Effects of Post Calcination Treatment on Photoluminescence and Structural Properties of Scheelite-type LiEuW 2 O 8 Nanoc
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0951-E03-29
Effects of Post Calcination Treatment on Photoluminescence and Structural Properties of Scheelite-Type LiEuW2O8 Nanocrystals Synthesized by Glycothermal Reaction Ryo Kasuya1, Tetsuhiko Isobe1, Shinobu Yamao2, and Hocine Sfihi3 1 Faculty of Science and Technology, Keio University, 3-14-1, Hiyoshi, Kohoku-ku, Yokohama, 223-8522, Japan 2 Idemitsu Kosan Co. Ltd., 1280, Kamiizumi, Sodegaura, 299-0293, Japan 3 Laboratoire de Physique Quantique, UMR CNRS 7142, ESPCI, 10 rue Vauquelin, Paris, 75231, France
ABSTRACT The scheelite-type LiEuW2O8 (LEW) nanoparticles of ~50 nm in diameter were synthesized from lithium acetate, europium (III) acetate and phosphotungstic acid in 1,4butylene glycol at 300oC for 2h by autoclave treatment. Post calcination treatment at 600 oC for 2 h enhanced the photoluminescence at 465 nm due to the 4f - 4f transition excitation of Eu3+ by a factor of 24.4, and due to charge transfer (CT) excitation from either O2- or WO42- to Eu3+ by a factor of 5.8. Convergent beam electron diffractmetry and the compositional analysis revealed the non-stoichiometric structure of the scheelite-type LEW nanocrystals. Raman spectroscopy also detected the WO4 vibrational modes due to the tetragonal scheelite-type LEW. Solid-state 7 Li single pulse excitation MAS NMR indicated the existence of at least three Li sites in the samples. We conclude that the following three factors are improved by calcination to increase the f-f transition probability of Eu3+: (i) a distribution of the Eu3+ – Eu3+ distance, (ii) the symmetry of Eu3+ polyhedra, (iii) the symmetry of tetrahedral WO4 units in the vicinity of Eu3+ polyhedra. Furthermore, oxygen is provided for non-stoichiometric LEW during calcination to enhance the CT probability. INTRODUCTION Recently, Odaki et al. reported that scheelite-type LiEuW2O8 (LEW) exhibits the higher red emission efficiency under blue light excitation than AEuW2O8 (A: Na, K, Rb and Cs) [1]. LEW has attractive optical properties listed in below: (i) the red emission due to the 4f-4f transition of Eu3+ occurs under excitation by ultraviolet, blue or green light, (ii) concentration quenching hardly occurs even if yttrium ions in the host material LiYW2O8 are completely substituted by europium ions. These features can be explained from its structural aspect. In scheelite-type LEW crystal, randomly distributed dodecahedra of LiO8 and EuO8 are surrounded by WO4 units [2]. Additionally, the distance between europium ions (4.1 Å) is longer than that of RbLa1-xEuxTa2O7 (3.8 Å), in which concentration quenching occurs at x > 0.5 [3]. Therefore, the migration of excitation energy between emission ions would be negligible. Micronsized LEW was synthesized by conventional solid-state reaction [1]. Here we applied the glycothermal method for the preparation of scheelite-type LEW, because we previously succeeded in the preparation of Y3Al5O12:Ce3+ nanocrystals of ~ 10 nm in diameter by glycothermal method [4].
EXPERIMENT Lithium acetate dihydrate (2.5 mmol), europium (III) acetate tetrahydrate (2.
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