LiBaPO4:Eu2+ and LiBaPO4:Eu2+, Pr3+ persistent phosphors were successfully synthesized via the solid-state reaction method. The persistent phosphors were investigated by XRD, diffuse reflectance, photolumine scence, persistent luminescence, and thermoluminescence spectra. Radiative energy transfer from Eu2+ to Pr3+ was observed due to the overlap between the Eu2+ emission and Pr3+ absorption spectra as well as the spin allowed nature of the relevant transitions of Pr3+. After being irradiated by ultraviolet light, these phosphors show a broad-band persistent luminescence located at similar to 470 nm at room temperature. The emissions for both photoluminescence and persistent luminescence are due to the 5d -> 4f transitions of Eu2+. The persistent luminescence of LiBaPO4:Eu2+ was greatly enhanced by the addition of Pr3+.

Herein, a kind of Tb3+-doped Sr3Al2O5Cl2 phosphor that features a bright green-yellow light afterglow property is first reported. The phase purity and luminescence properties of the obtained samples were characterized using the X-ray powder diffraction (XRD) method, photoluminescence (PL) spectra, decay curves, and thermoluminescence (TL) glow spectra. The XRD results reveal that all samples belong to the orthorhombic structure with the space group P2(1)2(1)2(1). In addition, the influence of Tb3+ content on PL properties was also investigated, and the optimal Tb3+ concentration of the PL properties was experimentally determined to be 0.0025. This is consistent with the optimal Tb3+ concentration (0.0025) for afterglow properties, in which one can clearly observe the longest green-yellow afterglow duration (similar to 3 h) for the sample in the dark with the naked eye. It is confirmed that the PL and afterglow properties are due to the characteristic transitions of Tb3+. According to the TL results, two broad bands at 80 degrees C and 150 degrees C were obtained, but it is suggested that the former band is ascribed to the contribution of the present afterglow properties. Finally, a feasible explanation for the afterglow generation is discussed in detail in this work.

A novel blue‐emitting phosphor Na2ZnGeO4 and a novel green‐emitting phosphor Na2ZnGeO4:Mn2+ have been newly developed via high‐temperature solid‐state reaction. The crystal structure of Na2ZnGeO4 has been identified. Energy transfer from Na2ZnGeO4 host to Mn2+ ions was affirmed. Undoped and Mn2+‐doped Na2ZnGeO4 phosphors exhibit blue and green long persistent luminescence (LPL) with persistent duration more than 40 min and 4 h, respectively. The traps created in host lattice were clarified. The LPL mechanism in Na2ZnGeO4 and Na2ZnGeO4: Mn2+ was discussed briefly. This investigation provides two new and efficient long persistent phosphors (LPPs).

The Sr3Al2O5Cl2:Eu2+, Pt3+ phosphor has been synthesized via high temperature solid state reaction. The X-ray powder diffraction confirms that the obtained samples are pure orthorhombic Sr3Al2O5Cl2 phases, with a space group of D2(4)-P2(1)2(1)2(1). Bluegreen emission is observed when the sample is doped with Pr3+ ions and an orange red emission with Eu2+ ions doping. Both of the samples show obvious afterglow emission. The intensity and lifetime of such afterglow can be substantially enhanced in the case of Pr3+ - Eu2+ co-doped, whose afterglow can last for approximately 300 min in the dark and the intensity is five times higher than the sample single doped with Eu2+. According to the thermoluminescence glow curves, the trap depth of these double-doped samples is about 0.95 eV, which is suitable for the generation of afterglow luminescence. And Pr3+ ions help to enhance the traps concentrations and modify the trap depth, which contributes to prolong the afterglow duration and increase the afterglow intensity of the phosphors. Finally, a feasible explanation of this afterglow generation is also discussed in this work. (C) 2015 Elsevier Ltd. All rights reserved.

