Europium doped mayenite (C12A7) powders of different concentrations (0.5, 1.0, 1.5, and 2.0 at.%) have been synthesized by a modified glycine/nitrate procedure - MGNP). Obtained samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE- SEM), and steady-state photoluminescence spectroscopy. The effect of doping concentration on photoluminescence properties of Eu3+ doped mayenite was studied and discussed. With the increasing of Eu3+ doping concentration, the red-emitting intensity exhibited behavior that increased firstly and then decreased. The optimal Eu3+ ion concentration is found to be 1.5%. High-pressure luminescence was measured in a Betsa high-pressure membrane diamond anvil cell up to 23 GPa

Dmitrović, Svetlana

Jelenković, Branislav

Luković, Jelena M.

Maletaškić, Jelena

Matović, Branko

Nikolić, Marko G.

Prekajski-Đorđević, Marija D.

Repository of the Vinča Nuclear Institute (VinaR)

Journal of Innovative Materials in Extreme Conditions  2020 Volume 1 Issue 1   12 Serbian Society for Innovative Materials in Extreme Conditions (SIM-EXTREME), Belgrade, Serbia.   LUMINESCENCE PROPERTIES OF Eu3+ DOPED MAYENITE UNDER HIGH PRESSURE   B. Matovic1*, M. Nikolic2, M. Prekajski Djordjevic1, S. Dmitrovic1, J. Lukovic1, J. Maletaskic1, B. Jelenkovic2 1Center for synthesis, processing, and characterization of materials for application in the extreme conditions “CextremeLab”, VINCA Institute of Nuclear Sciences - National Institute of thе Republic of Serbia, University of Belgrade, Belgrade, Serbia 2Institute for Physics, Belgrade University, Belgrade, Serbia Corresponding author*: mato@vinca.rs   Abstract: Europium doped mayenite (C12A7) powders of different concentrations (0.5, 1.0, 1.5, and 2.0 at.%) have been synthesized by a modified glycine/nitrate procedure - MGNP). Obtained samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and steady-state photoluminescence spectroscopy. The effect of doping concentration on photoluminescence properties of Eu3+ doped mayenite was studied and discussed. With the increasing of Eu3+ doping concentration, the red-emitting intensity exhibited behavior that increased firstly and then decreased. The optimal Eu3+ ion concentration is found to be 1.5%. High-pressure luminescence was measured in a Betsa high-pressure membrane diamond anvil cell up to 23 GPa.   1. Introduction  Mineral mayenite is one of the intermediary phases of the CaO-Al2O3 binary system, known as a constituent of calcium aluminate cement with a formula written as 12CaO·7Al2O3 or C12A7 in the cement-chemist notation. The first time it was originally reported from Eifel volcanic complex in Germany and is named after the place of its discovery, Mayen (Germany) by Hentschel 1. Recently, mayenite encouraged research interest because of its oxygen mobility 2, ionic conductivity 3, high anti-carbon and anti-sulfur characteristics 4-7 and catalytic properties 8. Mayenite possesses a distinct feature related to its crystal structure. The unit cell of C12A7 contains two molecules and could be expressed as a positively charged lattice framework (Ca24Al28O64)4+ and 2O2- negative one 9. The positive part of the framework consists of three-dimensional 12 crystallographic cages per unit cell, which are interconnected in such a way that the channels of complex shape form between them with an inner free space of 0.4 nm in diameter. The letter part, 2O2-, is known as “extra-framework oxide ions,” which are distributed within the cages to maintain charge neutrality 10, 11. It is assumed [2] that weakly bound oxygen anions can move through empty channels between cages and thereby create high oxygen conductivity in the material. Mayenite has been synthesized using various methods. The commonly used method is a solid-state reaction between CaCO3 and -Al2O3 12. Although it is a simple way that also provides mass production, the high firing temperature is a critical issue, and a new fabrication method was developed which is also capable of being utilized at a much lower temperature. Single crystals of C12A7 with regular morphologies can be obtained by applying