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Abu Yousuf, Tomoya Ohno, [Jian Xu](https://orcid.org/0000-0002-1040-5090), [Takayuki Nakanishi](https://orcid.org/0000-0003-3412-2842), [Koji Morita](https://orcid.org/0000-0001-6040-7054), [Byung-Nam Kim](https://orcid.org/0000-0003-4302-462X), [Tohru S. Suzuki](https://orcid.org/0000-0001-9458-6863), [Hiroaki Furuse](https://orcid.org/0000-0002-9008-1697)

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[Transparent Ce<sup>3+</sup>-doped fluorapatite (FAP) ceramics fabricated by spark plasma sintering (SPS)](https://mdr.nims.go.jp/datasets/fe188bfd-a0cb-4300-90c6-5c7c1e0028eb)

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Transparent Ce3+-doped fluorapatite (FAP) ceramics fabricated by spark plasma sintering (SPS)Research Article Vol. 14, No. 9 / 1 Sep 2024 / Optical Materials Express 2114Transparent Ce3+-doped fluorapatite (FAP)ceramics fabricated by spark plasma sintering(SPS)ABU YOUSUF,1,2,4 TOMOYA OHNO,2 JIAN XU,3 TAKAYUKINAKANISHI,1 KOJI MORITA,1 BYUNG-NAM KIM,1 TOHRU S.SUZUKI,1 AND HIROAKI FURUSE1,*1National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan2Kitami Institute of Technology, 165 Koen-cho, Kitami, Hokkaido 090-8507, Japan3International Center for Young Scientists (ICYS), National Institute for Materials Science (NIMS), Tsukuba305-0044, Japan4m3225480015@std.kitami-it.ac.jp*FURUSE.Hiroaki@nims.go.jpAbstract: Polycrystalline Ce3+-doped fluorapatite (Ce:FAP) transparent ceramics with finemicrostructures were fabricated through liquid-phase synthesis for the initial powder and sparkplasma sintering (SPS) for full densification. These ceramics were confirmed to have a single-phase crystal structure, and the average grain sizes were determined to be 139 and 135 nm forthe 1 and 2 at.% Ce-doping concentrations, respectively. Their emission spectra revealed thatthe fabricated ceramics convert UV light to visible light emission due to the 5d→4f electronictransition of Ce3+. These ceramics are expected to be useful in photonic applications.© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement1. IntroductionCerium (Ce)-doped materials are widely used as phosphors in the visible wavelength range forvarious applications such as white LED [1,2], bio-imaging [3,4], and scintillators [5–7]. Ce3+is considered as a highly suitable activator because its parity-allowed 5d-4f electron transitiongenerates a wide spectrum of colors in the visible wavelength range with high absorptioncoefficient. However, the emission wavelength strongly depends on the host material, and manyresearchers are investigating the fabrication of efficient Ce-doped phosphor materials.Fluorapatite (Ca10(PO4)6F2: FAP) is a fascinating host material because it exhibits excellentfluorescence properties, making it a valuable choice for laser applications [8]. Furthermore, sinceFAP is a prominent biomaterial, Ce-doped FAP phosphors are expected to be promising optionsfor bio-photonic applications. In comparison to hydroxyapatite (HAP), FAP exhibits superiorluminescence efficiency owing to the luminescence quenching characteristics of the OH− grouppresent in HAP [9]. Recent research efforts have been focused on exploring fluorapatite dopedwith rare earth (RE) ions such as Eu3+, Sm3+, Dy3+, and Pr3+ etc. to investigate their luminescentproperties [10,11].In 2006, the emission properties of Ce:FAP powder under UV and X-ray irradiation werereported [12]. The emission spectra were recorded at a wavelength of approximately 400 nm.A similar investigation on Ce:FAP nanorods under UV excitation was conducted in 2010 [13].The rapid decay curves observed in these studies suggest the possibility of using Ce:FAP as ascintillator material. However, to the best of our knowledge, transparent Ce:FAP materials havenot yet been reported.Typically, FAP has a hexagonal crystal structure and is not adequately transparent in polycrys-talline ceramics because of grain boundary scattering due to birefringence. In such non-cubic#530288 https://doi.org/10.1364/OME.530288Journal © 2024 Received 4 Jun 2024; revised 11 Jul 2024; accepted 22 Jul 2024; published 2 Aug 2024-- Compressed PDF version --https://orcid.org/0000-0002-9008-1697https://doi.org/10.1364/OA_License_v2#VOR-OAhttps://crossmark.crossref.org/dialog/?doi=10.1364/OME.530288&amp;domain=pdf&amp;date_stamp=2024-08-02Research Article Vol. 14, No. 9 / 1 Sep 2024 / Optical Materials Express 2115ceramics, grain boundary scattering can be described as follows [14]:γ(λ) =3π2d∆n22λ2 (1)where λ is wavelength of light, d is the average grain size, and ∆n represents average difference inthe refractive indices at grain boundaries. One way to realize transparent ceramics for non-cubicmaterials is to reduce the average grain size d. We previously reported Nd:FAP, Yb:FAP, andNd:S-FAP by controlling the grain size of the ceramics to approximately 100 nm and demonstratedtheir laser oscillation [15–17]. Therefore, it is expected to realize transparent Ce:FAP bulkceramics using a process similar to that used in our previous studies.The purpose of this study is to fabricate transparent Ce:FAP ceramics with fine microstructure forvisible-light emission phosphors and to measure their emission properties, including fluorescencetime decay. Ce:FAP transparent ceramics were fabricated using liquid-phase synthesis toobtain fine initial powders. Densification of the ceramics was accomplished through sparkplasma sintering (SPS), which allows precise control of the sintering behavior and grain size.Additionally, their emissions in the visible wavelength region were observed under UV lightexcitation. Finally, we conducted lifetime measurements to evaluate their practical applicability,especially as scintillators, owing to the short fluorescence decay time observed in the previouslyreported Ce:FAP nano powder [13].2. Experimental methodA schematic of the ceramic preparation process is shown in Fig. 1. A liquid-phase synthesiswas used to prepare the Ce:FAP initial powder. First, Ce-doped hydroxyapatite (Ce:HAP)precursor was synthesized using Ce(NO3)3.6H2O (99.99%, Sigma-Aldrich), Ca(OH)2 (99.9%,Kanto Chemical), and H3PO4 (85%, Kishida Chemical) as the raw materials. The Ce dopingconcentrations used were 1 and 2 at.%. Subsequently, an appropriate amount of trifluoroacetamide(CF3CONH2) (98%, Tokyo Chemical) was added to the Ce:HAP precursor, and the mixture wasthen heated to 600 °C in an electric furnace to obtain the Ce:FAP powder. The resulting powderwas sieved through a screen.A spark plasma sintering machine (LABOX-315, Sinter Land, Japan) was used to sinter theCe:FAP powder. Initially, the powder was inserted and uniaxially pressed into a 10 mm graphitemold using a graphite punch on each side. Two circular carbon sheets were sandwiched betweencubic materials is to reduce the average grain size d. We previously reported Nd:FAP, Yb:FAP, and Nd:S-FAP by controlling the grain size of the ceramics to approximately 100 nm and demonstrated their laser oscillation [15–17]. Therefore, it is expected to realize transparent Ce:FAP bulk ceramics using a process similar to that used in our previous studies. The purpose of this study is to fabricate transparent Ce:FAP ceramics with fine microstructure for visible-light emission phosphors and to measure their emission properties, including fluorescence time decay. Ce:FAP transparent ceramics were fabricated using liquid-phase synthesis to obtain fine initial powders. Densification of the ceramics was accomplished through spark plasma sintering (SPS), which allows precise control of the sintering behavior and grain size. Additionally, their emissions in the visible wavelength region were observed under UV light excitation. Finally, we conducted lifetime measurements to evaluate their practical applicability, especially as scintillators, owing to the short fluorescence decay time observed in the previously reported Ce:FAP nano powder [13]. 2. Experimental methodA schematic of the ceramic preparation process is shown in Fig.1. A liquid-phase synthesis was used to prepare the Ce:FAP initial powder. First, Ce-doped hydroxyapatite (Ce:HAP) precursor was synthesized using Ce(NO3)3.6H2O (99.99%, Sigma-Aldrich), Ca(OH)2 (99.9%, Kanto Chemical), and H3PO4 (85%, Kishida Chemical) as the raw materials. The Ce doping concentrations used were 1 and 2 at.