Transparent Lu3Al5O12:Dy3+ ceramics were fabricated for UV-pumped white light-emitting diodes (WLEDs) via solid-state sintering under vacuum. The color chromaticity of the ceramic-based phosphors were tuned by tailoring the Dy3+ concentration and incorporating Yb into the crystal lattice to form (Lu, Yb)3Al5O12:Dy3+ solid solutions. Phase composition, microstructure, optical and photoluminescence properties of the ceramics were investigated in detail by X-ray diffraction (XRD), Scanning electron microscopy (SEM), UV–vis-NIR spectrometer and fluorescence spectrophotometer, respectively. White light can be obtained by combining the UV-chip and the structure/property-optimized ceramic phosphors. The color hue was tuned from (0.4107, 0.4037) to (0.3647, 0.3299) with the increasing Yb content from 0 to 0.5 substituting Lu sites in the garnet structure. The (Lu0.5Yb0.5)3Al5O12: 0.01Dy3+ ceramic-based phosphor showed a relative low correlated color temperature of 4137 K. The decrease in PL intensities with Yb incorporation was also discussed via microstructure and fluorescence lifetime characterizations.

•Eu3+-doped La0.8Gd1.2Hf2O7 transparent ceramics were first reported.•The crystal structure, transmittance, and luminescence properties with different Eu3+ content were investigated.•The annealing process affected the transmittance as well as the luminescence behavior of the ceramics.Eu3+-doped Lanthanum gadolinium hafnates (La0.8Gd1.2Hf2O7) transparent ceramics with different Eu3+ concentration were fabricated by vacuum sintering. XRD results showed all the ceramics are cubic pyrochlore structure. The effects of annealing process on in-line transmittance and luminescence behavior of the Eu3+-doped La0.8Gd1.2Hf2O7 transparent ceramics were investigated. Before annealing, the in-line transmittance of the ceramics was low and the luminescence intensity was weak. As Eu3+ doping content increased, the transmittance as well as the luminescence intensity decreased. This was ascribed to oxygen vacancy and other defects in the ceramics resulted from the vacuum sintering. After annealing, the transmittance and luminescence intensity were raised, indicating the elimination of oxygen vacancy. Moreover, with the increase of Eu3+ doping content from 1 at% to 10 at%, the luminescence intensity increased without concentration quenching.

•transparent MgAl2O4 ceramics has been fabricated by reactive sintering method.•Effect of polymorphism of Al2O3 on densification process has been investigated.•Sample using γ-Al2O3 exhibited a bimodal grain size distribution and residue pores.Transparent MgAl2O4 ceramics were fabricated by reactive sintering in air followed by hot isostatic press treatment using commercial Al2O3 powder (γ-Al2O3 or α-Al2O3) and MgO powder as raw materials. The densification rate, microstructure and optical properties of the ceramics were investigated. Densification temperature of the sample from γ-Al2O3/MgO was lower than that from α-Al2O3/MgO. However, in-line transmission (2 mm thick) of the sample from α-Al2O3/MgO at the wavelength of 600 nm and 1100 nm were respectively 77.7% and 84.3%, higher than those (66.7%, 81.4%) of the sample from γ-Al2O3/MgO. SEM observation revealed that the sample from α-Al2O3/MgO exhibited a homogeneous and pore-free microstructure, while, the sample from γ-Al2O3/MgO showed an apparent bimodal microstructure containing pores.

•TMAH addition is effective to delay shear-thickening behavior of alumina slurry.•Solids loading of slurry was improved and gelling rate was accelerated.•One role of TMAH was hydrolysis enhancement by OH− groups.•Formed N(CH3)4 or COON(CH3)4 groups played a hydrophobic role.Tetramethyl ammonium hydroxide (TMAH) has been demonstrated to increase solids loading effectively and to accelerate gelling rate of alumina slurries gelled by Isobam (a copolymer of isobutylene and maleic anhydride) system. With 0.5 vol% addition of TMAH, solids loading of alumina slurry was increased from 56 vol% to 58 vol%. Meanwhile, storage modulus (G’) of the slurry with 56 vol% solids loading was increased from 964 Pa to 2850 Pa at a testing time of 30 min. The enhanced dispersing and gelling behavior was ascribed to the hydrolysis and hydrophobic modification of Isobam by TMAH. Flexural strength of the final alumina ceramics from 58 vol% solids loading was 543 ± 10 MPa, higher than that of the ceramics from 56 vol% solids loading (531 ± 13 MPa).