the Czochralski method 13 and floating zone (Fz) methods 14,15. Also, a low-temperature approach is possible, such as hydrothermal and sol-gel synthesis 16,17 and oxalate precursor route [18]. One of the most efficient and precise methods is a modified glycine/nitrate procedure [19], which is used in this paper. Since they share common planes in cages that are open and oxygen can move easily, it is assumed that the C12A7 structure is very sensitive to pressure. Moreover, in phosphors doped mayenite, the change can be followed by its luminescence properties due to subatomic motions or stress on a crystal. In this work, the results on synthesis and high-pressure dependent photoluminescence properties of Eu doped C12A7 powders are described.    Journal of Innovative Materials in Extreme Conditions  2020 Volume 1 Issue 1   13 Serbian Society for Innovative Materials in Extreme Conditions (SIM-EXTREME), Belgrade, Serbia.  2. Experimental  Precursors used for the production of the powder were Ca and Al nitrates, and Al and Ca acetate, (Aldrich, USA), and aminoacteic acid-glycine, (Fischer Scientific, USA). Europium is added as Eu nitrate in different concentrations (0.5, 1.0, 1.5, and 2.0 at.%) in order to obtain a solid solution in small concentrations. Starting chemical reactants were dissolved in a glass beaker according to the composition of C12A7, following thermal treatment on a hot plate until the reaction smoke is terminated. This reaction is based on self-combustion of the metal nitrate/acetate and glycine mixture which occurs at 180°C spontaneously and terminates rapidly [19]. The resulting powder was calcined to 600°C to burn any organic residues from precursors and at 1200°C to induces crystallization. The phases of samples were identified using X-ray powder diffraction (XRPD) on an Ultima IV Rigaku diffractometer using CuK radiation with a scanning step size of 0.02° and at a scan rate of 2 °/min. (2-range: 20 - 80°). Phase analysis was done by using the PDXL2 software (version 2.0.3.0) [20], regarding the patterns of the International Centre for Diffraction Data database (ICDD) [21], version 2018.  The microstructural observations of obtained powders were performed by scanning electron microscopy (SEM), Vega Tescan. The photoluminescence spectra measurements were done under continuous Nd YAG laser excitation at 532 nm. Data were collected from Ocean Optics USB2000 (200-800 nm) spectrometer. High-pressure luminescence was measured in a Betsa high-pressure membrane diamond anvil cell with steel gasket (T301). The sample chamber was 125 µm in diameter and 70 µm in thickness. A mixture of methanol and ethanol with a 4:1 volume ratio was selected as the pressure-transmitting medium. Ruby loaded with sample determines the pressure. For measuring Ruby R1 line shift HR2000 Ocean Optics spectrometer (600-800 nm) was used.  3. Results and discussion  The microstructure of synthesized powders exhibited a porous structure (Figure 1a). Since the reaction for C12A7 synthesis is based on self-combustion of the metal nitrate/acetate and glycine mixture which produced a huge amount of gaseous; normally, the highly porous texture is obtained. On the other side, the microstructure is changed after thermal treatment. Morphologies of calcined powders at 1200C are cemented in a form of agglomerated, mostly well-formed faceted grains. Agglomerates manifested layer structure with plate grains consisting of regular crystals of a narrow size range of 0.8–1 μm and thickness roughly estimated about 200-250 nm (Figure 1b).                                         a)                                   b) Figure 1. SEM images of samples C12A7 calcinated at (a) 600°C, and at (b) 1200°C.  The XRD study confirms the SEM results. The patterns of obtained powders by the MGNP method calcined at 600C show typical amorphous diffraction lines (Figure 2). Journal of Innovative Materials in Extreme Conditions  2020 Volume 1 Issue 1   14 Serbian Society for Innovative Materials in Extreme Conditions (SIM-EXTREME), Belgrade, Serbia.  