Highly efficient and thermally stable Eu3+ activated phosphate based phosphors for wLEDs: An experimental and DFT study

Abstract

Europium ions-activated alkali- alkaline phosphate phosphors with sharp red-light emission were designed via compositional ternary phase diagram for the first time. A number of polyphosphate phosphors including Eu3+ activated NaCa(PO3)3 were prepared via solid state reaction route. The synthesized phosphors exhibited narrow red light emissions around 612 nm initiating from Eu3+ f-f transitions. Corresponding excitation spectra of Eu3+ ions in the present host displayed characteristics f-f transition along with a charge transfer (CT) band. Origin of CT band was clarified utilizing band structure and density of states (DOS) analysis via Density functional theory (DFT). Experimentally calculated band gap was obtained as 5.59 eV and verified through band structure analysis. The concentration quenching mechanism was explored by varying the Eu3+ concentration, revealing 4 mol% as the optimum concentration and the quenching was controlled via dipole–dipole interaction. High thermal stability (~90.17%) was correlated with structural rigidity related to bond length distortion index evaluated using DFT. The Internal quantum yield (IQY) of prepared phosphors was obtained as ~86%. Additionally, Judd-Ofelt (JO) theory was utilized to assess local surrounding environment nearby europium ion in NaCa(PO3)3 host. In addition, near ultraviolet (NUV) chip was coated with the NaCa0.96Eu0.04(PO3)3 and commercial blue and green-light-emitting phosphors to fabricate the white light emitting diode (wLED). The fabricated LED showed high color-rendering-index (CRI) of 85.1 and suitable correlated-color-temperature (CCT) of 5661 K. The results suggested that the Eu3+ activated NaCa(PO3)3 phosphors were suitable for white LEDs. ? 2021