(Invited) Investigation of Eu Doped Silicate Phosphor Family M2Ln8(SiO4)6O2 (M=Mg, Ca, Sr; Ln=La, Y) with Apatite Structure for LED Applications
Authors: Kirm Marco , Vielhauer Sebastian , Tõldsepp Eliko , Mägi Henri , Nagirnyi Vitali , Juestel Thomas , Jansen Thomas , Khaidukov Nicolas M , Makhov Vladimir N 
Published: February 2020

Research and development of new high-efficiency phosphors for white light generation using light emitting diodes (LEDs) is presently one of the most important topics for development of light sources for general and special lighting. LED light sources offer significant advantages compared to other lighting technologies: low power consumption, high efficiency, long service life, compact size, environmentally-friendly (mercury-free) modules, high resistance to external conditions [1,2]. Search for new compounds with better performance is actively going on. Silicates with apatite structures have got attention for LED phosphor applications because of their stability and high efficiency having a potential for white light generation [3-6].

The complicated crystal structure of silicate apatites provides a good basis for advanced materials engineering. There are two crystallographic sites in the IXA14
VIIA26(SiO4)6O2 structure, which can be occupied by the alkaline earth (M2+) and rare earth (Ln3+) cations, 9 and 7-fold coordinated for A1 and A2, respectively. Besides that, silicate apatites can be doped and codoped with ions of different valence states, i.e. the conditions can be realized for various energy and charge transfer processes. Variation of alkaline earth ions in a row Mg – Ca – Sr changes occupancy of the 9 and 7-fold coordinated sites due to the properties of the apatite structure [7]. Small radius Mg2+ ions tend to occupy preferentially the A2 site and larger radius Sr2+ ions mainly the A1 site resulting in IXLn4
VIILn4
VIIMg2(SiO4)6O2 and IXSr2
IXLn2
VIILn6(SiO4)6O2 compounds, respectively. Medium size Ca2+ ions follow statistical distribution of cations populating both sites. These features of silicate apatites allow advanced materials engineering providing large potential for the development of phosphors for white light generation using LEDs. The motivation of our research is twofold: firstly, the study of the silicate apatite phosphor family enables deeper understanding how structural peculiarities influence electronic properties and secondly, evaluation of application potential for the developed rare earth doped materials in the LED field.

The studied phosphors, silicate apatites M2Ln8(SiO4)6O2 (where M = Mg, Ca, Sr; Ln = Y, La) doped with europium ions at various concentrations (0.5-2.0 at. %), were prepared by high-temperature solid-state reaction using the precursors synthesized under hydrothermal conditions (e.g see [8]). The selected samples were annealed in H2/Ar reducing atmosphere in order to convert Eu3+ into Eu2+ and bring more understanding between interplay of electronic subsystems of ions with different valence states.

The structural and luminescence properties of synthesized phosphors were characterized by XRD analysis, Raman spectroscopy, diffuse reflectance, time-resolved photoluminescence studies (under blue-to-VUV excitation), cathodoluminescence and site-selective high resolution spectroscopy in the temperature range of 3-400 K. Particle size and morphology of the samples were investigated by scanning electron microscopy. We followed the evolution of emission spectra and decay kinetics of the phosphors under excitation of different 4f levels of Eu3+, (O2- - Eu3+) charge transfer band (peaked near 275 nm) and into host absorption (< 160 nm). Based on experimental data, several emission centers due Eu3+ and Eu2+ ions as well as for host emissions were identified. Depending on the host compound and treatment procedure the phosphors can show a large variety of spectral properties which cannot be explained in many cases as only by the presence of two crystallographic sites for doping ions. Accordingly, additional mechanisms should be also considered for the detailed explanation of the observed properties. The relaxation and energy transfer processes for silicate apatites M2Ln8(SiO4)6O2 will be discussed. Also properties relevant for LED application will be reported.

This research project NANOLED # 361 was carried out within framework of the ERA.Net RUS Plus Programme supporting cooperation in 'Science & Technology' between European Union and Russia (RFBR Grant 16-52-76028 ERA_а).

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