Tuning of Photoluminescence and Local Structures of Substituted Cations in <i>x</i>Sr₂Ca(PO₄)₂–(1 – <i>x</i>)Ca₁₀Li(PO₄)₇:Eu²⁺ Phosphors

Local structure modification in solid solution is an essential part of photoluminescence tuning of rare earth doped solid state phosphors. Herein we report a new solid solution phosphor of Eu²⁺-doped <i>x</i>Sr₂Ca­(PO₄)₂–(1 – <i>x</i>)­Ca₁₀Li­(PO₄)₇ (0 ≤ <i>x</i> ≤ 1), which share the same β-Ca₃(PO₄)₂ type structure in the full composition range. Depending on the <i>x</i> parameter variation in <i>x</i>Sr₂Ca­(PO₄)₂–(1 – <i>x</i>)­Ca₁₀Li­(PO₄)₇:Eu²⁺, the vacancies generated in the M(4) site enable the nonlinear variation of cell parameters and volume, and this increases the magnitude of M(4)­O₆ polyhedra distortion. The local structure modulation around the Eu²⁺ ions causes different luminescent behaviors of the two-peak emission and induces the photoluminescence tuning. The shift of the emission peaks in the solid solution phosphors with different compositions has been discussed. It remains invariable at <i>x</i> ≤ 0.5, but the red-shift is observed at <i>x</i> > 0.5 which is attributed to combined effect of the crystal field splitting, Stokes shift, and energy transfer between Eu²⁺ ions. The temperature-dependent luminescence measurements are also performed, and it is shown that the photoionization process is responsible for the quenching effect

Temperature and Eu²⁺-Doping Induced Phase Selection in NaAlSiO₄ Polymorphs and the Controlled Yellow/Blue Emission

The union of temperature-dependent phase transition and relating structural transformation via modification of chemical compositions is of fundamental importance for the discovery of new materials or their functional properties optimization. Herein, the synthesis temperature and Eu²⁺-doping content induced phase selection and variations of the local structures in nepheline, low-carnegieite and high-carnegieite types of NaAlSiO₄ polymorphs were studied in detail. The luminescence of Eu²⁺ in low-carnegieite and nepheline phases shows blue (460 nm) and yellow (540 nm) broad-band emissions, respectively, under near-ultraviolet excitation. The photoluminescence evolution can be triggered by the different synthesis temperatures in relation to the Eu²⁺-doping concentration, as corroborated by density functional theory calculations on the local coordination structures and corresponding mechanical stabilities in terms of the Debye temperature. The fabricated white light-emitting diode device with high color rendering index demonstrates that the multicolor phosphors from one system provides a new gateway for the photoluminescence tuning

Hydrates [Na₂(H₂O)_x](2-thiobarbiturate)₂ (<i>x</i> = 3, 4, 5): crystal structure, spectroscopic and thermal properties

<p>The hydrates [Na₂(H₂O)₃(Htba)₂] (<b>1</b>) and [Na₂(H₂O)₄(Htba)₂] (<b>2</b>), where H₂tba is 2-thiobarbituric acid, were obtained under different thermal conditions from aqueous solutions and were structurally characterized. The molecular and supramolecular structures were compared to the known structure of [Na₂(H₂O)₅(Htba)₂] (<b>3</b>). In polymeric <b>1</b>–<b>3</b>, the Htba⁻ ions are linked to Na⁺ through O and S forming octahedra. The decrease of the number of coordination water molecules led to an increase of the total number of bridge ligands (μ₂-H₂O, Htba⁻) and a change of the Htba⁻ coordination. These factors induced higher distortion of the octahedra. It was assumed that hydrates, with a different number of coordinated water molecules, are more probable when the central metal has weaker bonds with O water molecules and with other ligands. The net topologies of <b>1</b>–<b>3</b> were compared. Thermal decomposition and IR spectra were analyzed for <b>1</b> and <b>2</b>.</p

Eu²⁺ Site Preferences in the Mixed Cation K₂BaCa(PO₄)₂ and Thermally Stable Luminescence

Site preferences of dopant Eu²⁺ on the locations of K⁺, Ba²⁺, and Ca²⁺ in the mixed cation phosphate K₂BaCa­(PO₄)₂ (KBCP) are quantitatively analyzed via a combined experimental and theoretical method to develop a blue-emitting phosphor with thermally stable luminescence. Eu²⁺ ions are located at K2 (M2) and K3 (M3) sites of KBCP, with the latter occupation relatively more stable than the former, corresponding to emissions at 438 and 465 nm, respectively. KBCP:Eu²⁺ phosphor exhibits highly thermal stable luminescence even up to 200 °C, which is interpreted as due to a balance between thermal ionization and recombination of Eu²⁺ 5d excited-state centers with the involvement of electrons trapped at crystal defect levels. Our results can initiate more exploration of activator site engineering in phosphors and therefore allow predictive control of photoluminescence tuning and thermally stable luminescence for emerging applications in white LEDs

Eu²⁺ Site Preferences in the Mixed Cation K₂BaCa(PO₄)₂ and Thermally Stable Luminescence

