Local Coordination, Electronic Structure, and Thermal Quenching of Ce3+ in Isostructural Sr2GdAlO5 and Sr3AlO4F Phosphors

SIGNIFICANCE

  

Research has revealed that phosphors employed in LED (Light Emitting Diode) or LD (Laser Diode) pumped white lighting highly rely on Cerium and ‎Europium activators’ parity- and spin-allowed d-f transitions, which permits broad band feature of photoluminescence excitation and emission. ‎Europium shows a slower decay since the excited state contains both spin octets and sextets while the ground state contains only spin octets. Compared to europium, the faster decay of cerium excited state to ground state reveal the advantage of avoiding a process of “excited state absorption”, which is the main cause of saturation in case of high excitation density. Alternatively, recent publications have shown that Sr2GdAlO5:Ce and Sr3AlO4F:Ce are isostructural phosphors in which the Cerium 4f-5d1 transition can be efficiently excited by a photon with energy lower than 3.1 eV. Unfortunately, further work is necessitated since the crystal field splitting of Cerium depends on the local environment and the wave function overlap between its 5d orbitals and the ligands, εcfs is affected by specific shape (symmetry) and size of the doping site.

Recently, researchers led by Professor Setsuhisa Tanabe from the Graduate School of Human and Environmental Studies at Kyoto University presented a study in which they focused on analyzing the crystal chemistry of the cerium local coordination, compare the thermal quenching property and construct the electronic structure of cerium in them. In their work, they paid attention to the oxide/oxyfluoride hosts and summarized the available structures/sites in which cerium can be efficiently excited by a relatively low energy photon. Their work is currently published in the research journal, Journal of the American Ceramic Society.

In brief, the local coordination of Cerium in the chosen Sr2GdAlO5/Sr3AlO4F (representatives of Cs3CoCl5-type oxide/oxyfluoride composition) was analyzed in a bid to explain the low energy 4f-5d transition. The site occupancy of cerium in both hosts was discussed based on the low-temperature photoluminescence excitation spectra; the transition energies from the 4f ground state to the lowest 5d1 state of Cerium in the local corresponding centers were assigned. Using the reflectance/photoluminescence spectra of Ce4+ and Cerium/‎Europium in Sr2GdAlO5 and Sr3AlO4F, the cerium 4f and 5d1 energy levels with respect to host bands were determined and the Vacuum Referred Binding Energy (VRBE) diagrams of cerium were constructed.

Based on the above approach, the researchers were able to explain the different thermal quenching behaviors based on the thermal ionization mechanism. Specifically, they reported that the cerium 5d-4f luminescence in Sr3AlO4F was much more stable against quenching than that in Sr2GdAlO5, as evidenced by the temperature-dependent luminescence intensity and luminescence decay studies.

In summary, isostructural Sr2GdAlO5 and Sr3AlO4F were selected as representative Cs3CoCl5-structural oxide and oxyfluoride and used to study the local coordination, electronic structure as well as thermal quenching of cerium. Altogether, a larger energy barrier ΔEdC between the cerium 5I1 level and the conduction band bottom in Sr3AlO4F was seen from the Vacuum Referred Binding Energy diagrams which explained the higher thermal quenching temperature by thermal ionization model.

      

Reference

Haipeng Ji, Jian Xu, Kazuki Asami, Jumpei Ueda, Mikhail G. Brik, Setsuhisa Tanabe. Local Coordination, Electronic Structure, and Thermal Quenching of Ce3+ in Isostructural Sr2GdAlO5 and Sr3AlO4F Phosphors. Journal of the American Ceramic Society, volume 102, issue 3, page 1316-1328.

Go To Journal of the American Ceramic Society

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