Abstract The Sr 3 Al 2 O 5 Cl 2 :Eu 2+ , Pr 3+ phosphor has been synthesized via high temperature solid state reaction. The X-ray powder diffraction confirms that the obtained samples are pure orthorhombic Sr 3 Al 2 O 5 Cl 2 phases, with a space group of D2 4 -P2 1 2 1 2 1 . Blue green emission is observed when the sample is doped with Pr 3+ ions and an orange red emission with Eu 2+ ions doping. Both of the samples show obvious afterglow emission. The intensity and lifetime of such afterglow can be substantially enhanced in the case of Pr 3+ -Eu 2+ co-doped, whose afterglow can last for approximately 300 min in the dark and the intensity is five times higher than the sample single doped with Eu 2+ . According to the thermoluminescence glow curves, the trap depth of these double-doped samples is about 0.95 eV, which is suitable for the generation of afterglow luminescence. And Pr 3+ ions help to enhance the traps concentrations and modify the trap depth, which contributes to prolong the afterglow duration and increase the afterglow intensity of the phosphors. Finally, a feasible explanation of this afterglow generation is also discussed in this work.

Pr3+-doped Cd2GeO4 phosphors were synthesized successfully via a traditional high temperature solid-state reaction method. It showed yellowish-pink long persistent luminescence (LPL) after the short UV-irradiation. The optimal doping concentration of Pr3+ ions for the brightest photoluminescence (PL) emission and the best LPL performance were experimentally to be about 1.5% and 1 mol%, respectively. The suitable trap depth for the generation of LPL was determined to be about 0.65 eV. Based on TL measurements, the trapping and re-trapping processes of charge carriers were studied. A model was proposed on the basis of experimental results to explain the mechanisms of PL and LPL. (C) 2014 Elsevier B.V. All rights reserved.

Stoichiometric phosphors LiGd1−xEux(PO3)4(x=0, 0.2, 0.4, 0.6, 0.8, 1.0) were synthesized via traditional solid state reactions. The X-ray powder diffraction measurements show that all prepared samples are isostructural with LiNd(PO3)4. Eu3+ doped phosphors can emit intense reddish orange light under the excitation of near ultraviolet light from 370 to 410 nm. The strongest two at 591 and 613 nm can be attributed to the transitions from excited state 5D0 to ground states 7F1 and 7F2, respectively. The typical chromaticity coordinates (x=0.620, y=0.368) of Eu3+ doped phosphors are in red area. The recorded absorbance spectra indicate that there is effective absorbance in the near UV region for all Eu3+ doped samples. Present research indicates that LiGd1–xEux(PO3)4 is a promising phosphor for white light-emitting diodes.

A series of Eu2+ and Ce3+ doped/co-doped Sr3Al2O5Cl2 afterglow phosphors that presented various bright colors were successfully synthesized via high temperature solid state reaction. The structure and luminescence properties of the obtained samples were characterized by X-ray powder diffraction (XRD), photoluminescence (PL) spectra and decay curves as well as the thermoluminescence (TL) glow curves. The XRD results showed that all the phase could be indexed to the orthorhombic structure with the space group P212121. After being exposed to a 254 nm or 365 nm mercury lamp, blue/yellow-orange afterglow emissions with broad bands peaking around 620 nm/435 nm, which were ascribed to the characteristic 4f65d–4f7/5d1–4f1 transitions of Eu2+/Ce3+, could be observed in phosphors of Sr3Al2O5Cl2:Eu2+/Sr3Al2O5Cl2:Ce3+, respectively. Because of the overlap spectral range between the Sr3Al2O5Cl2:Eu2+ and Sr3Al2O5Cl2:Ce3+ phosphors, the energy transfer (ET) from Ce3+ to Eu2+ occurred. The related ET process was discussed in detail. Moreover, the incorporation of Ce3+ could significantly prolong the afterglow duration of Sr3Al2O5Cl2:Eu2+ phosphor, which was due to the increase of trap concentration. Consequently, 6 h of the afterglow duration could be observed in Sr3Al2O5Cl2:1.0%Eu2+, 0.5%Ce3+ sample, exhibiting much longer than that of Sr3Al2O5Cl2: 1.0%Eu2+ (3 h). From the afterglow decay curves and the fitting results, the optimal concentration of Ce3+ for the enhanced afterglow property was experimentally determined to be 0.5%.

