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Deciphering the Role of Charge Compensator in Optical Properties of SrWO4:Eu3+:A (A = Li+, Na+, K+): Spectroscopic Insight Using Photoluminescence, Positron Annihilation, and X‑ray Absorption
journal contribution
posted on 2018-01-01, 16:03 authored by Santosh Kumar Gupta, Kathi Sudarshan, Ashok Kumar Yadav, Ruma Gupta, Dibyendu Bhattacharyya, Shambhu Nath Jha, Ramakant Mahadeo KadamStudies
have been carried out to understand the specific role of the alkali
charge compensator on the luminescence properties of an alkali ion
(Li+, Na+, and K+) codoped SrWO4:Eu phosphor. The oxidation state of the europium ion was
found to be +3 on the basis of X-ray absorption near edge structure
(XANES) measurements. This is the first report of its kind where opposite
effects of Li+ ion and Na+/K+ ions
on photoluminescence intensity have been observed. Li+ ion
codoping enhanced the photoluminescence intensity from SrWO4:Eu3+ phosphor while Na+/K+ ion
codoping did not. On the other hand, the luminescence lifetime is
maximum for the Na+ ion codoped sample and minimum for
the Li+ ion codoped sample. The results could be explained
successfully using time-resolved luminescence, positron annihilation
lifetime spectroscopy (PALS), and extended X-ray absorption fine structure
(EXAFS) spectroscopy measurements. Changes in the Eu–O bond
length and Debye–Waller Factor (σ2) upon Li+/Na+/K+ codoping were monitored through
EXAFS measurements. PALS also highlighted the fact that Li+ codoping is not contributing to reduction in the cation vacancies
and might be occupying interstitial sites rather than lattice positions
due to its very small size. On europium doping there is lowering in
symmetry of SrO8 polyhedra from S4 to C6, which is reflected in
an intense electric dipole transition in comparison to the magnetic
dipole transition. This is also corroborated using trends in Judd–Ofelt
parameters. The results have shown that the luminescence lifetime
is better when the vacancy concentration is lower as induced by Na+ and K+ codoping, while the emission intensity
is higher in the samples when distortion around Eu3+ is
reduced as induced by Li+ codoping.