Photoluminescence behavior of Al 3+ , Pr 3+ doped perovskite La 2/3 TiO 3 and pyrochlore La 2 Ti 2 O 7
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S.Y. Choi Department of Ceramic Engineering, Yonsei University, Seoul 120-749, Korea (Received 27 November 2000; accepted 19 June 2001)
The purpose of this study is to develop an understanding of the photoluminescence properties of Al3+, Pr3+ doped perovskite-type La2/3TiO3 and pyrochlore-type La2Ti2O7 phosphor, which is characterized by the red emission (1D2 â 3H4 transition) of the Pr3+ ion. The explanation for the energy transfer and the corresponding critical distance is proposed on the basis of the role of Al3+ ions in the perovskite-type La2/3TiO3:Pr phosphor. To clarify the distinction of photoluminescence properties between the perovskite-type La2/3TiO3 and the pyrochlore-type La2Ti2O7, the trap-involved process and the charge transfer band have been investigated, respectively.
I. INTRODUCTION
Pyrochlore-type La2Ti2O7 has recently found extensive application in various branches of technology. Because the ferroelectric properties result from the characteristic features of crystal structure, data on the structure of La2Ti2O7 are of considerable interest.1 Also, perovskite-type La2/3TiO3 has been studied by various authors due to the interesting dielectric and ferroelectric properties.2â4 In the pyrochlore La2Ti2O7 and the perovskite La2/3TiO3, the photocurrent, taking place mainly in the UV region, has been attributed to charge transfer occurring formally between oxygen and either lanthanum or titanium. In this view, it seems interesting to study the luminescence of Pr3+ in the perovskite La2/3TiO3 and pyrochlore La2Ti2O7. The luminescence process of La2/3TiO3:Pr and La2Ti2O7:Pr is characterized by the 1D2 â 3H4 transition of the Pr3+ ion acting as an activating center. In particular, the addition of an Al 3+ impurity to La2/3TiO3:Pr and La2Ti2O7:Pr was found to be most effective as an sensitizer in increasing the emission intensity, a possible explanation for the energy transfer between the Al3+ ion and the Pr3+ ion in La2/3TiO3 and La2Ti2O7. In this respect, the energy transfer and the corresponding critical distance deserve intensive consideration. II. EXPERIMENTAL PROCEDURE
Polycrystalline powders of Al3+, Pr3+ doped La2/3TiO3 and La2Ti2O7 were prepared by conventional solid-state reaction of La2O3, TiO2, Al(OH)3, and PrCl3 powders 2568
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J. Mater. Res., Vol. 16, No. 9, Sep 2001 Downloaded: 16 Mar 2015
mixed in the appropriate proportions, followed by an additional grinding and firing in oxygen. The phases of the powders were identified by using an x-ray diffraction (XRD) system with Cu K⣠radiation (Ni filter). The photoluminescence emission and excitation spectra of the powder samples were obtained using a Perkin-Elmer LS50 luminescence spectrometer with a xenon flash lamp.
III. RESULTS AND DISCUSSION
Trivalent Pr ions are expected to substitute for La3+ ions rather than Ti4+ ions solely on the basis of the ionic radii. Also, trivalent Al ions are expected to either substitute for Ti4+ ions or interstitial sites. The XRD patterns of the final phases of samples are sh
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