Color tuning in (Ca1−xSrx)8MgLu(PO4)7:Eu2+,Mn2+ phosphor via host composition design and energy transfer

In this work, a series of Eu²⁺-doped (Ca_1−xSr_x)₈MgLu(PO₄)₇ and Eu²⁺/Mn²⁺-codoped Ca_6.5Sr_1.5MgLu(PO₄)₇ phosphors were prepared via the combustion-assisted solid-state reaction process. XRD patterns and Rietveld refinements were used to verify the incorporations of Sr into Ca₈MgLu(PO₄)₇:Eu²⁺. Upon the same excitation wavelength of 380 nm, the emission peaks of Eu²⁺-doped (Ca_1−xSr_x)₈MgLu(PO₄)₇ (0≤x≤1) phosphors red-shifted from 453 to 519 nm with increasing Sr/Ca ratio. The red-shift of the Eu²⁺ emission with increasing Sr/Ca ratio was ascribed to the change of Eu²⁺ emission at different lattice sites. With variation of the Mn²⁺ content, the emission color of Eu²⁺/Mn²⁺ codoped Ca_6.5Sr_1.5MgLu(PO₄)₇ phosphors exhibited the luminescence tunable from greenish blue to white and eventually to red. The energy transfer from Eu²⁺ to Mn²⁺ in Ca_6.5Sr_1.5MgLu(PO₄)₇ host matrix was demonstrated to be of a resonant type via a dipole- dipole mechanism with the critical distance of ∼16.7 Å. By the Sr substitution for Ca and properly tuning by the relative composition change of Eu²⁺/Mn²⁺, chromaticity coordinates of (0.329, 0.326) can be reached at near UV light excitation. The combination of host composition design and energy transfer may provide a novel strategy to obtain white light and tunable luminescence.     

Luminescence enhancement of single-component Ca19Zn2(PO4)14:Dy3+ white-emitting phosphor powders through partial substitution of PO43− with SiO44− and BO33-

A series of Ca₁₉Zn₂(PO₄)_14-yAG_y:Dy³⁺ (AG = BO₃^3?, SiO₄^4?) white-emitting phosphors with partial PO₄^3? substitution were synthesized through high-temperature solid-state reaction. The BO₃^3? and SiO₄^4? anionic groups were introduced into Ca₁₉Zn₂(PO₄)₁₄:Dy³⁺ for enhancing the luminescence of Dy³⁺. The X-ray diffraction results exhibited that all samples were assigned to the standard trigonal Ca₁₉Zn₂(PO₄)₁₄ structure with R3c (161) space group. The white emission spectrum of Dy³⁺-doped phosphors was composed of several characteristic peaks located at 484 nm, 575 nm and 665 nm, corresponding to ⁴F_9/2 → ⁶H_15/2, ⁶H_13/2 and ⁶H_11/2 electron transitions, respectively. After partial SiO₄^4? and BO₃^3? substituting PO₄^3?, the luminescence intensity of Ca_18.62Zn₂(PO₄)_13.75(SiO₄)_0.25:0.38Dy³⁺ and Ca_18.62Zn₂(PO₄)_13.65(BO₃)_0.35:0.38Dy³⁺ samples were higher by 1.68 and 1.49 times than that of Ca_18.62Zn₂(PO₄)₁₄:0.38Dy³⁺ sample, respectively, due to the charge compensation and crystal field environment effect. The actual w-LEDs fabricated with as-prepared Ca_18.62Zn₂(PO₄)_13.75(SiO₄)_0.25:0.38Dy³⁺ and Ca_18.62Zn₂(PO₄)_13.65(BO₃)_0.35:0.38Dy³⁺ phosphors showed excellent optical performances. All results indicated that the single component Ca_18.62Zn₂(PO₄)_14-yAG_y:0.38Dy³⁺ white-emitting phosphors could have a potential application in w-LEDs.     

Crystal structure and luminescence properties of novel Sr10−x(SiO4)3(SO4)3O:xEu2+ phosphor with apatite structure

In this paper, a series of novel luminescent Sr_10−x(SiO₄)₃(SO₄)₃O:xEu²⁺ phosphors with apatite structure were synthesized by a high temperature solid-state reaction. The phase structure, photoluminescence (PL) properties, as well as the PL thermal stability were investigated. Sr_9.92(SiO₄)₃(SO₄)₃O:0.08Eu²⁺ phosphor exhibits better thermal quenching resistance, retaining the luminance of 66.55% at 150 °C compared with that at 25 °C. The quenching concentration of Eu²⁺ in Sr₁₀(SiO₄)₃(SO₄)₃O was about 0.08 (mol) with the dipole–quadrupole interaction. The Sr_10−x(SiO₄)₃(SO₄)₃O:xEu²⁺ phosphors exhibited a broad-band green emission at 538 nm upon excitation at 396 nm. The results indicate that Sr_10−x(SiO₄)₃(SO₄)₃O:xEu²⁺ phosphors have potential applications as near UV-convertible phosphors for white-light UV LEDs.     

