Luminescence from the EU³⁺ ion in D_4d symmetry

Eu³⁺ ions are extensively employed as spectroscopic probes because of the particular emissions due to the unique 4f⁶ electronic configuration. Herein, we have developed a series of interesting fluorescent powders of Eu³⁺ activated Sr₂MNbO₆ (M = Ga, In, Gd). When Sr₂GaNbO₆:Eu³⁺ is excited by the photons in the range of charge transfer band (CTB), the dark red ⁵D₀→⁷F₄ transition predominates the emission, exhibiting an anomalous intensity even higher than that of ⁵D₀→⁷F₁ or ⁵D₀→⁷F₂. Whilst, if the sample is pumped by f-f characteristic transitions, the hypersensitive ⁵D₀→⁷F₂ transition emerges as the strongest. However, Eu³⁺ doped Sr₂InNbO₆ and Sr₂GdNbO₆ never demonstrate such results, ⁵D₀→⁷F₁ acts as the most intense one regardless of the excitation wavelength. It is found that Eu³⁺ substitutes Sr²⁺ site, and cuboctaherdron [SrO₁₂] with O_h symmetry is likely to deform to triangular orthobicupola D_3h symmetry, leading to two non-equivalent crystal sites in Sr₂GaNbO₆ host. But Eu³⁺ replaces In³⁺ and Gd³⁺ position with O_h symmetry and has only one site in Sr₂InNbO₆ and Sr₂GdNbO₆. Based on the comparative study on the intensity of ⁵D₀→⁷F₀ transition, as well as the influence factors of temperature, the doping concentration on the relative intensity among ⁵D₀→⁷F_J transitions, and the band gaps of these three hosts, the origin of the abnormal intense ⁵D₀→⁷F₄ is ascribed to the mixing of CTB and low-lying 4f⁶ configuration as well as two non-equivalent Eu³⁺ sites.

Temperature is a fundamental physical quantity whose precise measurement is critical in research and technology. Temperature detection based on the optical response of the luminescence materials has piqued interest because it can overcome the limitations of classical contact thermometers and meet the requirements of non-contact thermometers that are not addressed by conventional contact thermometers. In this review, we looked at the most recent and significant advances made using various temperature sensing technologies based on the luminescence intensity ratio of thermally coupled energy levels of lanthanide ions. Temperature-dependent decay time, optical thermometry via temperature-induced spectral tuning of the characteristics charge transfer band (CTB) edge of lanthanides doped rare earth vanadate (Re: gadolinium [Gd], lutetium [Lu], yttrium [Y]) ReVO₄ (Re = Gd, Lu, Y) phosphors, and a temperature sensing method based on thermally populated low-lying energy levels of ions of samarium, europium, erbium, yttrium, holmium, thulium, or terbium (Sm³⁺, Sm²⁺, Eu³⁺, Er³⁺, Y³⁺, Ho³⁺, Tm³⁺, and Tb³⁺) doped phosphors have all been thoroughly reviewed. The physical mechanism, thermoluminescence characteristics, temperature detection range, and sensitivity of the materials are discussed in detail. The advantages and disadvantages of each approach have been compared. Finally, a further research direction based on temperature sensing has been suggested.

A novel effort has been given to transform the incipient ferroelectrics behavior to real in SrTiO₃ and to generate white-light emission by doping luminescent Eu³⁺ and Tb³⁺ ions. Ferroelectric study showed that upon doping Eu³⁺ ion, SrTiO₃ became pure ferroelectric in nature and upon co-doping Tb³⁺ ion there is a significant enhancement of the saturation polarization. Photoluminescence study showed that both the Eu³⁺ doped and Eu³⁺& Tb³⁺ co-doped compounds are white-light-emitting materials due to perfect combination of blue light originated from defect centers present in the host with the green and red emissions from the dopant ions. Such white-light-emitting materials with controlled piezo potential may not need external excitation energy and have potential application in many flexible optoelectronic devices. Lifetime study confirmed the existence of two components for both the dopant ions and suggested that there is an asymmetric environment created due to distortion of geometry, which is linked to ferroelectric property. Finally, all the experimental results were supported by Hybrid functional based theoretical calculation. We have observed that the bond angle variance (BAV) parameter and distortion index (DI), which are directly linked with the distortion in the matrix, are responsible for ferroelectric behavior and they follow the order with the dopant ions as Eu³⁺&Tb³⁺>Tb³⁺>Eu³⁺. This corroborates well with the experimentally observed trend of the saturation polarization.

