Radiative properties of Eu in BaGa2S4

A photoluminescence study of the blue-green emitting BaGa₂S₄:Eu²⁺ phosphor is reported. Diffuse reflectance, excitation and emission spectra were examined with the aim to enlarge the fundamental knowledge about the emission of the Eu²⁺ rare earth ion in this lattice. The thermal dependence of the radiative properties and the influence of the Eu²⁺ concentration were investigated. The Stokes shift, the crystal field splitting and the activation energy of the thermal quenching were determined. By combining these results with data available in literature, we discussed the radiative properties of the BaAl₂S₄:Eu²⁺ blue phosphor in relation with those determined in this study for the isostructural BaGa₂S₄:Eu²⁺ phosphor.     

Self-assembled CaMoO4 and CaMoO4:Eu hierarchical superstructures: Facile sonochemical route synthesis and tunable luminescent properties

Tetragonal CaMoO₄ and CaMoO₄:Eu³⁺ with various novel three-dimensional (3D) hierarchical architectures were successfully synthesized via a facile, efficient sonochemistry process in the absence of any surfactant or template. XRD, EDS, FE-SEM, and photoluminescence (PL) were employed to characterize the as-obtained products. It was found that morphology modulation could be easily realized by changing pH value of the precursor. The pH value of the precursor not only affected the substructures of the hierarchical structures, but also determined the size distributions of the final products. The formation mechanism for different hierarchical architectures was proposed on the basis of time-dependent experiments. The luminescence spectra showed that CaMoO₄:Eu³⁺ phosphors can be effectively excited by the near ultraviolet (UV) (396 nm) and blue (466 nm) light, and exhibited strong red emission around 615 nm, which was attributed to the Eu³⁺⁵D₀→⁷F₂ transition. Compared with Y₂O₃:Eu³⁺ phosphor, CaMoO₄:Eu³⁺ is much more stable, efficient and suitable, therefore, this phosphors could be a promising red component for possible applications in the field of LEDs.     

Blue and red long lasting phosphorescence (LLP) in β-Zn3(PO4)2:Mn,Zr

Multi-color long lasting phosphorescent (LLP) phenomenon in β-Zn₃(PO₄)₂:Mn²⁺,Zr⁴⁺ was systematically investigated. It is found that the red (λ_Em=616 nm) LLP performance of Mn²⁺ such as brightness and duration is largely improved, and that the blue (λ_Em=475 nm) LLP of Zr⁴⁺ with lower intensity appears when Zr⁴⁺ ions are co-doped into the matrix. The fluorescence, phosphorescence and thermoluminescence (TL) spectra show that Mn²⁺ ion is solely expected as a luminescent center, while Zr⁴⁺ ion not only acts as a luminescent center, but also induces an electron trap (Trap_Zr) associated with a TL peak at 344 K. The trap depth for Trap_Zr is 0.25 eV, while that for the intrinsic trap is 0.38 eV, associated with a dominant peak at 385 K for Zn₃(PO₄)₂:Mn²⁺. The Zr⁴⁺-induced trap with suitable depth is responsible for the improvement of the red LLP of Mn²⁺ ion and the appearance of the blue LLP of Zr⁴⁺ ion. The LLP mechanism is also investigated.     

Photoluminescence and gamma-ray irradiation of SrAl2O4:Euand Y2O3:Eu phosphors

Y₂O₃:Eu³⁺ phosphor is a very attractive material for use as a red phosphor in many fields. SrAl₂O₄:Eu²⁺ belongs to long lasting phosphor (LLP) and it is a useful bluish-green luminescence material, which can also be a promising candidate as a simple and easy-to-use radiation detection element for visual display of two dimensional radiation distributions. In the present study, both these two kinds of phosphors were synthesized using high temperature solid state reactions. In our work, the influence of gamma-ray irradiation on the properties of these two kinds of phosphors was studied by comparing photoluminescence, brightness and the decay curve of unirradiated and gamma-ray-irradiated samples. Conclusions from the present work can be briefly summarized as follows. In irradiated samples, the brightness is decreased without sensible change in the wavelength distribution of the luminescence spectrum and in the decay kinetic upon gamma exposure. Moreover, the emission due to Eu³⁺→Eu²⁺ conversion in Y₂O₃:Eu³⁺ phosphors was not observed in our sample after irradiation to high exposure. Also the brightness of SrAl₂O₄:Eu²⁺ phosphor turned out to decrease after the exposition to ionizing radiation while the luminescence wavelength distribution remained unchanged. The reason for the effect of gamma-ray irradiation on the properties of phosphors is also discussed in the paper.     

