Doped lanthanum cerate pyrochlore for multicolor luminescent phosphor: Decisive role of energy transfer and defects

Abstract

Designing of efficient luminescent materials requires proper understanding of energy transfer, defect evolution and dopant local structure. The present work is a perfect amalgam wherein we have thoroughly investigated the concentration dependent host sensitized energy transfer, defect evolution and local structure of europium ion in La2Ce2O7 (LCO) pyrochlore material. Raman spectroscopy suggested stabilization of defect fluorite structure for LCO as well as LCO:Eu3+ (LCOE) but degree of structural distortion as well as oxygen vacancies (OVs) increases with increase in europium ion concentration. Diffuse reflectance spectroscopy suggested band gap narrowing at higher doping level owing to enhanced density of OVs. Positron annihilation lifetime spectroscopy suggested an increase in the formation of oxygen vacancies and vacancy clusters near the surfaces at higher doping. As a result, LCO, on irradiation with 250 nm ultraviolet photon showed visible emission due to OVs as well charge transfer transition. On Eu3+ doping emission spectra was rich in host as well as europium emission and host to europium energy transfer increases with increase in europium ion concentration. This get's reflected as violet blue emission at lower doping and yellowish white at higher doping level ≥7.5% under host excitation and orange-red emission under 471 nm dopant excitation. Europium ion emission spectral profiles in LCOE suggested stabilization of Eu3+ in CeO6 octahedra as is also confirmed using lifetime spectroscopy. Judd-Ofelt measurement further reflected higher branching ratio for 5D0→7F2, internal quantum yield ~48% and high Ω2 compared to Ω4. This work highlights the importance of local dopant site, HSET, excitation photon and defects in designing color tunable luminescent materials and is expected to play an important role in designing the phosphor converted light emitting diodes (pc-LEDs).