Site preferences of dopant Eu²⁺ on the locations of K⁺, Ba²⁺, and Ca²⁺ in the mixed cation phosphate K₂BaCa­(PO₄)₂ (KBCP) are quantitatively analyzed via a combined experimental and theoretical method to develop a blue-emitting phosphor with thermally stable luminescence. Eu²⁺ ions are located at K2 (M2) and K3 (M3) sites of KBCP, with the latter occupation relatively more stable than the former, corresponding to emissions at 438 and 465 nm, respectively. KBCP:Eu²⁺ phosphor exhibits highly thermal stable luminescence even up to 200 °C, which is interpreted as due to a balance between thermal ionization and recombination of Eu²⁺ 5d excited-state centers with the involvement of electrons trapped at crystal defect levels. Our results can initiate more exploration of activator site engineering in phosphors and therefore allow predictive control of photoluminescence tuning and thermally stable luminescence for emerging applications in white LEDs

Light-Induced Charge Transfer to Achieve Deep-Red Emission in SrSc₂O₄:Bi toward Multiple Optical Applications

Bismuth (Bi) is used for luminescent materials due to its unique optical performance, but deep-red light from Bi-doped materials is rarely reported. In particular, establishing a design principle for Bi-doped red materials is considered to be a significant challenge. Herein, using a deep-red SrSc2O4:Bi material featuring Bi–Bi pair emission, light-induced charge-transfer from BiSc3+–BiSr3+ to BiSc4+–BiSr2+ enables the realization of Bi2+2P3/2(1) → 2S1/2 deep-red emission. Intriguingly, SrSc2O4:Bi displays an excellent zero-thermal-quenching performance from 298 to 423 K, with a peak intensity that retains 98% of the intensity at 298 K and an integrated intensity at 423 K that even reaches 110% of the initial intensity. The intriguing spectroscopic characteristics of SrSc2O4:Bi make it a promising candidate in the agricultural field, night-vision security, and the medical treatment area. This work advances the understanding of red luminescence in Bi-activated luminescent materials and thus can initiate more exploitation of red materials for emerging applications

Discovery of New Solid Solution Phosphors via Cation Substitution-Dependent Phase Transition in M₃(PO₄)₂:Eu²⁺ (M = Ca/Sr/Ba) Quasi-Binary Sets

The cation substitution-dependent phase transition was used as a strategy to discover new solid solution phosphors and to efficiently tune the luminescence property of divalent europium (Eu²⁺) in the M₃(PO₄)₂:Eu²⁺ (M = Ca/Sr/Ba) quasi-binary sets. Several new phosphors including the greenish-white SrCa₂(PO₄)₂:Eu²⁺, the yellow Sr₂Ca­(PO₄)₂:Eu²⁺, and the cyan Ba₂Ca­(PO₄)₂:Eu²⁺ were reported, and the drastic red shift of the emission toward the phase transition point was discussed. Different behavior of luminescence evolution in response to structural variation was verified among the three M₃(PO₄)₂:Eu²⁺ joins. Sr₃(PO₄)₂ and Ba₃(PO₄)₂ form a continuous isostructural solid solution set in which Eu²⁺ exhibits a similar symmetric narrow-band blue emission centered at 416 nm, whereas Sr²⁺ substituting Ca²⁺ in Ca₃(PO₄)₂ induces a composition-dependent phase transition and the peaking emission gets red shifted to 527 nm approaching the phase transition point. In the Ca_3–<i>x</i>Ba_<i>x</i>(PO₄)₂:Eu²⁺ set, the validity of crystallochemical design of phosphor between the phase transition boundary was further verified. This cation substitution strategy may assist in developing new phosphors with controllably tuned optical properties based on the phase transition

New Yellow-Emitting Whitlockite-type Structure Sr_1.75Ca_1.25­(PO₄)₂:Eu²⁺ Phosphor for Near-UV Pumped White Light-Emitting Devices

New compound discovery is of interest in the field of inorganic solid-state chemistry. In this work, a whitlockite-type structure Sr_1.75Ca_1.25­(PO₄)₂ newly found by composition design in the Sr₃(PO₄)₂–Ca₃(PO₄)₂ join was reported. Crystal structure and luminescence properties of Sr_1.75Ca_1.25­(PO₄)₂:Eu²⁺ were investigated, and the yellow-emitting phosphor was further employed in fabricating near-ultraviolet-pumped white light-emitting diodes (w-LEDs). The structure and crystallographic site occupancy of Eu²⁺ in the host were identified via X-ray powder diffraction refinement using Rietveld method. The Sr_1.75Ca_1.25(PO₄)₂:Eu²⁺ phosphors absorb in the UV–vis spectral region of 250–430 nm and exhibit an intense asymmetric broadband emission peaking at 518 nm under λ_ex = 365 nm which is ascribed to the 5d–4f allowed transition of Eu²⁺. The luminescence properties and mechanism are also investigated as a function of Eu²⁺ concentration. A white LED device which is obtained by combining a 370 nm UV chip with commercial blue phosphor and the present yellow phosphor has been fabricated and exhibit good application properties