A series of Bi3+ and Gd3+ doped ZnB2O4 phosphors were synthesized with solid state reaction technique. X-ray diffraction technique was employed to study the structure of prepared samples. Excitation and emission spectra were recorded to investigate the luminescence properties of phosphors. The doping of Bi3+ or Gd3+ with a small amount (no more than 3 mol%) does not change the structure of prepared samples remarkably. Bi3+ in ZnB2O4 can emit intense broad-band purplish blue light peaking at 428 nm under the excitation of a broad-band peaking at 329 nm. The optimal doping concentration of Bi3+ is experimentally ascertained to be 0.5 mol%. The decay time of Bi3+ in ZnB2O4 changes from 0.88 to 1.69 ms. Gd3+ in ZnB2O4 can be excited with 254 nm ultraviolet light and yield intense 312 nm emission. The optimal doping concentration of Gd3+ is experimentally ascertained to be 5 mol%. The decay time of Gd3+ in ZnB2O4 changes from 0.42 to 1.36 ms. (C) 2014 Elsevier B.V. All rights reserved.

The tunable emission color phosphors Ba3Gd(PO4)3: Ce3+, Mn2+ were synthesized by conventional high-temperature solid-state reaction method. The photoluminescence properties and energy transfer (ET) of the Ce3+ and Mn2+ co-doped Ba3Gd(PO4)3 phosphors were studied systematically. Through an efficient energy-transfer process, the obtained phosphors show both dual-emission including a ultraviolet (UV) to blue emission originating from Ce3+ and a reddish-orange emission from Mn2+ under UV excitation (~310 nm) with considerable intensity. When the Ce3+ doping content is fixed at 0.05, the emission color tone can be adjusted from blue through reddish-purple and ultimately to red region by tuning the contents of Mn2+ from 0.01 to 0.5. The critical distance between Ce3+ and Mn2+ was calculated to be 12.23 Å. The concentration quenching of Mn2+ was experimentally found to be 0.3. The energy-transfer efficiency was discussed and the ET mechanism between Ce3+ and Mn2+ ions was proved to be dipole–dipole interaction. The Ce3+ and Mn2+ co-doped Ba3Gd(PO4)3 phosphors are potential UV-convertible candidates in UV white-light LEDs for the energy transfer from Ce3+ to Mn2+ ions.

Bi3+-doped Zn2GeO4 were prepared by the high temperature solid-state reaction method. The phase purity and crystallinity of Zn2GeO4:Bi3+ samples were characterized by X-ray diffraction (XRD). The Photoluminescence and persistent properties of Zn2GeO4:Bi3+ phosphors were investigated through the excitation spectra, the emission spectra, the persistent luminescence, the persistent decay curves and thermoluminescence spectra. Excitation into the host absorption, Zn2GeO4:Bi3+ gives a blue broadband emission; a bluish-green emission of Zn2GeO4:Bi3+ is obtained for the direct excitation of Bi3+ under 300 nm excitation. Zn2GeO4:Bi3+ shows a weak persistent luminescence after irradiation by 254 nm UV light for 3 min. The photoluminescence and persistent luminescence mechanism of Zn2GeO4:Bi3+ were discussed in detail. (C) 2014 Elsevier B.V. All rights reserved.

The modification of optical properties of SrAl2O4∶Eu2+,Dy3+ phosphor by holmium doping was studied for the first time. The phosphor by holmium doping was synthesized by the combustion method at the temperature of 600℃.X-ray diffraction pattern indicates that the samples possess a monoclinic crystal structure.The emission spectra shows a continuous luminescence band with a peak position near to 510nm and excitation spectra shows only a peak at 356nm.The intensity decay curves were measured by a single photon counter. The results show the initial intensity and decay time can be modified by holmium doping. Meanwhile,the possible mechanism for the improvement of initial intensity was presented here.

