Introduction Tremendous efforts are devoted to improving the luminous performance of white light-emitting diodes (WLEDs) as so-called fourth generation lighting source. The most common approach for manufacturing WLEDs is to cover a blue LED chip with a yellow-emitting Y3Al5O12:Ce3+ (YAG:Ce3+) phosphor. This construction has an advantage of high lumen efficiency, but an insufficient red light component in the spectrum leads to a low color rendering index and a high color temperature. An effective solution is to enhance the red emission of YAG:Ce3+ via introducing red-emitting ions such as Pr3+, Eu3+, Sm3+, Mn2+, Mn4+ and Cr3+. In Y3Al5O12 and many other garnet-structured aluminates, Mn2+, Mn4+ and Cr3+ are suffered from serious luminescent thermal quenching, while Pr3+ and Sm3+ are severely limited by concentration quenching. In comparison, the luminescence of Eu3+ in YAG is highly anticipated because it has an ion radius similar to Y3+, which is beneficial to increasing doping concentration and reducing luminescence quenching. In this paper, a series of Ce3+-Eu3+ co-doped YAG transparent ceramics with different Eu concentrations were fabricated, and the effect of Eu3+-doping on the structure, optical transmittance and photoluminescence properties was also evaluated.Methods Ce0.01Y2.99-xEuxAl5O12 (0 < x < 0.75, Eu contents of 0-25%) fluorescent ceramics were synthesized in vacuum via high-temperature solid-state reaction. High purity Y2O3, α-Al2O3, Eu2O3, and CeO2 powders were used as starting materials. Oleic acid and tetraethoxysilane were used as a molding agent and a sintering aid, respectively. These materials were weighed in stoichiometric ratios, and ground in ethanol for 20 h. The obtained slurry was dried, ground and sieved through a 100?mesh sieve. The resulting powders were made into 20 mm discs under uniaxial pressure of 5-10 MPa, annealed in oxygen atmosphere at 800 ℃ for 10 h, and cold isostatically pressed under 200 MPa. Finally, the ceramics was mechanically thinned and polished to the thickness of 1mm for the subsequent structural and performance characterizations.Results and discussion For YAG:Ce3+/Eu3+ ceramics sintered at 1 600 ℃, a garnet phase appears when Eu content is in the range of 0-25%. However, for the ceramics sintered at 1700 ℃, the grains grow as Eu content (x) increases, and eventually become over-sintered and textured when x ≥ 15%. YAG:Ce3+/Eu3+ ceramics show some characteristic absorption/emission bands for both Ce3+ and Eu3+. For Ce3+, the PLE spectrum involves two broad bands at 320-370 nm and 400-520 nm, and the PL spectrum spans from 440 nm to 650 nm and is dominated by yellow-green emission. Unlike a broad band absorption/emission of Ce3+, the PLE/PL spectra of Eu3+ are composed of a series of lines originating form f-f transitions. There are some intense excitation lines around the maximum absorption locating at 395 nm, constructing a quasi-continuous broad band. It is therefore inferred that YAG:Ce3+/Eu3+ ceramics can be efficiently pumped by NUV LED chips. The PL spectrum of Eu3+ ions covers the spectral range from orange to deep red. The orange emission from 5D0→7F1 transition is more intense than the red emission from 5D0→7F2 transition because Eu3+ ions occupy the lattice sites with an inversion symmetry.The PL spectra of YAG:Ce3+/Eu3+ ceramics vary with excitation wavelength due to the difference in absorption spectra of Ce3+ and Eu3+. Therefore, the effect of excitation wavelength on the PL spectra of YAG:Ce3+/Eu3+ ceramics with various Eu3+ concentrations were investigated. Under the excitation at 363 nm and 466 nm, the emission bands/peaks of both Ce3+ and Eu3+ appear. The PL spectrum excited at 442 nm contains almost only a broadband emission of Ce3+ ions, as Eu3+ ions hardly absorb photons near this wavelength. In contrast, the PL spectrum under the excitation at 395 nm is dominated by Eu3+ emission due to a weak absorption of Ce3+. Clearly, the PL spectrum and color coordinates of YAG:Ce3+/Eu3+ exhibit an excitation wavelength dependence, which has a potential application. In addition, under excitation at 442 or 466 nm, the emission of Eu3+ is weak, while that of Ce3+ rapidly decays with increasing Eu content. As a result, the emission intensity of YAG: Ce3+/Eu3+ ceramics decreases rapidly with the increase of Eu content when excited by blue light. However, when excited by UV light (at 363 nm), the attenuation of Ce3+ emission is compensated by the enhancement of Eu3+ emission to some extent, and the overall photoluminescence intensity can be maintained at a high level. Especially, the samples with Eu contents of 1%-3% can emit an intense orange-yellow light, thus increasing the red component of YAG:Ce3+ effectively. This kind of phosphor has a certain practicability in WLEDs. Also, under the excitation at 395 nm, the PL spectrum originates mainly from the f-f transitions of Eu3+, and the quenching concentration of Eu3+ in YAG:Ce3+ ceramics is 9%, which is much higher than that of Pr3+ (i.e., 0.8%) and Sm3+ (i.e., 3%). For rare-earth ions with forbidden f-f transition, a high quenching concentration is crucial for achieving a high fluorescence emission intensity and meeting the inevitable requirements of LED applications.Thermal quenching is a key factor in evaluating luminescent materials. For the YAG:Ce3+/Eu3+ ceramics with Eu contents of 1% and 9%, the profile of PL spectrum does not change with temperature, leading to a good temperature stability of color coordinates. The relative PL intensity at 575 K remains 83% and 61% of that at room temperature (i.e., 300 K) for 1% and 9% Eu3+-doped samples, respectively, indicating that the photoluminescence thermal quenching in YAG:Ce3+/Eu3+ ceramics is rather weak. Furthermore, the relative PL intensity (IT/I0) and the ratio of emission arising from 5D0 to different 7FJ levels, including (5D0→7F1)/(5D0→7F2)、(5D0→7F4)/(5D0→7F2) and (5D0→7F1,2)/(5D0→7F4), vary linearly with temperature. This feature can be used in the field of fluorescent thermometers, which is one of the potential applications of YAG:Ce3+/Eu3+ transparent ceramics.Conclusions In YAG: Ce3+/Eu3+ transparent ceramics, Eu3+ could be excited by UV (at 363 nm), NUV (at 380-405 nm), and blue (at 466 nm) light, emitting a series of lines in the spectra range from 580 nm to 750 nm, i.e., from orange to deep red. These PL lines had an important application in lighting and display. Eu3+ showed the maximum emission when its concentration was between 5% and 9%. The high quenching concentration had a advantage in enhancing the red component in the emission spectrum of YAG:Ce3+. Furthermore, the red component in the spectrum could be regulated via adjusting the doping concentration of Eu3+. However, the increase in Eu content led to a decrease in Ce3+ emission intensity. The similar phenomena appeared in various dual/multi activators co-doped systems. The solution of this common problem was crucial for the application of this kind of luminescent materials in WLEDs. A highlight of YAG:Ce3+/Eu3+ transparent ceramics was that they could absorb NUV light from 380 nm to 405 nm and emit an orange-to-red light with a high quantum efficiency and a high thermal stability. Therefore, they could be used as red-emitting phosphors excited by NUV LED chips. Besides, the ratios of emission arising from 5D0 to different 7FJ levels varied linearly with temperature, having a promising potential application in the field of fluorescent thermometers.