Blue-light-excitable broadband yellow-emitting CaGd2HfSc(AlO4)3:Ce3+ garnet phosphors for white light-emitting diode devices with improved color rendering index

The current commercial white light-emitting diodes (LEDs) are generally based on the combination of blue LED chips and Y₃Al₅O₁₂:Ce³⁺ yellow phosphors. However, because of the lack of red component, such white LED devices exhibit cool white-light emissions with low color rendering index (Ra < 75, R9 < 0). Therefore, it is urgent to discover new blue-light-excitable yellow-emitting phosphors with enhanced red emissions for fabricating high color-quality white LEDs. In the present work, we demonstrate a novel broadband yellow-emitting CaGd₂HfScAl₃O₁₂:Ce³⁺ garnet phosphor for blue-light-excited white LEDs with improved color rendering index. The as-prepared CaGd₂HfScAl₃O₁₂:Ce³⁺ garnet phosphor possesses a cubic structure with Ia$\overline{3}$d space group, and the unit cell parameters of the representative CaGd₂HfScAl₃O₁₂:2%Ce³⁺ phosphor are a = b = c = 12.450 Å, α = β = γ = 90°, and V = 1,929.59(4) ų. Impressively, we find that the CaGd₂HfScAl₃O₁₂:Ce³⁺ garnet phosphor shows an intense absorption band in the 300–500 nm wavelength range with a maximum at 452 nm owing to the 4f→5d transition of Ce³⁺ ions. On 452 nm excitation, the optimal CaGd₂HfScAl₃O₁₂:2%Ce³⁺ sample exhibits a broad asymmetric yellow emission band in the wavelength range of 470–750 nm with peak at 564 nm and full width at half maximum of 151 nm. The Commission Internationale de l’Eclairage chromaticity coordinates and internal quantum efficiency of the CaGd₂HfScAl₃O₁₂:2%Ce³⁺ sample are (0.4485, 0.5157) and 30.4%, respectively. Finally, a white LED device is fabricated by combing a 450 nm blue LED chip with commercial Y₃Al₅O₁₂:Ce³⁺ yellow-emitting phosphor, which generates white light with low color rendering index (CRI; Ra = 74.7, R9 = ?12.7) and high correlated color temperature (CCT = 6,554 K) under the 60 mA driving current. In sharp contrast, another white LED device, which is made by coating our as-prepared CaGd₂HfScAl₃O₁₂:2%Ce³⁺ yellow-emitting phosphors onto the surface of a 450 nm blue LED chip, produces white-light emission with high CRI value (Ra = 84.5, R9 = 26.3) and relatively low CCT of 5,649 K. This work reveals that the newly discovered broadband yellow-emitting CaGd₂HfScAl₃O₁₂:Ce³⁺ phosphors can serve as a potential color converter in high-color-quality phosphor-converted white LEDs.     

Novel high-saturated red-emitting phosphor Sr9(Mg0.5Mn0.5)K(PO4)7: Eu2+ with great quantum efficiency enhancement by La3+ codoping for white LED application

In this work, a novel whitlockite-structure red-emitting phosphor host, Sr₉(Mg_0.5Mn_0.5)K(PO₄)₇, is designed and successfully synthesized via a solid-state reaction. Upon X-ray diffractometer Rietveld refinement, it is revealed that this compound possesses compact Eu²⁺-Mn²⁺ distance (3.6809 Å) and large intra-Mn²⁺ distance (8.9905 Å), which is beneficial to the high-efficient Eu²⁺-Mn²⁺ energy transfer. By Eu²⁺ sensitization, our new phosphor exhibits a high-saturated and bright red Mn²⁺ emission at 620 nm with high color purity of 97.9%. Great emission enhancement up to 245 times than host is achieved by La³⁺ heterovalent substitution, which can be ascribed to the La³⁺-induced further structural confinement effect. Moreover, the quantum efficiency is boosted by twofold. The as-fabricated white phosphor-converted LEDs device shows bright warm white light with correlated color temperature (CCT) of 3,487 K, color-rendering index (CRI) of 92.4, and luminous efficacy of 31.59 lm/W. This work proves the feasibility of chemical unit co-substitution strategy in emission engineering of Mn²⁺-based phosphors, which can stimulate further studies on the red-emitting phosphor materials.     

