Excitation-Dependent Photoluminescence of BaZrO3:Eu3+ Crystals

The elucidation of local structure, excitation-dependent spectroscopy, and defect engineering in lanthanide ion-doped phosphors was a focal point of research. In this work, we have studied Eu³⁺-doped BaZrO₃ (BZOE) submicron crystals that were synthesized by a molten salt method. The BZOE crystals show orange-red emission tunability under the host and dopant excitations at 279 nm and 395 nm, respectively, and the difference is determined in terms of the asymmetry ratio, Stark splitting, and intensity of the uncommon ⁵D₀ → ⁷F₀ transition. These distinct spectral features remain unaltered under different excitations for the BZOE crystals with Eu³⁺ concentrations of 0-10.0%. The 2.0% Eu³⁺-doped BZOE crystals display the best optical performance in terms of excitation/emission intensity, lifetime, and quantum yield. The X-ray absorption near the edge structure spectral data suggest europium, barium, and zirconium ions to be stabilized in +3, +2, and +4 oxidation states, respectively. The extended X-ray absorption fine structure spectral analysis confirms that, below 2.0% doping, the Eu³⁺ ions occupy the six-coordinated Zr⁴⁺ sites. This work gives complete information about the BZOE phosphor in terms of the dopant oxidation state, the local structure, the excitation-dependent photoluminescence (PL), the concentration-dependent PL, and the origin of PL. Such a complete photophysical analysis opens up a new pathway in perovskite research in the area of phosphors and scintillators with tunable properties.

Controlling the luminescence in K2Th(PO4)2:Eu3+ by energy transfer and excitation photon: a multicolor emitting phosphor

This work highlighted green, red, and white light emission from a single K₂Th(PO₄)₂ compound consisting of actinide and an alkali ion through defect, doping, excitation, and energy transfer manipulation.

Ultraviolet emission and electron spin characteristics of Th(C2O4)2·xH2O:Gd3+

UV emitting Gd³⁺ doped phosphors have recently attracted significant attention among materials scientists owing to their important applications in the areas of photothermal therapy, transilluminators and sensitizer based luminescent phosphors.

Thermally Induced Disorder-Order Phase Transition of Gd2Hf2O7:Eu3+ Nanoparticles and Its Implication on Photo- and Radioluminescence

Crystal structure has a strong influence on the luminescence properties of lanthanide-doped materials. In this work, we have investigated the thermally induced structural transition in Gd₂Hf₂O₇ (GHO) using Eu³⁺ ions as the spectroscopic probe. It was found that complete phase transition from the disordered fluorite phase (DFP) to the ordered pyrochlore phase (OPP) can be achieved in GHO with the increase of annealing temperature from 650 → 1100 → 1300 °C. OPP is the more stable structural form for the GHOE nanoparticles (NPs) annealed at a higher temperature based on the energy calculation by density functional theory (DFT). The asymmetry ratio of the GHOE-650 NPs was the highest, whereas the quantum yield, luminescence intensity, and lifetime values of the GHOE-1300 NPs were the highest. Emission intensity of Eu³⁺ ions increases significantly with the phase transition from the DFP to OPP phase and is attributed to the higher radiative transition rate (281 s^-1) of the ⁵D₀ level of the Eu³⁺ ion in the environment with relatively lower symmetry (C _2v ) because of the increase of crystal size. As the structure changes from DFP to OPP, radioluminescence showed tunable color change from red to orange. The Eu³⁺ local structure obtained from DFT calculation confirmed the absence of inversion symmetry in the DFP structure, which is consistent with the experimental emission spectra and Stark components. We also elucidated the host to dopant optical energy transfer through density of states calculations. Overall, our current studies present important observations for the GHOE NPs: (i) thermally induced order-disorder phase transition, (ii) change of point group symmetry around Eu³⁺ ions in the two phases, (iii) high thermal stability, and (iv) tunability of radioluminescent color. This work provides fundamental understanding of the relationship between the crystal structure and photophysical properties of lanthanide-doped materials and helps design a strategy for advanced optoelectronic materials.

Effect of Molten Salt Synthesis Processing Duration on the Photo- and Radioluminescence of UV-, Visible-, and X-ray-Excitable La2Hf2O7:Eu3+ Nanoparticles

Ln³⁺-ion-doped nanomaterials possess excellent properties because of their high color purity, longer excited state lifetime, narrow emission, and large Stokes shifts. In this work, we studied the correlation between the luminescence properties of La₂Hf₂O₇:Eu³⁺ pyrochlore nanoparticles (NPs) synthesized by a molten salt synthesis (MSS) method at a relatively low temperature and several MSS processing durations (from 1 to 12 h). We synthesized these NPs with different sizes just by changing the MSS processing time without subjecting to high temperature. Raman spectroscopy confirmed the stabilization of the ideal pyrochlore structure of the La₂Hf₂O₇:Eu³⁺ NPs at various MSS processing durations. The synthesized NPs exhibited bright red emission under UV, visible, and X-ray excitations, highlighting their potential applications as a red phosphor and scintillator. As the MSS processing time was increased from 1 to 12 h, a spectral change in the position of the charge transfer state in the La₂Hf₂O₇:Eu³⁺ NPs was observed. The sample processed by the MSS with a duration of 3 h exhibited the highest luminescence intensity, which was attributed to its optimum crystals with least surface defects and less agglomeration. The obtained results strongly and unambiguously indicate the brighter side of this new type of pyrochlore-based NPs in the fast growing field of solid-state lighting and scintillator materials.