Synthesis and photoluminescence properties of NaPbB5O9:Dy phosphor materials for white light applications

Given the recent increased interest in phosphor materials and their applications, we analyzed a new NaPbB₅O₉:Dy³⁺ phosphor material with different concentrations of Dy³⁺. In particular, we investigated the crystal structure, morphology, and luminescence properties of these materials. X-ray diffraction analyses confirmed the formation of NaPbB₅O₉:Dy³⁺ phosphor powder. The functional groups present in the phosphor materials were examined by Fourier transform infrared spectroscopy. Scanning electron microscope images showed that the size of the grains was in the micrometer range. Photoluminescence spectra were recorded at different excitation wavelengths for the phosphor materials and we analyzed the variation in the intensity of the emission bands with different concentrations of Dy³⁺ ions. The color co-ordinates were calculated and used to characterize the color of the phosphor. We found that the emission colors of the Dy³⁺-doped NaPbB₅O₉ powders depended on the Dy³⁺ ion doping concentration and the excitation wavelength.     

Luminescent properties of R2(MoO4)3:Eu (R=La, Y, Gd) phosphors prepared by sol-gel process

A series of red phosphors R_0.8Eu_1.2(MoO₄)₃ (R=La, Y, and Gd) have been synthesized by sol-gel method. The crystallization processes of the phosphor precursors were characterized by X-ray diffraction (XRD) and thermogravimetry-differential thermal analysis (TG-DTA), and the properties of these resulting phosphors have also been characterized by photoluminescence (PL) spectra and reflectance spectra. Field emission scanning electron microscopy (FE-SEM) was also used to characterize the shape and the size of the samples. The results of TG-DTA and XRD indicated that all of the R_0.8Eu_1.2(MoO₄)₃ (R=La, Y, and Gd) phosphors crystallized completely at 650 °C. Y_0.8Eu_1.2(MoO₄)₃ and Gd_0.8Eu_1.2(MoO₄)₃ have two structures, monoclinic and orthorhombic, while La_0.8Eu_1.2(MoO₄)₃ only adopts monoclinic structure. The luminescent properties of phosphors R_0.8Eu_1.2(MoO₄)₃ (R=La, Y, and Gd) are dependent on their structures to some extent. The orthorhombic Y_0.8Eu_1.2(MoO₄)₃ and Gd_0.8Eu_1.2(MoO₄)₃ phosphors show very similar luminescent properties, which differ from those of phosphors with monoclinic structure. For all of R_0.8Eu_1.2(MoO₄)₃ (R=La, Y, and Gd) phosphors, intense red emission is obtained by exciting at ∼394 and ∼465 nm which are owing to the sharp ⁷F₀→⁵L₆ and ⁷F₀→⁵D₂ lines of Eu³⁺. Two strongest lines at 394 and 465 nm in excitation spectra of these phosphors match well with the two popular emissions from near-UV and blue GaN-based LEDs, so they could be used as red components for white light-emitting diodes.     

Photoluminescence properties of BaMgAl10O17:Eu phosphor prepared by the flux method

BaMgAl₁₀O₁₇:Eu²⁺ phosphors were synthesized by the flux method. When the appropriate amounts of fluxes are added, the synthesis temperature reduced by at least 200 °C compared with the conventional solid-state reaction method. SEM images demonstrated that addition of the flux in the process of phosphor synthesis benefitted the size and morphology of BaMgAl₁₀O₁₇:Eu²⁺ phosphor particles. Photoluminescence measurements under VUV excitation indicated that the luminescent intensity of the phosphor enhanced by adding the flux system (BaF₂+Li₂CO₃). Addition of the flux system can not only enhance the luminescence efficiency and improve the stability, but also control the morphology and grain size of the phosphor. Replacement of Ba²⁺ by Li⁺ could generate traps, which result in slightly longer decay time.     