LnBaB9O16:Eu3+(Ln=La,Y)的结构与荧光性质

利用电子衍射、X射线衍射和荧光光谱等方法研究了LnBaB₉O₁₆(Ln=La,Y)的结构特性.LnBaB₉O₁₆为单斜晶系,其中LaBaB₉O₁₆的晶胞参数a=1.3660nm,b=0.7882nm,c=1.6253nm,β=106.15°;YBaB₉O₁₆的晶胞参数a=1.3476nm,b=0.7776nm,c=1.6040nm,β=106.38°.荧光光谱研究表明,这两种化合物结构不同,Y³⁺在YBaB₉O₁₆结构中处于中心对称格位,而LaBaB₉O₁₆中La³⁺的格位则无中心对称性.Gd³⁺部分取代LaBaB₉O₁₆:Eu³⁺中的La³⁺可改善Eu³⁺离子的发光性质.LaBaB₉O₁₆:Eu³⁺在真空紫外区的吸收比较弱,这可能与硼氧比较小有关.     

掺三价铈离子四偏磷酸盐NaLn(PO3)4 (Ln=La, Gd)的真空紫外光谱研究

用高温固相法合成了NaLn（PO3）4：1．0％（原子分数）Ce^3＋（Ln=La,Gd）两种粉末发光材料,在合肥国家同步辐射实验室（NSRL）测得两种样品中Ce^3＋的真空紫外光谱。根据真空紫外光谱图,得到了两种稀土偏磷酸盐中所掺Ce^3＋离子5d轨道的能级分裂图。结合NaLa（PO3）4和NaGd（PO3）4的单晶结构数据,发现随着Ln-O平均键长的减小,Ce^3＋离子5d轨道的晶场劈裂（εcfs）、重心位移（εc）和总的红移（total redshilt,D）均增大。根据配体极化模型,计算了两种化合物中掺杂Ce^3＋离子的有效平均键长（Reff）和光谱极化率（αsp）,发现随着平均键长Rav（Ln—O）变短,Ce^3＋离子的光谱极化率（αsp）也随之减小。     

A red-emitting Ca8MgLa(PO4)7:Ce3+,Mn2+ phosphor for UV-based white LEDs application

Ca(8)MgLa(PO(4))(7):Ce(3+),Mn(2+) phosphors have been prepared by a conventional solid state reaction under a weak reductive atmosphere. The crystal structure and photoluminescent properties were investigated. It was found that the red emission at 640nm originated from the (4)T(1)((4)G)→(6)A(1)((6)S) transition of Mn(2+) increases dramatically by a factor of 6.4 with the optimum Ce(3+) co-doping. The energy transfer from Ce(3+) to Mn(2+) was proposed to be resonance-type via an electric dipole-dipole mechanism and the energy transfer efficiency was also calculated by the relative emission intensity. With the broadband ultraviolet (UV) absorption of Ce(3+) and the suitable color coordinates, Ca(8)MgLa(PO(4))(7):Ce(3+),Mn(2+) phosphors might be a promising candidate as red phosphors in the field of UV-based white light-emitting diodes.     

The fabrication of one-dimensional Ca4Y6(SiO4)6O: Ln3+ (Ln=Eu, Tb) phosphors by electrospinning method and their luminescence properties

One-dimensional Ca(4)Y(6)(SiO(4))(6)O: Ln(3+) (Ln=Eu, Tb) microfibers were fabricated by a simple and cost-effective electrospinning method. X-ray diffraction (XRD) pattern and high-resolution transmission electron microscopy (HRTEM) confirmed that the fibers were composed of hexagonal Ca(4)Y(6)(SiO4)(6)O phase. Thermogravimetric and differential scanning calorimetry (TG-DSC) results showed that the Ca(4)Y(6)(SiO4)(6)O phase began to crystallize at 740°C. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) results indicated that the diameter of as-prepared microfibers ranged from 390 to 900 nm and the diameter of the microfibers annealed at 1000°C ranged from to 120 to 260 nm. Under ultraviolet and low-voltage electron beams (3-5 kV) excitation, the Ca(4)Y(6)(SiO(4))(6)O: Ln(3+) (Ln=Eu, Tb) samples showed the red and green emission, corresponding to (5)D(0)→(7)F(2) transition of Eu(3+) and (5)D(4)→(7)F(5) transition of Tb(3+), respectively.     

