Behaviour of electric and magnetic dipole transitions of Eu3+, 5D0 → 7F0 and Eu-O charge transfer band in Li+ co-doped YPO4:Eu3+

The effect of Li+ co-doping on the photoluminescence properties of YPO4:Eu is discussed. Interesting behaviours, such as the presence of intermediate bands, shifting of the Eu-O charge transfer band (Eu-O CTB) to a lower wavelength, variation in intensities of magnetic (D-5(0) -> F-7(1)) and electric dipole (D-5(0) -> F-7(2)) transitions of Eu3+ and shift of D-5(0) -> F-7(0) to higher energy with increasing excitation wavelengths are observed. The Eu3+ ion does not have an absorption band in the range 340-350 nm, but after excitation at these wavelengths, a broad emission band (370-570 nm), as well as sharp peaks of Eu3+, could be observed. This is due to strong energy transfer from the intermediate band of the host to the Eu3+ ion. X-ray photoelectron spectroscopy (XPS) study also confirms that intermediate band emission is not due to Eu2+ ion emission. The blue shifting of Eu-O CTB is because of the increase in the optical electronegativity of the Eu3+ ion on Li+ co-doping. The variation in intensities of the D-5(0) -> F-7(2) and D-5(0) -> F-7(1) dipole transitions is related to (i) overlapping interaction parameters within the ground and excited states, (ii) exchange interaction among atoms/ions and (iii) density of the incoming photons. Shift of D-5(0) -> F-7(0) to a higher energy with increasing excitation wavelengths is because of change in the second order crystal field parameter B-20 with excitation wavelength. The significant enhancement of luminescence intensity is found with Li+ co-doping due to the increase in crystallinity.