Role of Synthesis Method on Luminescence Properties of Europium(II, III) Ions in β-Ca2SiO4: Probing Local Site and Structure

The europium ion probes the symmetry disorder in the crystal structure, although the distortion due to charge compensation in the case of aliovalent dopant remains interesting, especially preparation involves low and high temperatures. This work studies the preparation of the β-Ca₂SiO₄ (from here on C₂S) particle from Pechini (C₂SP) and hydrothermal (C₂SH) methods, and its luminescence variance upon doping with Eu²⁺ and Eu³⁺ ions. The blue shift of the charge-transfer band (CTB) in the excitation spectra indicates a larger Eu³⁺-O^2- distance in Eu³⁺ doped C₂SH. The changes in vibrational frequencies due to stretching and bending vibrations in the FTIR and the Raman spectra and binding energy shift in the XPS analysis confirmed the distorted SiO₄^4- tetrahedra in C₂SH. The high hydrothermal temperature and pressure produce distortion, which leads to symmetry lowering although doping of aliovalent ion may slightly change the position of the Ca atoms. The increasing asymmetry ratio value from C₂SP to C₂SH clearly indicates that the europium ion stabilized in a more distorted geometry. It is also supported by Judd-Ofelt analysis. The concentration quenching and site-occupancy of Eu³⁺ ions in two nonequivalent sites of C₂S were discussed. The charge state and concentration of europium ions in C₂SP and C₂SH were determined using X-ray photoelectron spectroscopy measurements. The C₂S particles were studied by X-ray powder diffraction, FTIR, Raman, BET surface area, TGA/DTA, electron microscopy, XPS, and luminescence spectroscopy. The impact of citrate ion on the morphology and particle size of C₂SH has been hypothesized on the basis of the microscopy images. This study provides insights that are needed for further understanding the structure of C₂S and thereby improves the applications in optical and biomedical areas and cement hydration.

Crystal structure of Ba2(La0.727Ba0.182M0.091)MO6 (M = Nb, Sb, Bi): symmetry nuance identified in photoluminescence and IR spectroscopy studies

A one-third lanthanum deficiency was created in Ba₂LaM⁵⁺O₆ compounds (LaM compounds) to form Ba₂La_2/3M⁵⁺O_5.5 compounds (La2/3M compounds) for M = Nb, Sb, and Bi. The compounds were prepared by a gel-combustion method using citric acid as a fuel. All the compounds were characterized by powder X-ray diffraction (XRD). The XRD analysis showed that the space group of the La2/3M compounds remains the same for the Bi and Sb samples when compared to the reported LaM compounds, except for the Nb sample. La2/3Nb and La2/3Sb adopt a rhombohedral structure with the space group R3[combining macron], whereas La2/3Bi adopts a monoclinic structure with the space group I2/m. As the positron annihilation spectroscopy (PALS) technique is sensitive to cation deficiency, it was used to detect the presence of cation vacancies in the samples, which are formed due to the decrease in the lanthanum concentration. The PALS analyses indicated that the absence of cation deficiency in the La2/3M compounds is similar to that observed in the LaM compound. Thus, the crystal structure of the La2/3M compound was modeled, such that the cation deficiency at the La site is filled by Ba²⁺ and M⁵⁺ ions, and the crystal structure formula is given as Ba₂(La_0.727Ba_0.182M_0.091)MO₆. This model was confirmed by Rietveld refinement of the XRD data. The emission spectra of Eu³⁺ showed a strong dependence on its local site symmetry in the host material, in which it is being doped and this can be used as a spectroscopic probe for detecting any differences in the symmetry. Comparison of the local symmetry around La³⁺ cation was studied using photoluminescence (PL) by doping 2 atom% Eu³⁺ in LaM and La2/3M compounds. Infrared spectroscopy (IRS) analyses were also carried out for LaM and La2/3M compounds. There was complete agreement between the PL and IRS results and they were also in concordance with the predicted crystal structure model. Interestingly in these La2/3M compounds, the equilibrium structure prefers large Ba²⁺ ion to occupy the octahedral B-site rather than forming an octahedral vacancy at that site, making these perovskite compounds rare and novel in their class.