Surprising luminescent properties of the polyphosphates Ln(PO3)3:Eu (Ln = Y, Gd, Lu)

The optical emission properties of the lanthanoid catena-polyphosphates Ln(PO(3))(3) (Ln = Y, Gd, Lu) doped with europium were investigated. Incommensurately modulated β-Y(PO(3))(3):Eu (super space group Cc (0|0.364|0)0) and Gd(PO(3))(3):Eu (space group I2/a) show the usual emission characteristics of Eu(3+), while in Lu(PO(3))(3):Eu (space group Cc) the europium is unprecedentedly partially reduced to the divalent state, as proven by both a broad emission band at 406 nm excited at 279 nm and an EPR spectroscopic investigation. (151)Eu-M?ssbauer spectroscopy showed that only a very small part of the europium is reduced in Lu(PO(3))(3):Eu. An explanation for this unusual behaviour is given.     

Electrospinning synthesis and luminescence properties of one-dimensional La(9.33)(SiO4)6O2: Ln3+ (Ln = Ce, Eu, Tb) microfibers

One-dimensional La(9.33)(SiO(4))(6)O(2): Ln(3+) (Ln = Ce, Eu, Tb) microfibers were fabricated by a simple and cost-effective electrospinning method. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), photoluminescence (PL) and low voltage cathodoluminescence (CL) as well as kinetic decay were used to characterize the resulting samples. SEM and TEM results indicated that the diameter of the microfibers annealed at 1000 °C for 3 h was 200-245 nm. The microfibers were further composed of fine and closely linked nanoparticles. La(9.33)(SiO(4))(6)O(2): Ln(3+) (Ln = Ce, Eu, Tb) phosphors showed the characteristic emission of Ce(3+) (5d → 4f), Eu(3+) ((5)D(0)→(7)F(J)) and Tb(3+) ((5)D(3,4)→(7)F(J)) under ultraviolet excitation and low-voltage electron beams (3-5 kV) excitation. An energy transfer from Ce(3+) to Tb(3+) was observed in the La(9.33)(SiO(4))(6)O(2): Ce(3+), Tb(3+) phosphor under ultraviolet excitation and low-voltage electron beam excitation. Luminescence mechanisms were proposed to explain the observed phenomena. Blue, red and green emission can be realized in La(9.33)(SiO(4))(6)O(2): Ln(3+) (Ln = Ce, Eu, Tb) microfibers by changing the doping ions. So the La(9.33)(SiO(4))(6)O(2): Ln(3+) (Ln = Ce, Eu, Tb) phosphors have potential applications in full-color field emission displays.     

Luminescence Tuning of Sr8MgCe(PO4)7:Eu2+,Mn2+ Phosphors: Structure Refinement,Site Occupancy,and Energy Transfer

Sr₈MgCe(PO₄)₇:Eu²⁺,Mn²⁺ phosphor with whitlockite‐type structure was prepared by a combustion‐assisted solid‐state reaction. The crystal structure and luminescence properties were investigated. Under UV radiation, Sr₈MgCe(PO₄)₇ host exhibits a violet‐blue emission band from Ce³⁺ ions. When Eu²⁺/Mn²⁺ are doped into the host, the samples excited with 270 nm UV radiation present multicolor emissions due to the energy transfer (ET) from Ce³⁺ to Eu²⁺/Mn²⁺. The emitting color of Sr₈MgCe(PO₄)₇:Eu²⁺ can be tuned from violet‐blue to yellow‐green, whereas Sr₈MgCe(PO₄)₇:Mn²⁺ can emit red light. Under excitation with long wavelength at 360 nm, Sr₈MgCe(PO₄)₇:Eu²⁺ phosphor shows a broadband emission from 390 to 700 nm, which is attributed to the 4f⁶5d¹→4f⁷ transition of Eu²⁺ without the contribution from Ce³⁺ emission. Tunable full‐color emitting light can be achieved in the Eu²⁺ and Mn²⁺‐codoped Sr₈MgCe(PO₄)₇ phosphor by ET_Eu–Mn through control of the levels of doped Eu²⁺ and Mn²⁺ ions. These results suggest that Sr₈MgCe(PO₄)₇:Eu²⁺,Mn²⁺ phosphor has potential applications in NUV chip pumped white LEDs.     