As novel functional materials,long afterglow phosphors have drawn more and more attention in recent years because of a constantly growing market for their applications in traffic signs,emergency signage,watches and clocks,textile printing,among others.Amid the newly developed long afterglow materials which have already found commercial use,green-emitting SrAl2O4:Eu2+,Dy3+ is of special interest because of its high quantum efficiency,good stability and excellent persistent luminescence combined with an easy proces-sability.However the mechanism of afterglow phosphors is still unclear,even the effect of the reductive using on the producing process on the luminescence is not well studied.In this paper,compounds of SrAl2O4:Eu,Dy were prepared in air atmosphere by high temperature solid state reaction firstly,and then were treated in weak reductive atmosphere(95%N2+5%H2).Thirdly,in air atmosphere again,and finally,in weak reductive atmosphere(95%N2+5%H2).The effect of this process on the structure,luminescent properties,and themoluminescent spectra of the samples were studied by X-ray diffraction(XRD),fluorescent spectrophoto-meter and thermoluminescence dosimeter(TLD) respectively.The monoclinic structure remains the same irrespective of the process,and is in good agreement with that of the powder diffraction file(JCPDS) 34-0379(SrAl2O4).The reduction of Eu3+→Eu2+ was firstly observed in the aluminate phosphor of SrAl2O4:Eu,Dy synthesized in air condition and the reductive mechanism was also discussed in this paper.Eu3+ shows completely different luminescent properties with Eu2+ in SrAl2O4.Eu3+ has two narrow,intense spectra peaking at 592 nm and 612 nm,which resulted from the 5D0→7F1 transition and 5D0→7F4 transition respectively,implied that the Eu3+ may occupy two different lattice sites in the host crystal lattice;while Eu2+ has only one broad band spectra peaking at 513 nm,which resulted from the 4f65d→4f7 transition.The emission of Dy3+ was not found in all the four samples,indicated that the Dy3+ ions play a roll as trap centre in the phosphor,but not the luminescence centre.The concentration of Eu2+ in the samples treated in weak reductive atmosphere greatly increased whereas the concentration of Eu3+greatly decreased when compared to the samples treated in air atmosphere.The samples treated in weak reductive atmosphere have much higher thermoluminescent peak than that of the samples treated in air atmosphere.But all the samples,irrespective being treated in air atmosphere or reductive atmosphere,have the same trap depth(about 0.65 eV),which derived from Dy3+.All these phenomena indicated that,for the long after-glow phosphor material of SrAl2O4:Eu,Dy prepared in air atmosphere,the process of being treated in weak reductive atmosphere,air atmosphere and weak reductive atmosphere orderly,changes the valence of Eu ion and luminescence intensity,but doesn't change the trap depth in the material.

Long persistent phosphors SrxCa1−xAl2O4: Eu2+, Dy3+ (x=0, 0.2, 0.4, 0.6, 0.8, 1.0) were prepared through a solution-combustion method at 600°C. The phase structures of the phosphors were characterized by X-ray diffraction. The major phase was CaAl2O4 when x<0.8 with SrAl2O4 phase as a minor phase, whereas the major phase was SrAl2O4 when x≥0.8. The results indicated that the phase structure of the samples changed from CaAl2O4 to SrAl2O4 when x changed from 0 to 1.0. The emission colors changed accordingly from blue to green-blue and then to green in visible range, as evidenced from the emission spectra. The decay characteristics showed that the life times also varied with the value of x. These results revealed that the emission colors and life times can be adjusted by the value of x. Furthermore, the reason that the emission colors and decay times varied with x has been ascribed to the distortion of crystal lattice.

The modification effect of the doping of Yb3+ ions, as an auxiliary activator, onto the luminescent properties of SrAl2O4:Eu2+, Dy3+ phosphor was studied for the first time. The phosphorescent nanoparticles were prepared by the combustion method. The experimental results indicate that the appropriate doping of Yb3+ ions largely improves phosphorescence of the phosphors with more intense luminescence, higher brightness, and no change in emission spectrum peaked at 513nm. Meanwhile the decay speed of the phosphor nanoparticles rises increasingly with the doping ratio of Yb3+ ions, whereas an excessive Yb3+ ions doping leads to the disappearance of the pure monoclinic phase of SrAl2O4 and the appearance of the weak diffraction lines of the YbAlO3 phase. The phosphorescent mechanism of the phosphors could be well understood based on the hole, thermally released from the trap levels of Dy3+ and Yb3+.

Rare-Earth doped strontium aluminate phosphors were synthesized through combustion method.XRD results indicate that the dominant host phase is SrAl2O4 when n(Al)∶n(Sr)=2.With the increasing of Al∶Sr ratios,the new crystalline phase of Sr4Al14O25 was initially formed.The phase SrAl12O19 was formed when Al∶Sr ratios increased to 4,and this phase was the dominate phase when Al∶Sr ratios increased to 12.The emission spectra show that the peaks of SrAl2O4∶Eu2+,Dy3+ and SrAl12O19∶Eu2+,Dy3+ lied round 519 and 512nm,respectively.The decay curves show that the decay process of all the samples with different crystalline phases consist of the initial fast process and the latter slow process.Intensity and decay lifetimes of the samples vary with the different crystalline phases.