•A new series of La2-xGdxHf2O7 transparent ceramics were fabricated by vacuum sintering using combustion-synthesized powders.•All the ceramics are transparent and the in-line transmittance can reach to 76.1% at 800 nm when x=1.2.•The Gd content has effects on the crystal structure, in-line transmittance, microstructures and densities of the ceramics.•With high density (7.91~8.88 g/cm3) and effective atomic number, some of the La2-xGdxHf2O7 transparent ceramics are promising candidates for scintillator hosts.La2-xGdxHf2O7 (x=0–2.0) transparent ceramics were fabricated through vacuum sintering from nano-powders synthesized by a simple combustion method. The phase composition of the powders and final ceramics, the in-line transmittance, microstructures and density of the ceramics were investigated. With the increasing of Gd content, the ceramics maintained the cubic pyrochlore structure, and the lattice parameters decreased, whilst the densities increased linearly. All the ceramics were transparent. The highest in-line transmittance was 76.1% at 800 nm (x=1.2). With high density (7.91–8.88 g/cm3) and effective atomic number, some of the La2-xGdxHf2O7 (x=0–2.0) transparent ceramics are promising candidates for scintillator hosts.

The paper reports the use of Y2O3 and La2O3 co-doping as a composite sintering additive for the fabrication of γ-AlON transparent ceramics by pressureless sintering. The Y3+ enhanced the mobility of grain boundary and accelerated the grain growth, whereas the La3+ has the opposite effect on grain growth. The sintering additives could cause the formation of liquid phase during sintering, which would greatly promote the densification and eliminate pores. High transparent AlON ceramics with the in-line transmittance of 80.3% at 400 nm wavelength have been prepared when the concentration of sintering additives was 0.12 wt% Y2O3 and 0.09 wt% La2O3.

•Gelcasting forming and vacuum sintering was developed to prepare alumina ceramics.•A spontaneous gelling system and a powder with a low BET were employed.•High solids loading slurry ensured low porosity and small pore size in green body.•Optical microscopy revealed that there were less residual pores in sintered ceramics.•In-line transmittance of resultant ceramic reached as high as 53.6% at 600 nm.Translucent alumina ceramics with high transparency were successfully prepared via gelcasting and vacuum sintering. Effects of powder properties and copolymer content on solids loading of the slurry and then on the porosity, microstructures of both green bodies and sintered ceramics, as well as optical transmittance of the final ceramics were extensively investigated. Fine and de-agglomerated alumina powder was effective to prepare slurries with higher solids loading. They resulted in lower porosity (higher relative density) and smaller pore size in green bodies. After sintered at 1860 °C for 6 h in vacuum, the resultant ceramics from higher solids loading had less residual pores. In-line transmittance of the ceramic (1 mm thick) from 55 vol% solids loading reached as high as 53.6% at 600 nm, much higher than previously reported data (<30%).

•A epoxy gelling system was firstly employed to gelcast YAG transparent ceramics.•Transmittance of the ceramic reached 83.0% at 1064 nm, close to theoretical value.•Large, sheet-like ceramic (60 × 50 × 0.5 mm3) with high optical quality was obtained.A transparent yttrium alumina garnet (YAG) ceramic was successfully prepared by gelcasting a mixture of Al2O3 and Y2O3 powders aided by a reactive sintering process. A water-soluble epoxy resin gelling system was employed to fabricate large, sheet-like ceramic pieces. A stable slurry with 75 wt% solid loading and low viscosity was prepared. After casting, drying, and pre-sintering, vacuum sintering was performed at 1720 °C for 6 h. Homogeneous microstructures of both pre-sintered and sintered bodies were obtained. The resulting YAG ceramic (60 × 50 × 0.5 mm3) with an average grain size of 10 μm had an in-line transmittance of 83.0% and 80.7% at the wavelength of 1064 nm and 400 nm, respectively.

Wet alumina green bodies with a dimension of 400 mm ×50 mm ×10 mm, respectively gelcast by PIBM (a copolymer of isobutylene and maleic anhydride) and EA (epoxy-amine) gel system, were dried at controlled temperature and humidity. Microstructure evolution and drying behavior of the green bodies with different solids loading and organic network were investigated. Pores among alumina particles became smaller and the constant rate period (CRP) became shorter with the increased solids loading or organic network. Further, the shrinkage of the body using PIBM ceased earlier and was smaller than that of the body using EA gel system. The typical microstructure of the body using PIBM gel system was thin organic networks on the particles and gradually a cocooned structure evolved during drying. While, the body using EA gel system had dense organic networks which evolved into a dense layer and strand-like structure around the particles. Such microstructures played different roles in water transportation and stress relaxation. As a result, the PIBM body was successfully dried without malformation but the EA body was bowing.