20 30 40 50 60 70 80Intensity (a.u.)2 Degree Figure 2. X-ray diffraction patterns of the powder obtained by the MGNP and calcinated at 600 /1 h.  Unlike as-synthesized powder (Figure 2), the annealed powders exhibit clear diffraction lines that could be assigned to the cubic unite cell (SG I-43d with lattice parameter a = 11.989 Å, Z = 2; JCPDS: 48-1882). Peaks relating impurities or other Ca or Al-oxide phases were not observed (Figure 3). XRD patterns of obtained C12A7:Eu3+ powders with different doping concentrations are shown in Figure3. 10 20 30 40 50 60 70 80x = 2.0x = 1.5x = 1.0930932761664842840752831741800651642730631444710640620611530521510422332420400321310220Intensity (a.u.)2Degree211x = 0.5 Figure 3. XRD patterns of 12CaO·7Al2O3:x% Eu3+ (x = 0.5; 1.0; 1.5; .2.0) calcined at 1200C/2 h.  Journal of Innovative Materials in Extreme Conditions  2020 Volume 1 Issue 1   15 Serbian Society for Innovative Materials in Extreme Conditions (SIM-EXTREME), Belgrade, Serbia.  Emission spectra of 0.15 Eu3+ doped mayenite sample excited with 532 lasers are shown in Figure 4.a. In this picture, five bands associated with 5D0 -7FJ (J=4, 3, 2, 1, and 0) spin forbidden f–f transitions can be observed (Figure 4.b). For the 5D0 -7FJ transition we observe only one peak, which is expected knowing that the coordination difference between two Ca2+ non-equivalent crystallographic sites, where Eu3+ ion can be placed, is practically negligible.                     Figure 4. a) Emission spectrum Eu3+ doped mayenite with 532 nm laser excitation. b) Energy scheme levels of Eu3+    Pressure dependence of emission spectrum Eu3+ doped mayenite is shown in Figure 5a. The 5D0-7F2 is an electric dipole transition (ΔJ=2) very sensitive to the local environment around Eu3+, and its intensity depends on the symmetry of the crystal field around the europium ion. In contrast, the 5D0-7F1 transition is the parity allowed magnetic dipole transition (ΔJ=1), and its intensity does not vary with the host.                 Figure 5. a) Excitation spectrum Eu3+ doped mayenite; b) emission spectra of Eu3+ doped mayenite samples obtained with different Eu3+ concentrations.     b) a) b) a) Journal of Innovative Materials in Extreme Conditions  2020 Volume 1 Issue 1   16 Serbian Society for Innovative Materials in Extreme Conditions (SIM-EXTREME), Belgrade, Serbia.                   Figure 6. a) Pressure dependence of emission spectrum Eu3+ doped mayenite. b) emission spectra before, during maximum pressure, and after decompression   Figure 6a shows the intensity ratio of these emission lines. It can be seen that the intensity of the line 5D0-7F2 decreases relative to the intensity 5D0-7F1 line up to 15 GPa, which indicates an increase in the symmetry environment of Eu3+ ions. Also, splitting 5D0-7F1 line (Figure 6b) shows the increase of the crystal field in the Eu3+ surrounding.                  Figure 7. Pressure dependence of a) intensity ratio of 5D0-7F1 magnetic dipole and 5D0-7F2 electric dipole transitions. b) 5D0-7F1 emission line splitting.               b) a) a) b) Journal of Innovative Materials in Extreme Conditions  2020 Volume 1 Issue 1   17 Serbian Society for Innovative Materials in Extreme Conditions (SIM-EXTREME), Belgrade, Serbia.                    Figure 8. Pressure dependence of 5D0-7F2 line shift.   4. Conclusion  The effect of doping concentration on photoluminescence properties of Eu3+ doped mayenite under high pressure was studied. Pressure dependence of emission spectrum Eu3+ doped mayenite shows that it is very sensitive to the local environment around Eu3+, and its intensity depends on the symmetry of the crystal field around the europium ion. Obtained results suggest that C12A7: Eu phosphor may serve as promising red luminescent materials used in the fabrication of optical storage and illumination in dark environments.  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Luminescence Properties Of Eu3+ Doped Mayenite Under High Pressure

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Luminescence Properties Of Eu3+ Doped Mayenite Under High Pressure

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