A novel Sr3Y(PO4)3-based white light-emitting phosphor

The Sr₃Y(PO₄)₃:0.05Sm³⁺, Sr₃Y(PO₄)₃:0.005Tb³⁺, and Sr₃Y(PO₄)₃:0.005Tb³⁺, 0.05Sm³⁺ phosphors were synthesized using a conventional solid-state reaction technique at high temperature and their photoluminescence properties under ultraviolet (UV) excitation were studied. We observed the UV sensitization of Sm³⁺ emission (565, 600, and 648 nm) by Tb³⁺ in Sr₃Y(PO₄)₃:0.005Tb³⁺, 0.05Sm³⁺, that leads to a white light emission with the CIE coordinate (0.367, 0.312) of Sr₃Y(PO₄)₃:0.005Tb³⁺, 0.05Sm³⁺ phosphor under UV excitation. The emission is a result of partial energy transfer from Tb³⁺ to Sm³⁺, which is discussed in detail in terms of the corresponding excitation and emission spectra.     

Sr5(PO4)3Cl:Eu2+蓝色荧光粉合成新方法的研究

Sr5 (PO4)3Cl:Eu2+是一种重要的蓝色发射荧光材料,通常采用高温固相反应法来制备.本文利用Sr5(PO4)3(OH)与Sr5(PO4)3Cl结构相同的特点,采用沉淀法合成出羟基磷酸锶铕前体,经过氯化铵和助熔剂作用下的固相氯代反应合成出Sr5(PO4)3Cl:Eu2+荧光粉.考察了pH值与原料比例等对沉淀反应过程及产物的影响,并讨论了氯化铵作用下的氯代过程以及助熔剂对产物荧光粉形貌的作用机制.研究结果表明,本合成方法条件易控,且合成产物Sr5 (PO4)3Cl:Eu2+的物相纯度高,尺寸分布均匀,形貌规则,发光性能优良.     

Microwave synthesis and photoluminescent properties of Sr2CeO4/Ln3+ (Ln = Er, Ho, Tm)

Sr₂CeO₄/Ln³⁺ (Ln = Er, Ho, Tm) phosphors were synthesized with the microwave radiation method for the first time. The luminescent properties of the samples were investigated and the up-conversion luminescence of Er³⁺, Ho³⁺ and Tm³⁺ doped Sr₂CeO₄ phosphors was observed. The spectra indicate that the energy transfer takes place from the triplet excited state of MLCT (metal-to-ligand charge transfer) state for Sr₂CeO₄ (sensitizer) to the rare earth ions (activator). __________ Translated from Journal of Hebei Normal University (Natural Science Edition), 2007, 31(2): 212–216 [译自: 河北师范大学学报 (自然科学版)]     

Cation deficient layered Ruddlesden-Popper-related oxysulfides La2LnMS2O5 (Ln=La, Y; M=Nb, Ta)

The structures of the new oxysulfide Ruddlesden-Popper phases La2LnMS2O5 (Ln=La, Y; M=Nb, Ta) are reported together with an iodide-containing variant: La3-xNb1+xS2O5I2x (0相似文献   

Patterning of Gd2(WO4)3:Ln3+ (Ln=Eu, Tb) luminescent films by microcontact printing route