Alumina spherical green bodies (∅≈75 mm) prepared by gelcasting were presintered at 1000 °C. Green bodies׳ temperature difference between the center and surface as a function of solids loading, heating rate, and gel system were extensively investigated. Temperature in the center was lower than that on surface of the alumina spheres cast using PIBM gel system during presintering, and temperature difference (ΔT:Tcenter−Tsurface) slightly decreased with increased solids loading but significantly increased with heating rate. Temperature difference (ΔT<0) generated compressive stress near the surface and tensile stress in center, which led to a porosity difference between the center and the surface. Moreover, the porosity exponentially decreased with increased compressive stress. Tensile stress in the center of the green body cast using PIBM gel system was estimated as 0.3 MPa, which was less than the minimal strength. Therefore, the body was successfully presintered without cracking. However, cracking was found on the surface of the body cast by epoxy-amine gel system, which exhibited a more severe temperature difference fluctuation.

A series of transparent ceramics with the composition of La2−xLuxZr2O7 (x = 0−2.0) were prepared by solid-state reactive sintering in vacuum. With the increase of Lu content (x), phase transition from pyrochlore to defective fluorite occurred and a two-phase region existed in the range of x = 0.6−1.2. Grain sizes of the pyrochlore phase dominated samples (x < 0.5) were 11−14 μm, and that of the defective fluorite phase dominated samples were larger than 60 μm. However, grain sizes of the samples in the two-phase region were smaller than 3 μm. The La0.8Lu1.2Zr2O7 ceramic with the smallest grain size (∼2.5 μm) reached a highest in-line transmittance of 72.4% at 1100 nm among all the samples.

•Combustion method and vacuum sintering were used to fabricate La2-xGdxZr2O7.•The lattice parameters decreased with the increase of Gd3+ concent(x).•The absorption edge of the transmittance curves shifted to UV region from 0.4-2.0.• All the ceramics have high n (2.08), making them candidate for optical lens.•With the increase of x, the effective atomic number and density increased.Transparent La2−xGdxZr2O7 (x = 0–2.0) ceramics were prepared via vacuum sintering from nanometric powders synthesized by a simple combustion method. The changes of phase composition, morphology and in-line transmittance of the resulting ceramics with Gd3+ content’s variation were investigated. With the increase of Gd3+ content, the samples keep the pyrochlore structure, but the X-ray diffraction peaks of the powders and ceramics shift to higher angle as the lattice parameters become smaller. All the ceramics are transparent with high in-line transmittance and high refractive index (2.08 @ 632.8 nm, x = 0.4–1.6). These results indicate that La2−xGdxZr2O7 ceramics might be used as optical lens. Moreover, with the increase of Gd3+ content, the effective atomic number and density of the ceramics increase, therefore making them promising host candidates for scintillators.

Highly transparent aluminum oxynitride ceramic was fabricated by presintering at 1900 °C and further sintering by hot isostatic pressing (HIP) at 1900 °C under a 190 MPa argon atmosphere. For a 4.2 mm thick specimen doped with 0.1 wt.% Y2O3–La2O3, the inline light transmittance reached 78.8% at 600 nm and 84.4% at 1084 nm, respectively. Unlike the inhomogeneous microstructure of pressureless sintered specimen (grain size ∼150 μm), which contained substructure and twinning grains, the HIPed specimen had a homogeneous and refined microstructure with average grain size of ∼45 μm.

Highlights•The up-conversion emission intensity of NaYF4:Yb3+, Er3+ after silica coating was significantly enhanced.•The up-conversion intensity ratio between red emission and green emission was decreased after silica coating.•The mechanism of up-conversion luminescence and emission intensity change was illuminated.NaYF4:Yb3+, Er3+ nanoparticles were synthesized through co-precipitation using EDTA as the chelator. The monodisperse core–shell structure NaYF4:Yb3+, Er3+@sSiO2 were synthesized by sol–gel method using TEOS as silica source. The core–shell–shell structure NaYF4:Yb3+, Er3+@sSiO2@mSiO2 were prepared by coating mesoporous silica shell onto NaYF4:Yb3+, Er3+@sSiO2 through hydrolysis of TEOS using CTAB as mesoporous template. Effects of CTAB concentration, TEOS concentration and different addition ways of CTAB were studied. The results show that the NaYF4:Yb3+, Er3+@sSiO2@mSiO2 nanoparticles with uniform amorphous silica and mesoporous silica shell were successfully synthesized. Up-conversion spectra of these nanoparticles were recorded with 980 nm laser excitation under room temperature. There were no changes of the emission centers of nanoparticles before or after silica coating, but the emission intensities of nanoparticles after silica coating were significantly enhanced. Furthermore, the relative intensity ratio between red emission and green emission was decreased after silica coating.

