Dependence of Luminous Performance on Eu3+ Site Occupation in SrIn2(P2O7)2: The Effect of the Local Environment

Color tunability, concentration dependence, and temperature responsive afterglow of SrZn2(PO4)2: Eu2+, Mn2+ phosphors

Long persistent luminescence (LPL) materials with color adjustable afterglow have a wide application prospect in display and information encryption, yet there are few reports on such materials. In this paper, SrZn2(PO4)2 phosphors with tunable and thermal stimuli-responsive afterglow emissions were synthesized by a solid-state method. After ultraviolet (UV) excitation, the prepared samples exhibited emission at 419 nm, 533 nm, and 633 nm due to Eu2+: 4f65d1 → 4f7 transitions, Mn2+: 4T2(4G) →6A1(6S) transitions, and Mn2+: 4T1(4G) → 6A1(6S) transitions, respectively. The Commission Internationale de I' Eclairage (CIE) coordinates of samples can be changed among blue, white, and yellow by varying the Mn2+ concentration. Additionally, due to the differing thermal stability of Eu2+ and Mn2+, the phosphor exhibited a temperature-responsive afterglow, and its colors can be adjusted between white and yellow over a temperature range from 293 K to 383 K. Given its distinctive temperature-responsive afterglow, the prepared phosphor is suitable for use as a temperature sensor. Moreover, the tunable afterglow colors of the phosphors are advantageous for application in anti-counterfeiting.

Suppressed concentration quenching and tunable photoluminescence in Eu2+-activated Rb3Y(PO4)2 phosphors for full-spectrum lighting

Highly efficient inorganic phosphors are desirable for lighting-emitting diode light sources, and increasing the doping concentration of activators is a common approach for enhancing the photoluminescence quantum yield (PLQY). However, the constraint of concentration quenching poses a great challenge for improving the PLQY. Herein, we propose a fundamental design principle by separating activators and prolonging their distance in Eu2+-activated Rb3Y(PO4)2 phosphors to inhibit concentration quenching, in which different quenching rates are controlled by the Eu distribution at various crystallographic sites. The blue-violet-emitting Rb3Y(PO4)2:xEu (x = 0.1%-15%) phosphors, with the occupation of Rb1, Rb2 and Y sites by Eu2+, exhibit rapid luminescence quenching with optimum external PLQY of 10% due to multi-channel energy migration. Interestingly, as the Eu concentration increases above 20%, Eu2+ prefer to occupy the Rb1 and Y sites with separated polyhedra and large interionic distances, resulting in green emission with suppressed concentration quenching, achieving an improved external PLQY of 41%. Our study provides a unique design perspective for elevating the efficiency of Eu2+-activated phosphors toward high-performance inorganic luminescent materials for full-spectrum lighting.

A brief review of characteristic luminescence properties of Eu3+ in mixed-anion compounds

Trivalent europium (Eu3+) ions show red luminescence with sharp spectral lines owing to the intraconfigurational 4f-4f transitions. Because of their characteristic luminescence properties, various Eu3+-doped inorganic compounds have been developed to meet the demands of optoelectronic devices. Regardless of shielding by the outer 5s and 5p orbitals, the properties of the Eu3+:4f-4f transition depend on the local environment, such as the shapes of the coordination polyhedra, site symmetry, nephelauxetic effects, crystal field effects, and bonding character. Mixed-anion coordination, where multiple types of anions surround a single Eu3+ ion, can directly affect the optical properties of Eu3+. We review the luminescence properties of Eu3+ ions in mixed-anion compounds of the oxynitride YSiO2N and oxyhalides YOX (X = Cl or Br). Oxynitride and oxyhalide coordination results in characteristic transition probabilities and branching ratios of the 5D0 → 7F0-6 transitions due to distorted structural environments and red-shifted charge transfer excitation bands due to an upward shift of the valence band. The expected and experimentally observed features of Eu3+ luminescence in mixed-anion compounds are outlined based on band and Judd-Ofelt theories. Future applications of the intense red luminescence at ∼620 nm under near-ultraviolet light illumination in Eu3+-doped mixed-anion compounds are introduced, and material design guidelines for new functional Eu3+-doped phosphors are presented.

Evidence and Practical Applications of Site Occupancy Theory (SOT) of Eu3+ in Scheelite Compounds

The determination of the site occupancy of activators in phosphors is essential for precise synthesis, understanding the relationship between their luminescence properties and crystal structure, and tailoring their properties by modifying the host composition. Herein, one simple method was proposed to help determine the sites at which the doping of rare earth ions or transition metal ions occupies in the host lattice through site occupancy theory (SOT) for ions doped into the matrix lattice. SOT was established based on the fact that doping ions preferentially occupy the sites with the lowest bonding energy deviations. In order to provide detailed experimental evidence to prove the feasibility of SOT, several scheelite-type compounds were successfully synthesized using a high-temperature solid-phase method. When Eu3+ ions occupy a similar surrounding environment site, the photoluminescence spectra of the activators Eu3+ are similar. Therefore, by comparing the intensity ratio of photoluminescence spectra and the mechanism of all transitions of KEu(WO4)2, KY(WO4)2:Eu3+, Na5Eu(WO4)4, and Na5Y(WO4)4:Eu3+, it was proved that SOT can successfully confirm the site occupation when doped ions enter the matrix lattice. SOT was further applied to the sites occupied by Eu3+ ion-doped LiAl(MoO4)2 and LiLu(MoO4)2.

Preparation and Luminescence Properties of CaMoO4 :Eu3+ /Sm3+ phosphors

Eu³⁺ and/or Sm³⁺ doped CaMoO₄ phosphors were prepared by hydrothermal method. The XRD results show that the sample is a tetragonal CaMoO₄ phase, the space group is I41/a (88), the lattice constants are a = b = 5.226 Å, c = 11.430 Å, and V = 312.2 ų . We explored the effects of Eu³⁺ doping concentration, reaction temperature, preparation time, and the energy transfer relationship between Eu³⁺ and Sm³⁺ on the phosphors. The CIE calculation results indicated that under the excitation of ultraviolet light at 283 nm, the CIE coordinates of some CaMoO₄ :Eu³⁺ /Sm³⁺ materials are located in the near-white light region, which can be used as potential candidates for single-matrix white phosphors.