Experimental and theoretical approach to account for green luminescence from Gd2Zr2O7 pyrochlore: exploring the site occupancy and origin of host-dopant energy transfer in Gd2Zr2O7:Eu3+

Abstract

Pure and Eu³⁺-doped Gd₂Zr₂O₇ pyrochlore was synthesized using a gel combustion method using citric acid as a fuel. Samples of Gd₂Zr₂O₇ pyrochlore were characterized systematically using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS) and time-resolved photoluminescence spectroscopy (TRPLS). On irradiating the undoped Gd₂Zr₂O₇ pyrochlore with Ultraviolet (UV) light, an intense green emission was observed. Photoluminescence lifetime measurements and X-ray photoelectron spectroscopy (XPS) showed the presence of oxygen vacancies in the pyrochlore sample, which were responsible for the intense green emission in Gd₂Zr₂O₇. Calculations based on density functional theory (DFT+U) of the electronic density of states in the presence of charged oxygen defects qualitatively explained the origin of the green emission in undoped Gd₂Zr₂O₇. The emission spectrum of Eu³⁺ revealed that it was distributed at both Gd³⁺ and Zr⁴⁺ sites in Gd₂Zr₂O₇, which was also confirmed using lifetime measurements. DFT-based calculations of cohesive energy also showed that doping with Eu is almost equally favorable at Gd and Zr sites. Based on DFT calculations, it is proposed that the distribution of f and d states of Eu³⁺ atoms matches well with the total density of states (DOS) of ordered Gd₂Zr₂O₇ (o-Gd₂Zr₂O₇), which signifies efficient transfer of photon energy from the host to the Eu³⁺ dopant. The actual site symmetry of europium ions in gadolinium zirconate was also determined based on the Stark splitting pattern and was found to be D_2d, although it is O_h for Gd³⁺ in Gd₂Zr₂O₇. Calculations of the Judd–Ofelt parameters revealed that Ω₂ > Ω₄, which indicates high covalency and low symmetry around Eu³⁺, which is in agreement with the results of emission spectroscopy. The high intensity of the red emission corresponding to the ⁵D₀ → ⁷F₂ transition and good fluorescence yields (51%) highlight the unexplored potential of Gd₂Zr₂O₇:Eu³⁺ as a promising red phosphor.