•The double-emitting blue phosphor Sr3(PO4)2: Eu2+, Dy3+ was prepared.•A broad excitation band between 250 and 450 nm was observed.•Two emission peaks located 410 nm and 485 nm were observed.•Luminescence center Eu2+ occupies both 6-coordinated and 10-coordinated Sr2+ sites.•The phosphor is emission-tunable by varying the Dy3+ concentration.Double-emitting blue phosphor Sr3(PO4)2: Eu2+, Dy3+ was synthesized by solid state reaction under H2 atmosphere. XRD exhibited the pure hexagonal phase of the prepared phosphor. The photoluminescence results showed that all samples had intense broad absorption band between 250 and 450 nm, which matched well with the near-UV (350–420 nm) emission band of InGaN-based chips. The emission spectrum of Sr3(PO4)2: Eu2+, Dy3+ consisted of two broad bands, peaked at 485 nm and 410 nm, which originated from two luminescent centers, related to 4f65d1 → 4f7 transition of Eu2+ in six-coordinated Sr(I) and ten-coordinated Sr(II) sites respectively. The intensity ratio of two emission bands could be easily tuned by adjusting Dy3+ co-doping content, which resulted in color-tunable luminescence in bluish green region to purplish blue region.

Eu2+ doped γ-AlON transparent ceramics have been prepared by the solid-state reaction sintering method. The influences of Eu concentration on both strength, transparency and luminescence properties of the as-prepared samples were discussed. The strength and transparency decreased as Eu content increased. Two bands were observed in the emission spectrum of each sample. One (B1) was narrow and centered at around 401 nm, the other (B2) was comparatively broader, and the location of its center as well as the intensity ratio of peak values of B2 to that of B1 varied with Eu content.Highlights► Eu2O3 was an effective sintering aid in fabrication of transparent γ-AlON ceramics. ► Eu-doped transparent γ-AlON ceramics exhibited broad emission spectra composed of two bands. ► The relationship between crystal position of Eu2+ ions and luminescent properties was given.

•Gd3+ and/or Mn2+ doped NaYF4 possessed cubic structure was synthesized by a solvothermal route with PEI as the surfactant.•The luminescence intensity of 20 mol% Gd3+ doped NaYF4 was stronger than that of any other concentration under the 272 nm excitation.•The luminescence intensity of Mn2+ around 561 nm regions in NaYF4: Gd3+ was increased with the increase of Gd3+ doped concentration.•The strong green emission can be observed with naked eyes.•The energy level of 6PJ of Gd3+ ions played a key role in the energy transfer process between Gd3+ and Mn2+.NaYF4 co-doped with Gd3+ and Mn2+ nanocrystals were synthesized by a solvothermal route with PEI as the surfactant. X-ray diffraction analyses evidenced that the phases of synthesized NaYF4: Gd3+, Mn2+ nanocrystals were all pure cubic structure. The luminescence intensity of 20 mol% Gd3+ doped NaYF4 was stronger than that of any other concentration under the 272 nm excitation. The photoluminescence emission intensity of NaYF4: Gd3+, Mn2+ at 561 nm was increased with the increase of Gd3+ doping concentration. The energy transfer mechanisms between Gd3+ and Mn2+ were discussed. The results showed that the energy level of 6PJ of Gd3+ ions played a key role in the energy transfer process. Furthermore, the Mn2+ ions doped NaYF4: Gd3+ nanocrystals with a strong green emission and proper paramagnetism originated from Gd3+ and Mn2+ may find potential applications in bio-probes and magnetic resonance imaging.

A simple combustion method was used to synthesize LaGdZr2O7 powder and LaGdZr2O7 transparent ceramic was prepared by vacuum sintering at 1850 °C for 6 h. The final transparent ceramic, with a density of 6.46 g/cm3, has an in-line transmittance of 70.7% at 1000 nm and a refractive index of 2.08 at 632.8 nm.