Photoluminescence and LED application of β-SiAlON:Eu2+ green phosphor

We successfully prepared β-SiAlON:Eu²⁺ phosphors with composition of Eu_xSi_6?zAl_zO_yN_8?y (y = z ? 2x, x = 0.018, z = 0.23) by gas-pressured synthesis for application to LED. The crystal phase, microstructure, PL emission and thermal quenching properties were investigated in detail. The β-SiAlON:Eu²⁺ phosphors absorbed broad UV–vis spectral region, and showed a single intense broadband emission near 538 nm. The Stokes shift and zero-phonon line were calculated mathematically, and also estimated from the spectral data. The β-SiAlON:Eu²⁺ green phosphor showed superior thermal quenching properties compared to commercial silicate (SrBaSiO₄:Eu²⁺) green phosphor. The white light-emitting diode (LED) using the prepared β-SiAlON:Eu²⁺ green phosphor exhibited high color gamut, and good optical stability in high working temperature.     

A single-phase white-emitting La10(SiO4)6O3：Eu2+/Eu3+ phosphor for near-UV LED-based application

A novel single-phase white-emitting phosphor La₁₀(SiO₄)₆O₃ (LSO): xEu has been synthesized by high-temperature solid-state reaction. Its crystal structure, luminescence properties, fluorescence decay time and oxygen vacancies have been characterized by X-ray diffraction (XRD) and photoluminescence (PL) spectra. XRD result shows a typical oxyapatite structure with the space group of P6₃/m. Characteristic excitation and emission peaks of Eu²⁺ and Eu³⁺ were observed from PL studies. The optimum doping concentration of Eu was found to be 7.5 mol% (x = 0.075). In this work, the lifetimes of Eu³⁺ and Eu²⁺ were considerably longer than those from some references. Under the excitation of different near ultraviolet (n-UV) longer wavelengths (λex = 360, 370, and 380 nm), the white light emission can be realized with the CIE chromaticity coordinates (0.3907, 0.3595), (0.3472, 0.3282), and (0.3504, 0.3062) for the phosphor LSO: 0.075Eu. The chromaticity coordinates of the phosphor were all located in the white region. Therefore, it is suggested that the explored LSO: 0.075Eu phosphor can be a good candidate for white light-emitting diodes (W-LEDs) application.     

Influence of Bi2O3 flux in the structural and photoluminescence properties of SrAl2O4:Eu2+ phosphors

SrAl₂O₄:Eu²⁺ phosphors with various content of Bi₂O₃ flux were synthesized and analyzed. It was observed that the crystallinity and the particle size of the phosphors were increased with the addition of Bi₂O₃ flux. These phenomena are considered to be caused via the melting of the Bi₂O₃ flux particles during the synthesis of the phosphors. The melted Bi₂O₃ flux increased the mobility and homogeneity of solid reactants, thereby enhancing the photoluminescence intensity of the phosphors. SrAl₂O₄:Eu²⁺ phosphors with Bi₂O₃ as the flux exhibited a broad green emission with a peak at 520 nm. The highest photoluminescence emission intensity was observed when 5 mol% Bi₂O₃ flux was added into the phosphors. The emission is due to 4f⁶5d→4f⁷ (⁸S_7/2) transitions of the Eu²⁺ ions. Moreover, Bi₂O₃ flux extended the application of the ultraviolet excited phosphors toward the blue-light excited phosphors. Nevertheless, the influence of Bi₂O₃ on the afterglow and the emission color of SrAl₂O₄:Eu²⁺ phosphors were not significant. This research indicated that Bi₂O₃ flux is effective flux for synthesizing SrAl₂O₄:Eu²⁺ phosphors.     

Novel Mn4+ doped Ca2GdSbO6 red–emitting phosphor: A potential color converter for light-emitting diodes

As potential color converter towards white/red light–emitting diodes, novel Ca₂GdSbO₆:Mn⁴⁺ phosphors with excellent optical performances were prepared by a conventional solid–state reaction route. The as–prepared phosphors with monoclinic crystal system had abundant [SbO₆] octahedrons for Mn⁴⁺ ions to occupy and stably exist. With an excitation of 356 nm, an intense red emission peaking at 676 nm attributed to ²E_g→⁴A_2g transition of Mn⁴⁺ ions can be observed in the emission spectrum. The critical concentration of Mn⁴⁺ ions was found to be 0.6 mol% and the concentration quenching mechanism was also discussed in detail. Importantly, the Ca₂GdSbO₆:0.6%Mn⁴⁺ phosphors exhibited a high internal quantum efficiency of 38.9%.     