Energy transfer dynamics and time resolved photoluminescence in BaWO4:Eu3+ nanophosphors synthesized by mechanical activation

Red purity and high quantum efficiency of BaWO₄:Eu³⁺ projects it as a new red phosphor for application in white LEDs.

Preparation and crystal structure of K2Ce(PO4)2: a new complex phosphate of Ce(iv) having structure with one-dimensional channels

In this manuscript we report crystal structure of a new complex binary phosphate K2Ce(4+)(PO4)2 in K2O-P2O5-CeO2 system prepared by solid state reaction at moderate temperature conditions. The prepared material was characterized by powder X-ray diffraction using lab source and synchrotron radiation as well as thermal analyses, Raman scattering, FTIR, and X-ray photoelectron spectroscopic studies. The crystal structure of the compound has been determined from powder XRD data by ab initio structure solution in direct space followed by Rietveld refinements. K2Ce(PO4)2 crystallizes in a monoclinic (P21/n) lattice with unit cell parameters: a = 9.1060(4), b = 10.8160(5), c = 7.6263(4) Å, β = 111.155(2)°, V = 700.50(6) Å(3). The unit cell contains two distinguishable PO4 tetrahedra and one CeO8 distorted square anti-prism. Raman spectroscopy confirmed the presence of isolated PO4(3-) groups in the structure. These PO4 tetrahedra are connected to one CeO8 polyhedra by sharing one edge and three other CeO8 polyhedra by sharing corners to form the three dimensional structure and empty channels parallel to a-axis. The channels are occupied by two crystallographically distinguishable K(+) ions which maintain the charge neutrality. Contrast to the earlier reported composition K4Ce2P4O15, this study revealed the composition in actual is K4Ce2P4O16 with Ce in 4+ oxidation state and is also supported by X-ray photoelectron spectroscopic and X-ray absorption near edge structure studies. Differential scanning calorimetric studies revealed a structural transition around 525 °C which reverts on cooling with a large thermal hysteresis. At higher temperature it undergoes a loss of oxygen atom and subsequently loss of phosphorus as P2O5. These thermal effects are also supported by in situ high temperature XRD studies. Finally the crystal chemistry of complex phosphates with tetravalent cations is also discussed.

Why host to dopant energy transfer is absent in the MgAl2O4:Eu3+ spinel? And exploring Eu3+ site distribution and local symmetry through its photoluminescence: interplay of experiment and theory

Local site occupancy of Eu³⁺ in combustion synthesized MgAl₂O₄ spinel and host → Eu³⁺ energy transfer dynamics is investigated using photoluminescence and DFT calculations.

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+

Origin of green emission in undoped Gd₂Zr₂O₇ and photophysical characteristics such as local site and energy transfer dynamics of Gd₂Zr₂O₇:Eu³⁺ is investigated using PL and DFT calculations.

Crystal structure of a mixed-ligand terbium(III) coordination polymer containing oxalate and formate ligands, having a three-dimensional fcu topology

The title compound, poly[(μ 3-formato)(μ 4-oxalato)terbium(III)], [Tb(CHO2)(C2O4)] n , is a three-dimensional coordination polymer, and is isotypic with the La(III), Ce(III) and Sm(III) analogues. The asymmetric unit contains one Tb(III) ion, one formate anion (CHO2 (-)) and half of an oxalate anion (C2O4 (2-)), the latter being completed by application of inversion symmetry. The Tb(III) ion is nine-coordinated in a distorted tricapped trigonal-prismatic manner by two chelating carboxyl-ate groups from two C2O4 (2-) ligands, two carboxyl-ate oxygen atoms from another two C2O4 (2-) ligands and three oxygen atoms from three CHO2 (-) ligands, with the Tb-O bond lengths and the O-Tb-O bond angles ranging from 2.4165 (19) to 2.478 (3) Å and 64.53 (6) to 144.49 (4)°, respectively. The CHO2 (-) and C2O4 (2-) anions adopt μ 3-bridging and μ 4-chelating-bridging coordination modes, respectively, linking adjacent Tb(III) ions into a three-dimensional 12-connected fcu topology with point symbol (3(24).4(36).5(6)). The title compound exhibits thermal stability up to 623 K, and also displays strong green photoluminescence in the solid state at room temperature.

Eu3+ local site analysis and emission characteristics of novel Nd2Zr2O7:Eu phosphor: insight into the effect of europium concentration on its photoluminescence properties

New Color tunable Nd₂Zr₂O₇:Eu³⁺ (0–5 mol%) phosphor was synthesized using gel-combustion and its photophysical characteristics such as energy transfer, point group symmetry and Judd–Ofelt parameter were investigated using time resolved photoluminescence.

Multifunctional pure and Eu3+ doped β-Ag2MoO4: photoluminescence, energy transfer dynamics and defect induced properties

The photoluminescence and electrocatalytic properties of pure and europium doped β-Ag₂MoO₄ are explored. The host dopant energy transfer process is explained theoretically.