A novel blue-emitting long-lasting proyphosphate phosphor Sr2P2O7:Eu,Y

By introducing the Y³⁺ into Sr₂P₂O₇:Eu²⁺, we successfully prepared a kind of new phosphor with blue long-lasting phosphorescence by the high-temperature solid-state reaction method. In this paper, the properties of Sr₂P₂O₇:Eu²⁺,Y³⁺ were investigated utilizing XRD, photoluminescence, luminescence decay, long-lasting phosphorescence and thermoluminescence (TL) spectra. The phosphor emitted blue light that was related to the 4f⁶5d¹-⁸S_7/2 transition of Eu²⁺. The bright blue phosphorescence could be observed by naked eyes even 8 h after the excitation source was removed. Two TL peaks at 317 and 378 K related to two types of defects appeared in the TL spectrum. By analyzing the TL curve the depths of traps were calculated to be 0.61 and 0.66 eV. Also, the mechanism of LLP was discussed in this report.     

White-light long-lasting phosphorescence from Tb-activated Y2O2S phosphor

The white-light long-lasting phosphors Y₂O₂S:Tb³⁺, Sr²⁺ or/and Zr⁴⁺ were prepared and studied. The white-light afterglow emission after the irradiation with 254 nm UV are composed of the blue light emission and the yellowish-green light emission, originating from the transitions of ⁵D₃→⁷F₅, ⁵D₄→⁷F₅ in Tb³⁺ when the Tb³⁺ concentration is not higher than 0.3 at%. The codoped Sr²⁺ and Zr⁴⁺ ions act as trap-creating ions. The afterglow can last over 21 min in the dark for Y₂O₂S:Tb³⁺0.3%, Sr²⁺4%, Zr⁴⁺4% after irradiation by 254 nm ultraviolet light. Y₂O₂S:Tb³⁺ may be a promising material for the development of white-light long-lasting phosphor since the Tb³⁺ has a high luminescent efficiency and the dominant excitation band of 4f →5d is located at 220-300 nm.     

EPR study of Ce ions in lutetium silicate scintillators Lu2Si2O7 and Lu2SiO5

Two Ce³⁺-doped scintillator crystals, LSO (Lu₂SiO₅:Ce) and LPS (Lu₂Si₂O₇:Ce), are studied by EPR spectroscopy. The analysis indicates that Ce³⁺ substitutes for Lu³⁺ ion in a C₂-symmetry site for LPS and in two C₁-symmetry sites for LSO, with a preference for the largest one, with 6+1 oxygen neighbors. Angular dependence of the EPR spectrum shows that the electronic ground state of Ce³⁺ is different in these two matrices. It is mainly composed of |M_J|=5/2 state in LPS and |M_J|=3/2 state in LSO. The temperature dependence of the linewidth shows a noticeably long spin lattice relaxation time, especially in LPS, which is the result of a stronger crystal field in LPS than in LSO.     

Magnetic properties for the Cu2MnSnSe4 and Cu2FeSnSe4 compounds

Magnetic susceptibility χ measurements in the range from 2 to 300 K were carried out on samples of the Cu₂FeSnSe₄ and Cu₂MnSnSe₄ compounds. It was found that Cu₂FeSnSe₄ was antiferromagnetic showing ideal Curie-Weiss behavior with a Néel temperature T_N of about 19 K and Curie-Weiss temperature θ=−200 K, while for Cu₂MnSnSe₄ the behavior was spin-glass with a freezing temperature T_f of about 22 K and Curie-Weiss temperature θ=−25 K. The spin-glass order parameter q(T), determined from the susceptibility data, was found to be in agreement with the prediction of conventional spin-glass theory.     