Effect of Sr/Ca substitution on phase structure and photoluminescence properties of micro-SrxCa1−xAlSiN3: Eu2+ phosphor for high CRI white LEDs

Sr_xCa_1−xAlSiN₃: Eu²⁺ phosphors were prepared by using the high temperature solid state reaction in a 1.1 Mpa N₂ atmosphere. The phase structures, photoluminescence (PL) properties, and chromaticity properties of the phosphors affected by Sr/Ca Substitution have been investigated in detail. With increasing Sr content (x value), the crystal grain became bigger and the average grain size increased from 5 µm to 10 µm. PL emission bands of Sr_xCa_1−xAlSiN₃: Eu²⁺ showed a blue-shift from 660 (x=0) to 617 nm (x=0.8), the shoulder of the excitation spectra around 550 nm showed a slightly blue-shift and decay lifetime shortened from 776.96 (x=0.2) to 642.35 ns (x=0.8). Both the emission and excitation intensity of peak position increased with Sr content increased. The ideal white light with high CRI (Ra>88) can be obtained by mixing the Sr_xCa_1−xAlSiN₃: Eu²⁺ phosphors and commercial green phosphors with appropriate proportion of the components.     

Deep-red emission of Mn4+ and Cr3+ in (Li1−xAx)2MgTiO4 (A=Na and K) phosphor: Potential application as W-LED and compact spectrometer

Mn⁴⁺ doped and Mn⁴⁺/Cr³⁺ co-doped alkali metal titanate phosphors have been prepared by solid state reaction method. A part of Li⁺ ions in the Li₂MgTiO₄: Mn⁴⁺ are substituted with Na⁺ and K⁺ ions and consequently the intensity of Mn⁴⁺ emission at 678 nm is enhanced by 1.7 and 2.5 times, respectively. In the Mn⁴⁺/Cr³⁺ co-doped (Li_0.95K_0.05)₂MgTi_0.999O₄, both emission of Cr³⁺at 726 nm and emission of Mn⁴⁺ at 678 nm of Mn⁴⁺ are observed. It is interesting to find that the intensity ratio of 726–678 nm emissions in the Mn⁴⁺/Cr³⁺ phosphor continually increases with excitation wavelength increasing from 290 nm to 455 nm, which means that the intensity ratio in turn can be used to identify the excitation light wavelength. This refers a possible approach to design novel compact light-wavelength detector or spectrometer based on the phosphor. The mechanism of Na⁺ or K⁺ substitution induced luminescence enhancement in the Mn⁴⁺ phosphor and the competition between the Cr³⁺ and Mn⁴⁺ emissions in the Mn⁴⁺/Cr³⁺ co-doped has been discussed.     

Significantly improved photoluminescence of the green-emitting β-sialon:Eu2+ phosphor via surface coating of TiO2

The β-sialon:Eu²⁺ phosphor particles were successfully coated by TiO₂ nanoparticles via the sol-gel method. The TiO₂-coated β-sialon:Eu²⁺ phosphor had a significantly improved photoluminescence (PL) performance under the 365 nm excitation, due to the localized surface plasmon resonance (LSPR) at the interface between the TiO₂ coating layer and phosphor surface. The emission intensity of the TiO₂-coated β-sialon:Eu²⁺ prepared with the titanium (IV) tetrabutoxide (Ti(OC₄H₉)₄, TTBO):H₂O = 1:0.5 volume ratio was dramatically increased by ~24%. When the preparation temperature was 500°C, it was responsible for superior PL intensity by considering the important domination factors of higher anatase content and spherical particle shape of the TiO₂ coating layer to the LSPR effect. The coating around the phosphor surface by the TiO₂ nanoparticles would be an effective technique to improve the PL efficiency of phosphor for the application in the white light-emitting diodes (LEDs), by utilizing the LSPR effect of the semiconductor coating layer, instead of conventional metal plasmonic materials.     

