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The spectrum consists of a broad and strong band in the 220–300 nm range with a maximum at 270 nm due to host absorption (the O 2 À –Nb 5+ charge transfer band), 23 indicating
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The photoluminescence (PL) properties of two Dy3+ doped ternary niobates, LaNbO4 and CaNb2O6 with fergusonite and columbite structures, respectively are investigated and compared. When the emission line corresponding to electric dipole transition of Dy3+ at 574 nm is monitored, an intense band due to host excitation is observed in both LaNbO4:Dy3+C...
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In order to evaluate the quantum efficiency of the cooperative energy transfer (CET) process in the Bi³⁺–Yb³⁺ co-doped Gd2O3 phosphor, direct measurements of external quantum yield (QY) were performed along with studies of photoluminescence (PL), photoluminescence excitation (PLE) and photoluminescence decay kinetics. The absolute QY measurements r...
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In this work, Dy3+-doped SrNb2O6 phosphors were fabricated by the molten salt process, which avoids high sintering temperatures, prolonged reaction time and poor compositional homogeneity. All samples crystallized to the orthorhombic columbite structure with space group, P21/c, while a rod-like morphology was observed by scanning electron microscopy (SEM). PL (photoluminescence) and RL
(radioluminescence) spectra of SrNb2O6:Dy3+ exhibited a strong blue emission peak at 576.0 nm related to 4F9/2 - 6H15/2 transition of Dy. The high RL emission of the 4F9/2 - 6H15/2 (electric dipole) transition
upon X-ray-induced excitation led to a decrease in Dy3+ local environmental symmetry. The Judd–Ofelt (J–O) theory was applied to the PL excitation spectra for the calculation of optical data such as Q2, Q4, and Q6 parameters, radiative transition probability (Ar), branching ratios (b, bexp) and stimulated emission cross-section (se). The quantum efficiencies (nQE) varied between 35.47 and 31.93%, which are compatible with theoretical quantum efficiencies based on the Einstein relation. The CIE chromaticity coordinates (x, y) and CCT (correlated color temperature) parameter for 3 mol% phosphor were defined (0.385, 0.432) and 4209 K, respectively.
A new series of single phase pyrochlore based white phosphors, CaLa1−xSnNbO7:xDy³⁺ (x = 0.01, 0.02, 0.03, 0.04) and Ca1−ySryLa0.97SnNbO7:0.03Dy³⁺ (y = 0.1, 0.2, 0.3) were developed by a high temperature ceramic route. The phase purity and the crystalline structure of the as-prepared samples were characterised by powder X-ray diffraction method and the phosphors were found to crystallize into the cubic pyrochlore type structure. The photoluminescence properties were investigated and the results indicated that phosphors show a strong excitation levels in near UV (388 nm) and blue (452 nm) regions and emits intense complimentary blue (487 nm and 477 nm) and yellow (589 nm) light upon UV or blue excitation which can be assigned to the characteristic ⁴F9/2 → ⁶HJ (J = 13/2 and 15/2) transitions of Dy³⁺ respectively. Thus the suitable combination of the above complimentary colors, yellow and blue is an approach to realize a full color emission and the Commission Internationale de I’Eclairage (CIE) color coordinates of the resultant phosphors lie in the white light region (0.38, 0.36) close to the standard chromaticity coordinates (white light) of NTSC. The substitution of bigger ion like Sr²⁺ for Ca²⁺ enhanced significantly the photoluminescence properties by way of their excitation and emission intensities due to increased polarizability and distortion of the Dy³⁺ environment. These results demonstrate that CaLa1−xSnNbO7:xDy³⁺ and Ca1−ySryLaSnNbO7:0.03Dy³⁺ are promising single phased white phosphor for pc-WLED applications.