Tunable luminescent properties by adjusting the Sr/Ba ratio in Sr1.97−xBaxMgSi2O7:Eu0.01, Dy0.02 phosphors

The long afterglow phosphors Sr_1.97−xBa_xMgSi₂O₇:Eu²⁺_0.01, Dy³⁺_0.02 (x=0, 0.4, 0.8, 1.2, 1.6 and 1.97) were synthesized via high temperature solid-state reaction. The phase identification reveals that the crystal plane spacing becomes greater with the decrease in the Sr/Ba ratio. Phase transition occurs when x=1.97. A nonlinear relationship between the emission peak and the crystal plane spacing is obtained with the decrease of the Sr/Ba ratio. This ascribes to the splitting of the 5d level of the Eu²⁺ and the change of the crystal field strength. The duration of the afterglow becomes shorter with the decrease of the Sr/Ba ratio. It may ascribe to deeper trap depth, lower trap concentration and the embarrassment of the transfer of carriers.     

Luminescence and EPR studies of YBO3:U

Yttrium borate doped with uranium was prepared by mixing and heating yttrium oxide obtained through oxalate precipitation route, boric acid and requisite amount of nuclear-grade uranium oxide at high temperature. Photoluminescence (PL), thermally stimulated luminescence (TSL) and electron paramagnetic resonance (EPR) studies were carried out on gamma-irradiated doped/undoped yttrium borate samples in the temperature range 300-600 K. TSL studies showed the presence of two glow peaks at 414 and 471 K. PL studies along with lifetime decay investigation suggested uranium goes in the matrix as UO₂²⁺. EPR studies showed the presence of O₂⁻radical ion along with electron trapped in defect centres, which might have been produced for charge compensation. Apart from this, CO₂⁻ radical was also observed in the system having its origin from residual oxalate ion. Temperature dependence EPR studies of the observed radical confirmed the involvement of the CO₂⁻ and dioxide radical ion in the observed glow peaks. By correlating the TSL, PL and ESR data, probable mechanism is proposed for the observed TSL glow in the system.     

The luminescence and structural characteristics of Eu- doped NaSrB5O9 phosphor

A red-emitting phosphor NaSrB₅O₉:Eu³⁺ was synthesized by employing a solid-state reaction (SSR) method. The structures of the phosphors were analyzed by X-ray diffraction (XRD), Fourier-transform infrared (FTIR) and Raman studies. The band at ~282 nm in the excitation spectra indicated the charge transfer band (CTB) of B-O in the host, whereas the CTB of Eu-O was observed at ~275 nm for the NaSrB₅O₉:Eu³⁺ (Eu³⁺=1 at.%) phosphor, which was supported by diffuse reflectance spectroscopy (DRS) measurements. The photoluminescence (PL) measurements exhibited a strong red emission band centered at about 616 nm (⁵D₀→⁷F₂) under an excitation wavelength of 394 nm (⁷F₀→⁵L₆). Upon host excitation at 282 nm, the pristine NaSrB₅O₉ exhibited a broad UV emission centered at ~362 nm. The energy transfer from host to Eu³⁺ ions was confirmed from luminescence spectra, excited with a 355 nm Nd:YAG laser. In addition, the asymmetric ratios indicate a higher local symmetry around the Eu³⁺ ion in the host. The calculated CIE (Commission International de l′Eclairage) coordinates displayed excellent color purity efficiencies (around 99.7%) compared to other luminescent materials.     

Study of the structural, dielectric and magnetic properties of Bi2O3 and PbO addition on BiFeO3 ceramic matrix

In this paper we studied the effects of Bi₂O₃ and PbO addition on BiFeO₃ (BFO) ceramic matrix. The structural, dielectric and magnetic properties of fifteen BFO samples were discussed in view of possible applications in RF and microwave devices. The present work also reports the preparation of the samples. Polyvinyl alcohol (PVA) and tetraethyl orthosilicate (TEOS) were also added as a binder in the fabrication procedure. The samples have been studied by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and magnetic hysteresis measurements. Further, a study based on impedance spectroscopy also has been done. Dielectric permittivity (ε′) and dielectric loss (tan δ) were measured at room temperature in the frequency range 100 Hz-10 MHz, as well as a.c. conductivity. The -Im[Z(f)] versus Re[Z(f)] plot has been obtained. The samples were investigated in view of possible applications like miniaturized filters, diplexers and dielectric resonator antennas (DRA). In the RF and MW frequency region, the application of magneto-dielectric and multiferroic perovskite composite materials is desirable for the miniaturization of components.     

