO4

Abstract

Undoped and europium doped CaMoO₄ and SrMoO₄ scheelites are synthesized using a complex polymerization method. The phase purity of the sample is confirmed using powder X-ray diffraction (PXRD). X-ray photoelectron spectroscopy (XPS) was carried out to confirm the oxidation states of various constituents and dopant elements and also the presence of oxygen vacancies. Interestingly both CaMoO₄ and SrMoO₄ on irradiation with UV light give blue and green emission respectively. On europium doping, it was found that molybdate to Eu³⁺ ion energy transfer is more efficient in SrMoO₄:Eu compared to CaMoO₄:Eu. It is also justified using a luminescence lifetime study which shows biexponential decay in the case of CaMoO₄:Eu corresponding to both the host and europium ion; whereas a single lifetime is observed in the case of SrMoO₄:Eu. Anomalies in host–dopant energy transfer are suitably explained using density functional theory (DFT) calculations and XPS. The actual site symmetry for the europium ion in CaMoO₄ and SrMoO₄ was also evaluated based on a Stark splitting pattern which turns out to be D₂ and C_2v respectively although it is S₄ for Ca/Ba²⁺ in AMoO₄. This is also reflected in higher Ω₂ values for SrMoO₄:Eu than CaMoO₄:Eu.