Phase and melting relationships of β, α and α′-Ca3(PO4)2 polymorphs in the Ca3(PO4)2-Zn3(PO4)2 system

In order to provide an exact knowledge of the phase transitions and melting relationships of Ca₃(PO₄)₂ (TCP) in the presence of zinc, a revisited version of the rich-Ca₃(PO₄)₂ region of the phase diagram of the system Ca₃(PO₄)₂-Zn₃(PO₄)₂ has been established in the present work. Experimental determination of this diagram was carried out by solid-state reactions of samples prepared from pure NH₄H₂PO₄, CaCO₃ and ZnO raw materials. X-ray Diffraction, Differential Thermal Analyses and Field Emission Scanning Electron Microscopy studies allowed to revise the α, β, α + β-TCP phase stability fields, delimitating for the first time the biphasic α + α′-TCP field and the melting relationships in the high temperature region of the system. The results allowed to determine two peritectic invariant points, at ≈1400 °C for 95 mol% Ca₃(PO₄)₂ and at ≈1490 °C for ≈99.5 mol% Ca₃(PO₄)₂.     

Ionic conductivity of apatite-type solid electrolyte ceramics Ca2−xBaxLa4Bi4(SiO4)6O2 (0≤x≤2)

A complex impedance of oxyapatites Ca_2?xBa_xLa₄Bi₄(SiO₄)₆O₂ (0≤x≤2) prepared by solid state reaction has been investigated. The formation of apatites has been checked by X-ray diffraction, FTIR, Raman and ²⁹Si MAS-NMR techniques. The electric impedance data indicate that relaxation phenomena are strongly dependent on temperature in the 923–1048 K range. The bulk resistance decreases with increasing temperature, showing a typical negative temperature coefficient of resistance (NTCR). ac-Conductivity measurements have been performed on a wide range of frequencies and temperatures. The complex modulus plots have confirmed the presence of bulk contributions. The complex impedance analysis suggests the presence of non-Debye relaxations that would be associated with correlation on ions motion.     

Charge states of atoms in ceramic superconductors HgBa2Ca n − 1Cu n O2n + 2, Tl2Ba2Ca n − 1Cu n O2n + 4, and Bi2Sr2Ca n − 1Cu n O2n + 4 (n = 1–3)

The parameters of the electric-field-gradient tensor for copper sites in the HgBa₂Ca _{n ? 1}Cu _n O_{2n + 2}, Tl₂Ba₂Ca _{n ? 1}Cu _n O_{2n + 4}, and Bi₂Sr₂Ca _{n ? 1}Cu _n O_{2n + 4} (n = 1–3) lattices have been determined using ⁶⁷Cu(⁶⁷Zn) Mössbauer emission spectroscopy and calculated in the framework of the point-charge approximation. The agreement between the experimental and calculated parameters has been achieved under the assumption that the holes formed as a result of the decrease in the oxidation state of a part of the mercury, thallium, or bismuth atoms are distributed over the oxygen sites in the Cu-O or adjacent planes. It has been demonstrated that the oxidation state of cations can be controlled in high-temperature superconducting ceramic materials.     

Colossal dielectric permittivity and electrical properties of the grain boundary of Ca1−3x/2YbxCu3−yMgyTi4O12 (x=0.05, y=0.05 and 0.30)

Dielectric properties of Ca_1−3x/2Yb_xCu_3−yMg_yTi₄O₁₂ (x=0.05, y=0.05 and 0.30) prepared using a modified sol–gel method and sintered at 1070 °C for 4 h were investigated. The mean grain sizes of the CaCu₃Ti₄O₁₂ and co-doped Ca_0.925Yb_0.05Cu_3−yMg_yTi₄O₁₂ (y=0.05 and 0.30) ceramics were ≈15.86, ≈3.37, and ≈2.32 μm, respectively. Interestingly, the dielectric properties can be effectively improved by co-doping with Yb³⁺ and Mg²⁺ ions to simultaneously control the microstructure and properties of grain boundaries, respectively. These properties were improved over those of single-doped and un-doped CaCu₃Ti₄O₁₂ ceramics. A highly frequency−independent colossal dielectric permittivity (≈10⁴) in the range of 10²–10⁶ Hz with very low loss tangent values of 0.018–0.028 at 1 kHz were successfully achieved in the co-doped Ca_0.925Yb_0.05Cu_3−yMg_yTi₄O₁₂ ceramics. Furthermore, the temperature stability of the colossal dielectric response of Ca_1−3x/2Yb_xCu_3−yMg_yTi₄O₁₂ was also improved to values of less than ±15% in the temperature range from −70 to 100 °C.     