Gd(2)(WO(4))(3) doped with Eu(3+) or Tb(3+) thin phosphor films with dot patterns have been prepared by a combinational method of sol-gel process and microcontact printing. This process utilizes a PDMS elastomeric mold as the stamp to create heterogeneous pattern on quartz substrates firstly and then combined with a Pechini-type sol-gel process to selectively deposit the luminescent phosphors on hydrophilic regions, in which a Gd(2)(WO(4))(3):Ln(3+) (Ln=Eu, Tb) precursor solutions were employed as ink. X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence (PL) spectra, as well as low voltage cathodoluminescence (CL) spectra were carried out to characterize the obtained samples. Under ultraviolet excitation and low-voltage electron beams excitation, the Gd(2)(WO(4))(3):Eu(3+) samples exhibit a strong red emission arising from Eu(3+)(5)D(0,1,2)-(7)F(1,2) transitions, while the Gd(2)(WO(4))(3):Tb(3+) samples show the green emission coming from the characteristic emission of Tb(3+) corresponding to (5)D(4)-(7)F(6,5,4,3) transitions. The results show that the patterning of rare earth-doped phosphors through combining microcontact printing with a Pechini-type sol-gel route has potential for field emission displays (FEDs) applications.     

Crystal structures of Ln4Ni3O8 (Ln = La, Nd) triple layer T'-type nickelates

Single-phase Ln4Ni3O8 (Ln = La, Nd) nickelates were synthesized and their crystal structures were determined by Rietveld refinement of powder neutron diffraction data. The crystal structures of these mixed-valent Ni1+/Ni2+ phases belong to the T'-type and are built by intergrowth of LnO2 fluorite layers with triple NiO2 infinite-layer structural blocks. The major driving force of transformation of the LnO rock-salt block of the parent Ln4Ni3O10-delta Ruddlesden-Popper phases to the fluorite arrangement in the reduced Ln4Ni3O8 phases is attributed to internal structural stress. This transformation allows longer Ni-O bonds in Ln4Ni3O8 without overstretching of the Ln-O bonds, especially in the equatorial plane. The observed displacement of Ni atoms from the outer NiO2 planes toward the Ni atom of the central NiO2 plane in Ln4Ni3O8 is ascribed to large electrostatic repulsion from the fluorite part of the structure. X-ray absorption spectra near the K-edge of Ni suggest that the charge density on the nickel ion is similar for all members of the T'-type Lnn+1NinO2n+2 homologous series, which correlates with nearly constant Ni-O bond lengths observed in all the reduced nickelates. This suggests that the formal changes in the valence state of Ni affect the covalency of the Ni-O bond.     

A tunable single-component warm white-light Sr3Y(PO4)3:Eu2+,Mn2+ phosphor for white-light emitting diodes

The photoluminescence properties and energy transfer of the Eu(2+) and Mn(2+) co-doped Sr(3)Y(PO(4))(3) phosphors are investigated in detail. Two main emission bands attributed to the Eu(2+) and Mn(2+) ions are observed under UV light excitation via an efficient energy transfer process. When the Eu(2+) doping content is fixed, the emission chromaticity can be varied by simply adjusting the content of Mn(2+). The study of the behavior as a function of doping concentration indicates that the warm white-light can be obtained in a single host lattice. Furthermore, the analysis of the fluorescence decay curves based on the Inokuti-Hirayama theoretical model reveals that the dipole-quadrupole interaction is mainly responsible for the energy transfer mechanism from the Eu(2+) to Mn(2+) ions in the Sr(3)Y(PO(4))(3) phosphor. The developed phosphor exhibits a strong absorption in UV spectral region and white-light emission which may find utility as a single-component white-light-emitting UV-convertible phosphor in white LED devices.     

Conductivity of nonstoichiometric fluorides LnF2+x (Ln = Sm,Eu, Yb)

Nonstoichiometric fluorides LnF_2+x (Ln = Sm, Eu, Yb) were synthesized by reduction of trifluorides with the above rare-earth elements. The resulting phases were identified by chemical and X-ray phase analyses, their composition and structure were determined, and their lattice constants were found. The ac bridge method at a frequency of 70 kHz was used to study the conductivity of the synthesized compounds and starting trifluorides in the temperature range 773–298 K. The temperature dependence of the conductivity of these compounds is satisfactorily approximated by the Arrhenius-Frenkel equation. A kink is observed on the plots of the electrical conductivity against temperature for all compounds. With decreasing content of fluorine, this kink shifts to higher temperatures. The highest conductivity is observed for the phases with low crystal packing density. With increasing content of fluorine, the conductivity of all nonstoichiometric phases not belonging to substitution solid solutions approaches the conductivity of the corresponding trifluorides.     

