Effect of alkali charge compensator on luminescent properties in Eu3+ doped β-dicalcium silicate

Abstract

Recent studies have shown that, Eu3+ doped silicates have gained lot of importance as an ideal red phosphor material in White Light Emitting Diode (WLED) application, due to Eu3+ electronic configuration. Studies have been carried out in the literature to observe the effect of charge compensation on photoluminescence properties of silicates. Alkali metals (Li+, Na+, K+, Cl−) and other halogens are used as charge compensators. The Li+ ion is known to be a good charge compensator because of its small size and such studies are well documented. Keeping this in view, we wanted to carry out such compensation using alkali metals but also curious to understand, the way a combination of alkali pair for such compensation process and the effect of charge compensator on the PL properties of this phosphor. We specifically chose the alkali pair, where charges are same but their size and mass ratio being vastly different. Europium doped calcium silicate (Ca2SiO4: Eu3+) has been prepared with charge compensators using alkali metals such as Li, Rb and Li-Rb. To start with, β-dicalcium silicate (Ca2SiO4) has been prepared by solution combustion technique in the muffle furnace at a temperature of 500 °C. Diformyl hydrazine (DFH) was used as a fuel. In the second step, we synthesized Ca2SiO4 mixed with Eu3+ and their mixture with required alkali metals by solid state reaction followed by calcinations at 900ºC for 2 h. Subsequently obtained β-Ca2SiO4: Eu3+ phosphor powders were characterized by powder X-ray diffraction (PXRD), Scanning electron microscopy (SEM), Fourier transform Infra-Red (FTIR) spectroscopy. PXRD analysis confirmed the monoclinic phase with P2/m space group. However, SEM observation revealed agglomerated wafer and stony cluster morphology. The PL spectra of Ca2SiO4: Eu3+ was carried out with varying concentration of Eu3+. The samples with 5 mol% showed maximum PL intensity peak compared to other concentrations and spectra revealed PL at 588, 611, 620, 648, 685 and 701 nm. The same procedure was adapted for β-Ca2SiO4: Eu3+, Li where Li act as a charge compensator. Surprisingly a new intense luminescent peak is observed at 574 nm and 588 nm, besides the intense red luminescent peak. The Hyper Sensitive Transition (HST) peaks around 612 nm and 621 nm have got their intensity pattern inverted indicating the effect of charge compensation on the Eu3+ environment which is a result of crystallographic effects and splitting of energy levels with changed transition probability.