Vacuum Referred Binding Energy Scheme, Electron–Vibrational Interaction, and Energy Transfer Dynamics in BaMg₂Si₂O₇:Ln (Ce³⁺, Eu²⁺) Phosphors

The host structure and the synchrotron radiation VUV–UV luminescence properties of samples BaMg₂Si₂O₇ (BMSO):Ln (Ce³⁺, Eu²⁺) at different doping levels and different temperatures were investigated in detail. Three important aspects are studied to elucidate the luminescence properties of samples: (1) the vacuum referred binding energy (VRBE) scheme is constructed with the electron binding in the BMSO host bands and in the Ce³⁺ and Eu²⁺ impurity levels with the aim to explain the different thermal stabilities of Ce³⁺ and Eu²⁺ emissions; (2) the electron–vibrational interaction analysis on the narrow Eu²⁺ emission indicates a weak electron–phonon interaction in the current case; (3) by using three models (Inokuti–Hirayama, Yokota–Tanimoto, and Burshteĭn models) at different conditions, the energy transfer dynamics between Ce³⁺ and Eu²⁺ was analyzed. It reveals that the energy transfer from Ce³⁺ to Eu²⁺ via electric dipole–dipole (EDD) interaction is dominant while energy migration between Ce³⁺ is negligible. Finally, the X-ray excited luminescence spectra of samples BMSO:Ce³⁺/Eu²⁺ are collected to evaluate their possible scintillator applications

Optical Properties of Ce-Doped Li₄SrCa(SiO₄)₂: A Combined Experimental and Theoretical Study

Investigation of optical properties of Ce³⁺-activated phosphors is not only of practical importance for various applications but also of fundamental importance for providing a basis to understand relevant properties of other lanthanide ions in the same host. We report herein a combined experimental and theoretical study of optical properties of Ce³⁺ in Li₄SrCa­(SiO₄)₂. Photoluminescence properties of the material prepared by a solid-state reaction method are investigated with excitation energies in the vacuum-ultraviolet (VUV) to ultraviolet (UV) range at low temperatures. The band maxima in the excitation spectra are assigned with respect to 4f → 5d transitions of Ce³⁺ at the Sr and Ca sites, from comparison between experimental and <i>ab initio</i> predicted transition energies. As a result of the two-site occupation, the material displays luminescence at 300–500 nm with a high thermal quenching temperature (>500 K), consistent with the calculated large gaps (∼1.40 eV) between the emitting 5d levels and the bottom of the host conduction band. On the basis of experimental and calculated results for Ce³⁺ in Li₄SrCa­(SiO₄)₂, the energy-level diagram for the 4f ground states and the lowest 5d states of all trivalent and divalent lanthanide ions at the Sr and Ca sites of the same host is constructed and discussed in association with experimental findings

The Effect of Sr²⁺ on Luminescence of Ce³⁺-Doped (Ca,Sr)₂Al₂SiO₇

A series of Ce³⁺-doped (Ca,Sr)₂Al₂SiO₇ phosphors with different Ce³⁺ and Ca²⁺/Sr²⁺ concentrations were prepared by a high temperature solid-state reaction technique. To get insight into the structure–luminescence relationship, the impact of incorporation of Sr²⁺ on structure of (Ca,Sr)₂Al₂SiO₇ was first investigated via Rietveld refinement of high quality X-ray diffraction (XRD) data, and then the VUV–UV excitation and UV–vis emission spectra of (Ca,Sr)₂Al₂SiO₇:Ce³⁺ were collected at low temperature. The results reveal that the crystal structure evolution of (Ca,Sr)₂Al₂SiO₇:Ce³⁺ has influences on band gaps and Ce³⁺ luminescence properties including 4f–5d_<i>i</i> (<i>i</i> = 1–5) transition energies, radiative lifetime, emission intensity, quantum efficiency, and thermal stability. Moreover, the influence of Sr²⁺ content on the energy of Eu³⁺–O^2– charge-transfer states (CTS) in (Ca,Sr)₂Al₂SiO₇:Eu³⁺ was studied in order to construct vacuum referred binding energy (VRBE) schemes with the aim to further understand the luminescence properties of (Ca,Sr)₂Al₂SiO₇:Ce³⁺. Finally, X-ray excited luminescence (XEL) spectra were measured to evaluate the possibility of (Ca,Sr)₂Al₂SiO₇:Ce³⁺ as a scintillation material

Site Occupation of Eu²⁺ in Ba_2–<i>x</i>Sr_<i>x</i>SiO₄ (<i>x</i> = 0–1.9) and Origin of Improved Luminescence Thermal Stability in the Intermediate Composition

Knowledge of site occupation of activators in phosphors is of essential importance for understanding and tailoring their luminescence properties by modifying the host composition. Relative site preference of Eu²⁺ for the two distinct types of alkaline earth (AE) sites in Ba_{1.9995–<i>x</i>}Sr_<i>x</i>Eu_0.0005SiO₄ (<i>x</i> = 0–1.9) is investigated based on photoluminescence measurements at low temperature. We found that Eu²⁺ prefers to be at the 9-coordinated AE2 site at <i>x</i> = 0, 0.5, and 1.0, while at <i>x</i> = 1.5 and 1.9, it also occupies the 10-coordinated AE1 site with comparable preference, which is verified by density functional theory (DFT) calculations. Moreover, by combining low-temperature measurements of the heat capacity, the host band gap, and the Eu²⁺ 4f⁷ ground level position, the improved thermal stability of Eu²⁺ luminescence in the intermediate composition (<i>x</i> = 1.0) is interpreted as due to an enlarged energy gap between the emitting 5d level and the bottom of the host conduction band (CB), which results in a decreased nonradiative probability of thermal ionization of the 5d electron into the host CB. Radioluminescence properties of the samples under X-ray excitation are finally evaluated, suggesting a great potential scintillator application of the compound in the intermediate composition