•Novel particle size and shape manipulation mechanism was studied.•Size, shape, crystal structure and composition could be controlled by a single parameter.•Emergent property was observed in the mesocrystal.It is interesting that the natural phenomenon of chelator citrate stabilizing nanocrystals in bone could be learned and utilized in manipulating man-made nano-materials. During the synthesis of upconversion luminescent fluorides, it appeared that, the more citrate participated, the more difficult the resultant material transformed from cubic phase KY3F10 nanocrystals, through hexagonal KYF4, to orthorhombic YF3 crystals or mesocrystals, which clearly indicates the critical role of citrate in stabilizing KY3F10 nanocrystals. Particles with different size, morphology, composition, and phase were controlled through the only variation of citric acid content. Meanwhile, the self-assembled mesocrystals showed emergent luminescent property with 63 times higher intensity, compared with the nanocrystals. The same sized YF3 single crystals and YF3 mesocrystals also exhibited dramatic difference in luminescent intensity.

Highly transparent Er:YAG ceramics with different Er doping concentrations were fabricated by a reactive sintering method under vacuum. The optical properties and the microstructures of the Er:YAG ceramics were investigated. For 3 mm thickness samples, the in-line transmittances of the as-fabricated Er:YAG ceramics at the wavelength of 1100 nm and 400 nm were about 84% and 82%, respectively. The micrograph of the Er:YAG transparent ceramics exhibited a pore-free structure and the average grain size was about 10 μm. The grain boundary of the ceramics was clean and no secondary phase was detected. The absorption and emission spectra, the fluorescence decay traces of the Er:YAG ceramics were measured and discussed. The ceramics obtained may have potential use for eye-safe solid-state lasers partly replacing Er:YAG single crystals.Highlights► High quality transparent Er:YAG ceramics were fabricated by a reactive sintering method. ► The in-line transmittances of the Er:YAG ceramics at the wavelength of 1100 nm and 400 nm were about 84% and 82%, respectively. ► The absorption and emission spectra, the fluorescence lifetimes of the Er:YAG ceramics were measured and discussed in detail.

Persistent luminescence phosphors, Tb3+, Sr2+ co-doped Lu2O3 ceramics were fabricated under vacuum at 1870 °C. A strong green color afterglow could be observed after excitation with 254 nm UV radiation. The afterglow was found to be roughly doubled in intensity upon stimulation with violet or blue-green photons (∼400–600 nm) as well as upon IR radiation (∼800–980 nm). Hence, the enhancement of the green afterglow could also be attained by means of natural sunlight. Short wavelength UV radiation was found to induce an extrinsic absorption band below about 600 nm and extending into the deep UV. This was mirrored by a simultaneous decrease of the absorption by Tb3+ around 230–320 nm. The extrinsic absorption seemed to contain at least two overlapping components which supposedly resulted from a band related to Tb4+/Tb3+-h and a feature resulting from F/F+ centers created upon irradiation with short UV. This absorption was shown to be reversible, as it disappeared after a short heat treatment at about 300 °C as well as with continuous irradiation of ∼400 nm radiation.Highlights► Lu2O3 was sintered in vacuum, unlike previous atmosphere, at higher temperature. ► Post stimulation with visible and NIR lights for the afterglow was newly studied. ► Short UV radiation was found to induce an extrinsic structured absorption. ► The absorption could disappear upon further light or heat treatment.

Yttrium hafnate (Y2Hf2O7) precursor was prepared by mixing yttrium nitrate and hafnium (IV) chloride with ammonium oxalate aqueous solution. Yttrium hafnate powders with different morphologies were obtained by decomposition of the precipitate precursor at 800 °C for 4 h. The as-prepared particles were characterized by thermo gravimetric analysis (TGA), X-ray diffraction analysis (XRD) and field-emission scanning electron microscopy (FE-SEM). The concentration of the surfactant polyethylene glycol 6000 (PEG6000) was found to play a key role on the morphology of the particles. The possible mechanism of the morphology variation was proposed.

Barium hafnate (BaHfO3) precursor was prepared by mixed barium nitrate and hafnium (IV) chloride with ammonium oxalate aqueous solution. Barium hafnate powders were obtained by decomposition of the precipitate precursor at 800 °C for 2 h. As a dispersant, polyethylene glycol (PEG6000) was used in the precipitation reaction and the influence of dispersant dosage on particle size was studied. X-ray diffraction analysis (XRD) showed the powders was pure cubic BaHfO3 without impurities. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) demonstrated that the size distribution was uniform, and the average grain size was about 50 nm.