Ionic current in proton-conducting polycrystalline ceramics is often hampered a great deal by the grain boundaries, limiting their prospective applications as solid electrolytes for next-generation solid oxide fuel cells. To elucidate the conduction mechanism at the grain boundaries, we use a linear diffusion model and impedance spectroscopy to report complete current-voltage (I-V) characteristics of the grain boundaries in 0.5 mol % Sr-doped LaNbO 4 . We provide the first experimental evidence of complete annihilation of the space charge-induced proton depletion at the grain boundaries upon applying moderate bias voltages. We also show that it is possible to distinguish between the grain boundary resistance caused by the space charge and other sources by analyzing the I-V characteristics. The analysis is equally valid for other solid ionic conductors such that it can serve as an important tool to guide further optimization of the conductivity of polycrystalline solid electrolytes.
A 2-D dysprosium glutarate [Dy2(glu)3(phen)2]n (1, glu = glutarate, phen = 1,10-phenanthroline) has been synthesized from the hydrothermal reaction of DyCl3, glutaric acid and phen ligand. Compound 1 displays a classical 2-D (4,4) grid structure constructed by the linkages of Dy³⁺ ions and three types of glutarate ligands. Compound 1 shows slow magnetic relaxation and luminescent properties. Although some lanthanide glutarates have been reported, they do not exhibit slow magnetic relaxation properties. Compound 1 offers the only example of a dysprosium coordination polymer with field-induced SMMs based on aliphatic dicarboxylic acids in the absence of ancillary aromatic carboxylic acids.
CaY1-xEuxTiNbO7 compositions with pyrochlore structure were synthesized by solid state reaction technique and photoluminescence was studied. Photoluminescence emission results reveal that the ⁵D0→⁷F1 magnetic dipole transition has higher intensity indicating that Eu³⁺ occupies a centrosymmetric site in the host lattice. The critical concentration of Eu³⁺ was found to be x=0.5. The CaY0.5Eu0.5TiNbO7 phase shows higher orange-red emission when compared to that of commercial red phosphor, Y2O3:Eu³⁺.
La1−xNbO4:xDy3+ phosphors were synthesized through sol–gel route. XRD analysis confirms pure fergusonite monoclinic structure of La1−xNbO4:xDy3+ phosphor. The FTIR and Raman spectra show the vibrational modes present in La1−xNbO4:xDy3+ phosphor. The surface morphology and internal microstructure of the prepared phosphor were analysed by scanning electron microscopy and transmission electron microscopy and its microstructure is useful for better luminescence efficiency. UV–VIS-NIR spectrum of La1−xNbO4:xDy3+ phosphor was analysed on the basis of Judd–Ofelt theory and the J–O intensity parameters (Ωλ) were calculated. The radiative properties such as radiative transition probability, branching ratio, stimulated emission cross-section and optical gain were investigated using J–O intensity parameters. Energy transfer process between host and Dy3+ ions was studied at host excitation of 251 nm and tunable color emission from blue to near white was obtained with increasing concentration of Dy3+ ion. Excitation into 4f levels of Dy3+ ions at 351 nm show cool white light emission and the corresponding decay time, color coordinates and color correlation temperature were calculated. The results indicate that the prepared phosphor produce white light emission from a single phase host and can be used for near ultraviolet pumped white light emitting diode (NUV WLED) applications.
A single-phase white-light emitting phosphor LuNbO4:Dy³⁺ was synthesized using the solid state method in air for the first time. X-ray diffraction (XRD) along with excitation spectra, emission spectra, decay times and thermal stability were exploited to characterize the asprepared phosphors. Under ultraviolet (UV) excitation (261 nm), the self-activated emission of the LuNbO4 host is peaked at 402 nm with a broad emission band ranging from 320 nm to 600 nm, ascribing to the charge transfer in NbO4³⁻ groups, which has spectral overlapping to the excitation of f-f transitions of Dy³⁺ in LuNbO4:Dy³⁺ phosphors. They show both the broad host emission and sharp emission lines due to the characteristic f-f transitions of Dy³⁺ ions, which exhibit tunable white light emissions due to the energy transfer from the NbO4³⁻ groups in the host to Dy³⁺ with increased Dy³⁺ content. The optimal chromaticity coordinates and Correlated Color Temperature (CCT) in LuNbO4:Dy³⁺ are (x= 0.336, y=0.311) and 5299 K, respectively, which occur when the doping Dy³⁺ is 0.01. The decrease of decay lifetime for host emission in LuNbO4:Dy³⁺ with raised Dy³⁺ content demonstrates the energy transfer from the host to Dy³⁺. The energy transfer mechanism in LuNbO4:Dy³⁺ phosphors was determined to be a resonant type via dipole-dipole mechanism. Moreover, good thermal stability was also identified in Lu0.99NbO4:0.01Dy³⁺ and its emission intensity was reduce to 85% of its initial value at 100 °C and 62% at 200 °C, the chromaticity color coordinate values of Lu0.99NbO4:0.01Dy³⁺ had a slight shift with raised temperature. The current research suggests that LuNbO4:Dy³⁺ could potentially serve as a single-phase white-light emitting phosphor in solid-state lighting and display fields.