Effect of codoping Ce3+ on the luminescence properties of Sr9Mg1.5(PO4)7:Eu2+ orange–yellow phosphor

Sr₉Mg_1.5(PO₄)₇:Eu²⁺ has recently been reported as a promising blue light-excited orange–yellow phosphor that can be used in white LED device. Here, Ce³⁺-codoping is found to be an effective strategy to improve the luminescence performance of Sr₉Mg_1.5(PO₄)₇:Eu²⁺ phosphor. The coexistence of Eu²⁺ and Eu³⁺ ions has been verified via photoluminescence spectral analysis. The reduction of Eu³⁺ to Eu²⁺ in Sr₉Mg_1.5(PO₄)₇ lattice cannot be completed in a reducing atmosphere, but can be promoted through codoping with Ce³⁺ ions to a great extent, which finally increase the effective concentration of Eu²⁺ in the crystal lattice. The Eu³⁺−Eu²⁺ reduction mechanism is analyzed using a charge compensation model. This work not only achieves enhanced luminescence of the Sr₉Mg_1.5(PO₄)₇:Eu²⁺ phosphor by codoping with Ce³⁺ ions, but also provides new insights into the design of Ce³⁺/Eu²⁺ codoped luminescent materials.     

Structure and photoluminescence properties of Ca0.99−xSrxAlSiN3:0.01Ce3+ solid solutions

Nitride phosphors of Ca_0.99−xSr_xAl_1.01Si_0.99N₃:0.01Ce³⁺ (0 ≤ x ≤ 0.9) were synthesized by conventional solid-state method. XRD data analysis shows that all samples are single phase with CaAlSiN₃-type structure. Under blue or near-ultraviolet (~400 nm) light excitation, the emission peak can be tuned largely from 615 to 568 nm by increasing Sr content, and the emission intensity is maximal at x = 0.8. With the Sr content increase, the emission band blue-shifts due to the decreased crystal field splitting and the reduced centroid shift; while the thermal luminescence quenching resistance is almost unchanged. The quenching temperature (T₅₀) is well above 500 K for all samples, which satisfies the requirement of commercial application. The quenching process is mainly attributed to the radiationless transitions by thermally activated crossover from the 5d excited state to 4f ground state in the configurational coordinate diagram. The luminescence properties show that the (Ca,Sr)AlSiN₃:Ce³⁺ phosphors are very promising for use in blue and near-ultraviolet light excited white-light-emitting diodes.     

Oxide ionic conductivity of apatite type Nd9·33(SiO4)6O2 single crystal

Single crystal of hexagonal apatite type Nd_9·33(SiO₄)₆O₂ which is an oxide ionic conductor was prepared by the FZ method and an anisotropy of its conductivity was investigated. The conductivity of a parallel component to a c-axis (2·1×10⁻⁸ S cm⁻¹ at 30°C) was higher about one order of magnitude, compared with that of perpendicular component.     

Ca3(VO4)2:Eu3+的燃烧合成及光致发光

以稀土氧化铕、偏钒酸铵、硝酸钙为原料,以尿素作燃烧剂,采用燃烧法合成了Ca3(VO4)2:Eu3+.利用XRD,SEM,荧光分光光度计对其结构、形貌和发光性能进行了表征,并探讨其实际应用的可能性.结果表明,燃烧法合成的稀土Eu3+掺杂ca3(VO4)2荧光粉颗粒规则、均一,发射主峰位于614.0 nm,是现有PDP商品(Y,Gd)BO3:Eu3+红色荧光粉的良好替代品.     

Synthesis of a novel organic–inorganic hybrid flame retardant based on Ca(H2PO4)2 and hexachlorocyclotriphosphazene and its performance in polyvinyl alcohol

To optimize the poor thermal stability and flammable of polyvinyl alcohol (PVA), a novel environmental-friendly organic–inorganic hybrid flame retardant Ca(H₂PO₄)₂@HCCP was successfully designed and synthesized via surface treatment technology and used to advance the flame retardancy of PVA. The thermogravimetric analysis implied that Ca(H₂PO₄)₂@HCCP can enhance significantly the thermal stability and char forming ability of PVA. Combustion results demonstrate that Ca(H₂PO₄)₂@HCCP could effectively suppress the melt dripping of PVA in the process of combustion. The presence of Ca(H₂PO₄)₂@HCCP can sharply reduce peak heat release rate and the total heat release up to 75% and 22.9%, respectively, in the microscale combustion calorimeter measurement. The results manifested that Ca(H₂PO₄)₂@HCCP could endow PVA with superior flame retardancy. Moreover, char residues analysis explained the flame retardant mechanism in condensed and gas phase, which is mainly attributed to the strong catalytic char formation, free radical trapping, and gas barrier effect. Therefore, the green flame retardant has great applications prospect in fire safety.