Porous silica ceramics with tri-modal pores were prepared, based on the generation of foams from silica/starch composite slurry and the subsequent stabilization of the structure by starch consolidation. The rheology of the original slurry and the foamed one were evaluated and compared. After drying, the green bodies were debindered and sintered at 1250 °C for 5 h. The resulting materials consisted of a hierarchical structure with large-sized cells, moderate-sized pores in cell wall and small-sized voids among silica grains. The compressive strength of the sintered samples varied within the range of 4–17 MPa, corresponding to relative densities of 18–30%.

Sm3+/Eu3+ doped and co-doped Lu2O3 phosphors were prepared via a sol–gel combustion route using citric acid as the fuel. XRD analysis and FTIR spectra showed that the amorphous precursor crystallized in a cubic structure upon heating at 500 °C. TEM images revealed that the particle size of the prepared powders was about 80 nm. Intense reddish orange and red lights were detected at room temperature when the samples were exposed into UV or blue light excitation. Based on the emission spectra of Sm3+/Eu3+ doped and co-doped samples, the possible energy transfer process between Sm3+ and Eu3+ in this system was proposed.

YAG:Tm3+and YAG:Yb3+, Tm3+ nanocrystals were synthesized by a co-precipitation method using ammonium hydrogen carbonate as precipitator. Pure nanocrystals with a crystallite size of 40 nm were obtained after the precipitates were calcined at 800 °C. YAG:Tm3+ and YAG:Yb3+, Tm3+ powder compacts were excited by 796 and 940 nm, respectively. No upconversion emissions were observed either in YAG:Tm3+ and YAG:Yb3+, Tm3+ nanocrystals excited at 796 nm or in YAG:Tm3+ nanocrystals excited at 940 nm. However, the blue emission of YAG:Yb3+, Tm3+ nanocrystals excited by 940 nm was strong enough to be seen by the naked eyes. The emission intensity of this material was also influenced by Yb3+ concentration as well as the calcination temperature.

YAG (Y3Al5O12):Yb3+,Er3+ pure phase nanopowders were prepared by a co-precipitation method in which ammonium hydrogen carbonate was adopted as precipitator. Upconversion emissions were investigated on the nanopowder compacts pumped by a 980 nm continuous wave diode laser, and red emission centered at 660 nm and green emission centered at 563 nm were observed. Energy transfer from Yb3+ to Er3+ ions and cooperative energy transfer between two nearby Er3+ ions were the main mechanisms of upconversion luminescence. Red emission relative to green emission (R/G) was strikingly enhanced with the increase of Yb3+ dopant concentration, and dominant red emission appeared when 20 at% Yb3+ ions were doped. On the other hand, the decrease of calcining temperature was beneficial to the red emission dominance. The reasons for the red emission enhancement were discussed based on the mechanisms of upconversion luminescence.

•Gd3+ and/or Mn2+ doped NaYF4 possessed cubic structure was synthesized by a solvothermal route with PEI as the surfactant.•The luminescence intensity of 20 mol% Gd3+ doped NaYF4 was stronger than that of any other concentration under the 272 nm excitation.•The luminescence intensity of Mn2+ around 561 nm regions in NaYF4: Gd3+ was increased with the increase of Gd3+ doped concentration.•The strong green emission can be observed with naked eyes.•The energy level of 6PJ of Gd3+ ions played a key role in the energy transfer process between Gd3+ and Mn2+.NaYF4 co-doped with Gd3+ and Mn2+ nanocrystals were synthesized by a solvothermal route with PEI as the surfactant. X-ray diffraction analyses evidenced that the phases of synthesized NaYF4: Gd3+, Mn2+ nanocrystals were all pure cubic structure. The luminescence intensity of 20 mol% Gd3+ doped NaYF4 was stronger than that of any other concentration under the 272 nm excitation. The photoluminescence emission intensity of NaYF4: Gd3+, Mn2+ at 561 nm was increased with the increase of Gd3+ doping concentration. The energy transfer mechanisms between Gd3+ and Mn2+ were discussed. The results showed that the energy level of 6PJ of Gd3+ ions played a key role in the energy transfer process. Furthermore, the Mn2+ ions doped NaYF4: Gd3+ nanocrystals with a strong green emission and proper paramagnetism originated from Gd3+ and Mn2+ may find potential applications in bio-probes and magnetic resonance imaging.