Towards Natural White Luminescence: Synthesis Control of the Oxidation States of Europium in Glassy Matrices and Its Effect on Photoluminescence Spectra

It is well known that the photoluminescence (PL) of lanthanides (Ln = Eu, …) centers in inorganic matrices depends on their oxidation states. Therefore, it is desirable to control that factor at the stage of synthesis of the PL materials which can be promising phosphors in many applications, e.g., in lighting. The experience of our group with controlling the oxidation states of transition metals present in prospective glassy and nanostructured cathode materials for Li-ion and Na-ion [1], has helped us to use similar methods to take control of the oxidation states of lanthanides centers embedded in transparent glassy matrices. Lanthanides are widely used in today's technology – among others, in lasers based on Ln-doped crystals or glasses. This is due to a large variety of their <i>4f</i> configurations of electrons in Ln atoms, which leads to a wide range of fluorescent states and wavelengths [2, 3].<br/>Our recent work [4] proved that it is possible to synthesize Eu-doped glassy materials whose photoluminescence spectra can be tuned depending on synthesis conditions by using melt-quenching process. During that work, we found out that one can control the relative Eu³⁺/Eu²⁺ ions concentrations. As both Eu³⁺/Eu²⁺ are photoluminescent in different parts of the visible range, the possibility of controlling their relative concentrations gives an opportunity to "tune” photoluminescence spectrum of the material and makes it similar to that of natural daylight. It is also possible to adjust phosphors, based on glassy matrices doped with Eu, to other desired colors. However, further studies are needed to improve this method of europium ion reduction and to specify the exact influence of melting time and temperature on the final properties of the samples.<br/>In order to do that, a series of glassy matrices (10 samples in total) with nominal composition Na₃Al₂(PO₄)₂F₃ doped with 1 wt% of Eu₂O₃ were successfully synthesized by a melt-quenching process, using a double crucible method [5] – 9 samples, and 1 sample without reducing atmosphere. Samples synthesis condition differed, temperature was controlled in the range of 1000 °C, 1100 °C and 1200 °C and time was set as 5 min, 10 min or 15 min. Obtained glasses were carefully investigated using X-ray diffractometry (XRD), photoluminescence spectroscopy (PL), photoluminescence excitation (PLE) and absorption spectroscopy.<br/>X-ray diffractometry confirmed amorphousness of all samples. Photoluminescence spectroscopy results (samples excited with 270 nm and 396 nm) showed gradual increase of Eu²⁺ emission line in the spectrum together with increase of time and temperature of melting. At the same time, the opposite was observed for Eu³⁺ line – it decreased with time and temperature. Photoluminescence excitation and absorption spectroscopy confirmed photoluminescence spectroscopy data with the difference of increase/decrease of characteristic absorption peaks for Eu³⁺/Eu²⁺.<br/><b>References</b><br/>[1] T.K. Pietrzak <i>et al.,</i> Materials Science and Engineering B 213 (2016) 140-147.<br/>[2] M. C. Goncalves <i>et al., </i>Comptes Rendus Chimie, 5 (2002) 845–854.<br/>[3] B. M. Walsh: <i>Advances in Spectroscopy for Lasers and Sensing. Judd-ofelt theory: principles and practices</i>. [red.] Baldassare Di Bartolo, Ottavio Forte. Erice Italy, Springer, 2005<br/>[4] T.K. Pietrzak, A. Golebiewska <i>et al</i>., Journal of Luminescence 208 (2019) 322–326.<br/>[5] K. Hirose <i>et al</i>., Solid State Ionics, 178 (2007) 801–807.<br/><b>Acknowledgments: </b>This research was funded by POB FoTech of Warsaw University of Technology within the Excellence Initiative: Research University (IDUB) program.