A novel approach of defect engineering in Eu³⁺ doped MgF₂ compounds has been carried out in order to tune the optical properties of Eu³⁺ ion. Upon doping various co-dopant ions such as Li⁺ and Zr⁴⁺ ions, it has been observed for the first time in the photoluminescence (PL) study that such co-dopant ion would control the characteristics transition lines of Eu³⁺ ions and results in tunable color characteristics of the phosphor materials. While Li⁺ co-doping results in a higher intense magnetic dipole (MD) line (⁵D₀ → ⁷F₁), doping with Zr⁴⁺ results in an intense electric dipole (ED) line (⁵D₀ → ⁷F₂). Further, Li⁺ co-doping also results in an unusual highly intense ⁵D₀ → ⁷F₄ line. From PL decay profile it has been observed that although for both cases of the co-dopant ions two different Eu³⁺ components are found to exist, their contribution is completely different. From positron annihilation lifetime study it was observed that various Mg-vacancies and their cluster forms are responsible for the tunable emission characteristics. First principle based theoretical study has also been carried out to understand the configuration of various cluster vacancies as suggested by their respective positron lifetime values. The short-lived Eu³⁺ component, which gives rise to highly intense ⁵D₀ → ⁷F₄ line, originates from a Mg-site at the surface of the particle and close to a vacancy cluster VC1. While the long lived Eu³⁺ component is responsible for highly intense ED line and close to Mg mono- or di-vacancies in the bulk of the particle. Therefore, defects (both dopant impurity ions and lattice vacancies) play a significant role in tuning the emission characteristics of Eu³⁺ doped MgF₂ compounds.

The novel red-emitting Ca₉MY_0.667(PO₄)₇ (M = Li, Na) (CMYPO):xEu³⁺ (x = 0.02, 0.04, 0.07, 0.12, 0.16, 0.26, 0.35, and 0.44) phosphors were successfully synthesized by a high-temperature solid-phase reaction. The samples were evaluated by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), fourier transform infrared (FT-IR) spectra, ultraviolet–visible diffuse reflectance (UV–Vis DR) spectra, photoluminescence excitation (PLE) and emission (PL) spectra. It is found that the samples can be excited by near-ultraviolet (NUV) and blue light. The intense PL peaks were centered at 614 nm due to the ⁵D₀ → ⁷F₂ transition. Interestingly, ⁵D₀ → ⁷F₄ (700 nm) far-red emission matches with the far-red absorption wavelength required for plant photosensitive pigments red (P_R) and far-red (P_FR) light. Moreover, the PL intensity of Ca₉NaY_0·667(PO₄)₇ (CNYPO):Eu³⁺ is higher than that of Ca₉LiY_0·667(PO₄)₇ (CLYPO):Eu³⁺. In addition, the band gaps of CLYPO and CNYPO were determined to be 3.81 and 3.61 eV, respectively. Finally, the International Commission on Illumination (CIE) chromaticity of phosphors are located in red region. The results showed that the phosphors are considered as the potential luminescence materials in white light emitting diodes (w-LEDs). Impressively, the as-prepared samples can provide reference for the development of Eu³⁺-doped phosphors that can be used for plant growth LEDs.

Two unique lanthanide coordination polymers, namely, {[Ln₂(μ₃-OH)₂(Tfpht)₂ (H₂O)₂·0.5bipy]}_n (Ln = Gd (1) and Eu (2), H₂Tfpht = 3,4,5,6-Tetrafluorophthalic acid, bipy = 4,4′-bipyridine) have been afforded successfully. The coordination polymers 1 and 2 are isostructural and have the same phase, both crystallizing in the triclinic system, with space group of P₋₁. They both exhibit one-dimensional (1-D) ribbon-double chains composed of trinuclear lanthanide clusters bridged by μ₃-hydroxyl bridges and carboxyl oxygen atoms of Tfpht²⁻ ligands, and the chains are further linked by hydrogen bonding interactions to give rise to a 2-D corrugated layer. The solid state luminescence emission spectra of coordination polymer 1 is based on the H₂Tfpht acid ligand, whereas 2 displays the characteristic f-f transitions of Eu(III) ions. The direct-current (dc) magnetic susceptibility studies reveal the weak ferromagnetic interaction between the Gd(III) ions in coordination polymer 1.