Optical spectroscopy of Yb/Er codoped NaY(WO4)2 crystal

Erbium and ytterbium codoped double tungstates NaY(WO₄)₂ crystals were prepared by using Czochralski (CZ) pulling method. The absorption spectra in the region 290-2000 nm have been recorded at room temperature. The Judd-Ofelt theory was applied to the measured values of absorption line strengths to evaluate the spontaneous emission probabilities and stimulated emission cross sections of Er³⁺ ions in NaY(WO₄)₂ crystals. Intensive green and red lights were measured when the sample were pumped by a 974 nm laser diode (LD), especially, the intensities of green upconversion luminescence are very strong. The mechanism of energy transfer from Yb³⁺ to Er³⁺ ions was analyzed. Energy transfer and nonradiative relaxation played an important role in the upconversion process. Photoexcited luminescence experiments are also fulfilled to help analyzing the transit processes of the energy levels.     

Influence of different annealing conditions on the luminescent properties of Li2B4O7:Mn single crystals

The influence of reducing annealing and repeated oxidizing annealing of the Li₂B₄O₇:Mn single crystals on their thermostimulated luminescence (TSL) and X-ray luminescence (XL) has been studied. Because of the oxygen vacancies formation and the dopant ions reduction, the reducing annealing results in a drastic decrease of the luminescence intensity along with the simultaneous shift of a part of the maxima and redistribution of the peak intensities. The repeated annealing in the oxidizing environment leads only to the partial reduction of the luminescent properties of the Li₂B₄O₇:Mn crystals.     

Study of phase transition in ZrO2 doped with Pb3O4

Electrical conductivity of ZrO₂ doped with Pb₃O₄ has been measured at different temperatures for different molar ratios (x=0, 0.01, 0.02, 0.03, 0.04, 0.05 and 0.06). The conductivity increases due to migration of vacancies, created by doping. The conductivity increases with increase in temperature till 180 °C and thereby decreases due to collapse of the fluorite framework. A second rise in conductivity at higher temperatures beyond 500-618 °C is due to phase transition of ZrO₂. DTA and X-ray powder diffraction were carried out for confirming doping effect and transition in ZrO₂.The addition of Pb₃O₄ to ZrO₂ shifted the phase transition of ZrO₂ due to the interaction between Pb₃O₄ and ZrO₂.     

Impurity-induced magnetic order in the mixture of two spin gap systems (CH3)2CHNH3CuCl3 and (CH3)2CHNH3CuBr3

The ground state of the solid solution of the two spin gap systems (CH₃)₂CHNH₃CuCl₃ and (CH₃)₂CHNH₃CuBr₃ has been investigated by ¹H NMR. The existence of a magnetic ordering in the sample with the Cl-content x=0.85 was clearly demonstrated by a drastic splitting in a resonance line at low temperatures below T_N=13.5 K. The observed NMR spectra in the ordered state was qualitatively consistent with the simple antiferromagnetic state.     

Density functional theory study on two-peak emission of Eu activators in Sr2SiO4

The supercells of pure and Eu-doped Sr₂SiO₄ were theoretically analyzed by density functional theory (DFT) calculations to investigate the typical two-peak emission of Sr₂SiO₄:Eu²⁺, which originates from two different Sr²⁺ (or Eu²⁺) sites in the Sr₂SiO₄ host structure. The Perdew-Wang generalized-gradient approximation (GGA) functional and the double numerical plus d-functions (DND) basis set with effective core potentials (ECP) were employed in the calculations of electronic properties. The electron transfer between Eu²⁺ ions placed at two different crystallographic Sr²⁺ sites was understood based on the accurate assignment of deconvoluted peaks of the two-peak emission to their corresponding crystallographic sites. This study ought to be instructive as a basic guideline to improve the color chromaticity of Sr₂SiO₄:Eu²⁺ for use in white light emitting diodes (WLEDs).