A series of Dy3+/Tm3+ singly-doped, Dy3+/Tm3+ co-doped, and Dy3+/Tm3+/Eu3+ tri-doped InNbO4 phosphors have been synthesized by a high-temperature solid-state reaction method. Powder X-ray diffraction (XRD), structure refinement, UV-visible diffuse reflectance (UV-DR) spectrum, photoluminescence excitation (PLE) and emission (PL) spectrum, decay lifetimes and CIE chromaticity coordinate were employed to understand the origins of luminescence properties for synthesized phosphors. Rare-earth activators were determined to be completely dissolved into the host lattice and occupied the 2e sites deviated from an inversion center by virtue of XRD, structure refinement and photoluminescence features. The photoluminescence excitation spectra of phosphors exhibited a broad excitation band ascribed to charge-transfer of O2--Nb5+, ranging from 200 to 300 nm, and some characteristic excitation peaks associated with energy level transition of RE ions. Under the excitation of UV light, phosphors presented characteristic emission originating from f-f transition within the 4f configuration of RE ions. With increasing Dy3+ concentration, emission colors of InNbO4:Tm3+, Dy3+ phosphors can be appropriately tuned from blue to yellow, including almost all the white light region, by means of energy transfer from Tm3+ to Dy3+. Energy transfer rates and efficiencies were calculated according to decay lifetimes and energy transfer mechanism followed a resonant type dipole-dipole interaction with critical distance of 22.23 angstrom between Tm3+ and Dy3+. By compensating a red component of Eu3+ ion, single-component warm-white-light emitting was realized in InNbO4:Tm3+,Dy3+,Eu3+ phosphors. They might be promising candidates for color-tunable light-conversion components in the fabrication of ultraviolet-pumped warm-white-light emitting diodes (WLEDs) for solid-state lighting. Relationship of crystal structures and luminescence properties for InNbO4 and YNbO4 phosphors is as well explicated based on structural analysis.
Single crystal GdNbO4:Ln3+ (Ln = Dy, Eu) phosphors were prepared via a high-temperature high-pressure hydrothermal procedure at 650 °C under autogenous pressure. X-ray diffraction, field emission scanning electron microscopy, photoluminescence, Raman and XPS were utilized to characterize the as-synthesized phosphors. XRD reveals that the samples begin to crystallize at 550 °C and a pure GdNbO4 phase can be obtained at 650 °C. FE-SEM images indicate that GdNbO4:Ln3+ (Ln = Dy, Eu) samples consist of fine sheets with a size of 50–100 μm. Under UV excitation, the GdNbO4:Eu3+ and GdNbO4:Dy3+ phosphors showed the characteristic emissions of Eu3+ are 5D0 → 7FJ (J = 0, 1, 2, 3, 4), and Dy3+ (4F9/2 → 6H15/2 and 4F9/2 → 6H13/2 transitions), respectively. The preparation method presented here dramatically lowered the traditional temperature of 2300 °C or above in the Czochralski method. The GdNbO4:Dy3+ single crystals showed a bright white emission under different excitation wavelengths with a relatively high quantum yield of 21.7%. The GdNbO4:0.05Eu3+ exhibited excellent bright red luminescence at 612 nm under near-UV excitation, narrowed emission spectra, room temperature luminescence lifetimes of milliseconds and maximum quantum efficiencies of 43.2%.