The thermal characteristics of potassium oxalato lanthanates: K3LnOx3 · n H2O (Ln=La−Tb) and K8Ln2Ox7 · 14 H2O (Ln=Tb−Yb,Y)

The title compounds were studied by TG, DTA, DSC, IR and absorption spectroscopy. The complexes go through dehydration (70–200°C), an irreversible exothermic process (in air or N₂ atmosphere, 250–300°C) and decomposition to a mixture of oxides and carbonates (385–700°C). The exothermic process occurs without weight loss and corresponding heats of reaction fall in the range 0–26 kJ/mol. The absorption spectrum of the Nd complex in the range 5000–6000 Å was employed to monitor perturbations in the coordination sphere of Nd³⁺ arising from the exothermic process. Involvement of the Nd³⁺ cation is implied and the heats of reaction show a close relationship to the radii of Ln³⁺. The interpretation of these data was made with the aid of valuable structural information obtained previously.     

Two new groups of homoleptic rare earth pyridylbenzimidazolates: (NC12H8(NH)2)[Ln(N3C12H8)4] with Ln = Y,Tb, Yb,and [Ln(N3C12H8)2(N3C12H9)2][Ln(N3C12H8)4](N3C12H9)2 with Ln = La,Sm, Eu

The compounds (NC(12)H(8)(NH)(2))[Ln(N(3)C(12)H(8))(4)], Ln = Y, Tb, Yb, and [Ln(N(3)C(12)H(8))(2)(N(3)C(12)H(9))(2)][Ln(N(3)C(12)H(8))(4)](N(3)C(12)H(9))(2), with Ln = La, Sm, Eu, were obtained by reactions of the group 3 metals yttrium and lanthanum as well as the lanthanides europium, samarium, terbium, and ytterbium with 2-(2-pyridyl)-benzimidazole. The reactions were carried out in melts of the amine without any solvent and led to two new groups of homoleptic rare earth pyridylbenzimidazolates. The trivalent rare earth atoms have an eightfold nitrogen coordination of four chelating pyridylbenzimidazolates giving an ionic structure with either pyridylbenzimidazolium or [Ln(N(3)C(12)H(8))(2)(N(3)C(12)H(9))(2)](+) counterions. With Y, Eu, Sm, and Yb, single crystals were obtained whereas the La- and Tb-containing compounds were identified by powder methods. The products were investigated by X-ray single crystal or powder diffraction and MIR and far-IR spectroscopy, and with DTA/TG regarding their thermal behavior. They are another good proof of the value of solid-state reaction methods for the formation of homoleptic pnicogenides of the lanthanides. Despite their difference in the chemical formula, both types (NC(12)H(8)(NH)(2))[Ln(N(3)C(12)H(8))(4)], Ln = Y (1), Tb (2), Yb (3), and [Ln(N(3)C(12)H(8))(2)(N(3)C(12)H(9))(2)][Ln(N(3)C(12)H(8))(4)](N(3)C(12)H(9))(2), Ln = La (4), Sm (5), Eu (6), crystallize isotypic in the tetragonal space group I4(1). Crystal data for (1): T = 170(2) K, a = 1684.9(1) pm, c = 3735.0(3) pm, V = 10603.5(14) x 10(6) pm(3), R1 for F(o) > 4sigma(F(o)) = 0.053, wR2 = 0.113. Crystal data for (3): T = 170(2) K, a = 1683.03(7) pm, c = 3724.3(2) pm, V = 10549.4(14) x 10(6) pm(3), R1 for F(o) > 4sigma(F(o)) = 0.047, wR2 = 0.129. Crystal data for (5): T = 103(2) K, a = 1690.1(2) pm, c = 3759.5(4) pm, V = 10739(2) x 10(6) pm(3), R1 for F(o) > 4sigma(F(o)) = 0.050, wR2 = 0.117. Crystal data for (6): T = 170(2) K, a = 1685.89(9) pm, c = 3760.0(3) pm, V = 10686.9(11) x 10(6) pm(3), R1 for F(o) > 4sigma(F(o)) = 0.060, wR2 = 0.144.