Enhanced luminous efficiency in Pr3+ activated Ca0.5La(MoO4)2 red phosphor with blue excitation for WLED applications
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In this article, we have investigated the down-conversion luminescence properties of Ca0.5La(MoO4)2 doped with trivalent Pr3+ (substitute for La3+ site) and codoped with Mn2+ (substitute for Ca2+ site). The powder X-ray diffraction patterns confirm that the single crystalline phosphor powders belong to the scheelite-type tetragonal crystal structure. The photoluminescence (PL) excitation spectra of Ca0.5La(MoO4)2:Pr3+ exhibits a charge transfer band centered at 276 nm along with three intense sharp absorption bands identified at 449, 475, and 488 nm which are attributed to the f–f electronic transitions of 3H 4 (1) → 3P2, 3H 4 (1) → 3P1, 3H 4 (1) → 3P0 of Pr3+, respectively. Upon optical excitation, the depopulation of excited 3P0 level into the 3F2 lower levels of Pr3+ is governed by radiative transfer of energy, as a result, high intense red emission peak was observed at 647 nm. The change in PL emission intensity as a function of Pr3+ concentrations, Mn2+ concentrations, and different excitation wavelengths have been investigated in detail. Further, the codoping of alkali metal chlorides MCl (M = Na, K, Li) into the Ca0.5La(MoO4)2:Pr3+ phosphor greatly improves the luminescence intensity of the transition 3P0 → 3F2 which can be explained by charge compensation effect. To ensure the colour richness and quality of its emission, the photometric parameters were estimated using spectral energy distribution functions of Ca0.5La(MoO4)2:Pr3+. The luminous efficacy of radiation for NaCl codoped Ca0.5La(MoO4)2:Pr3+ phosphor was estimated to be 154 lm/W and the percentage of luminous efficiency is 23 %.
KeywordsEnergy Transfer Process Luminous Efficiency Colour Correlate Temperature Colour Render Index Alkali Chloride
The authors acknowledge the financial support provided by Science and Engineering Research Board (SR/FTTP/PS-135/2011), Govt. of India. Also, the authors gratefully acknowledge Alagappa University-Karaikudi, SAIF IIT-Bombay, and DST unit of Nano Science Centre, IIT-Madras, for extending their instrumentation facilities for characterization.
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