%. Subsequently, an appropriate amount of trifluoroacetamide (CF3CONH2) (98%, Tokyo Chemical) was added to the Ce:HAP precursor, and the mixture was then heated to 600 °C in an electric furnace to obtain the Ce:FAP powder. The resulting powder was sieved through a screen.A spark plasma sintering machine (LABOX-315, Sinter Land, Japan) was used to sinter the Ce:FAP powder. Initially, the powder was inserted and uniaxially pressed into a 10 mm graphite mold using a graphite punch on each side. Two circular carbon sheets were sandwiched between the Ce:FAP powder and the punches. The electrically conductive die and punches were heated at 5 °C/min heating rate under vacuum; uniaxial pressure of 80 MPa was applied to ensure that the particles get compacted during sintering. A thermocouple was used to measure the temperature of the mold; sintering temperature and holding time were maintained at 950 °C and 20 minutes, respectively. Upon completion of sintering, the ceramic surfaces were mirror polished to remove surface roughness for characterization.Figure 1. Schematic diagram of fabricating of Ce:FAP ceramics.The phase structure of the Ce:FAP powder and the sintered ceramics were investigated using X-ray diffraction (XRD; Ultima IV, Rigaku, Japan). The microstructural characterization of the transparent Ce:FAP ceramics was performed using field-emission scanning electron Ce:HAP PrecursorSuspensionMaterialH3PO4MaterialCa(OH)2MaterialCe(NO3)3Ce:FAP CeramicsCe:FAP PowderFluorine CompoundCF3CONH2 Calcination Spark Plasma SinteringFig. 1. Schematic diagram of fabricating of Ce:FAP ceramics.-- Compressed PDF version --Research Article Vol. 14, No. 9 / 1 Sep 2024 / Optical Materials Express 2116the Ce:FAP powder and the punches. The electrically conductive die and punches were heated at5 °C/min heating rate under vacuum; uniaxial pressure of 80 MPa was applied to ensure that theparticles get compacted during sintering. A thermocouple was used to measure the temperatureof the mold; sintering temperature and holding time were maintained at 950 °C and 20 minutes,respectively. Upon completion of sintering, the ceramic surfaces were mirror polished to removesurface roughness for characterization.The phase structure of the Ce:FAP powder and the sintered ceramics were investigated usingX-ray diffraction (XRD; Ultima IV, Rigaku, Japan). The microstructural characterization of thetransparent Ce:FAP ceramics was performed using field-emission scanning electron microscopy(FE-SEM; JSM-6701F, JEOL, Japan). The average grain size d was calculated from themicrostructural images by analyzing the sizes with more than 500 grains, and each grain wasassumed to be spherical in shape.An UV/VIS/NIR spectrometer (UV-3600Plus, Shimadzu, Japan) was used to analyze theoptical in-line transmittance spectra T; 5-mm optical aperture was used for the measurements.The total loss coefficient of the ceramics δ, including scattering and absorption coefficient, wascalculated using the following equation:T = (1 − R)2exp(−δt) (2)where t is the thickness of the samples and R represents the Fresnel loss of FAP, which is writtenas R= (1-n)2/(1+ n)2 and calculated using refractive index dispersion n(λ) [18].The photoluminescence excitation (PLE) and photoluminescence (PL) spectra, including theluminescence decay curves, were recorded using a spectrometer (FLS1000, Edinburg Instruments,UK) equipped with a xenon arc steady-state / flash lamp as the excitation source.3. Results and discussion3.1. Crystal structure and microstructureFigure 2 shows the XRD patterns of the 1 and 2 at.% Ce:FAP powder and ceramic samples,respectively. The XRD diffraction peaks of the synthesized powder and fabricated ceramics werecompared with the standard data in JCPDS file No. 15-0876. All the diffraction peaks of both thepowders and ceramics were indexed to the pure hexagonal apatite phase of calcium fluorapatite.The peaks of the powder and ceramics are in the same position matching well with the standardcard confirming the single phase of both as-prepared powder and ceramics.However, slight discrepancies could be observed in the peak intensities of the sintered ceramics.For example, the relative peak intensities of the (002) and (112) peaks for the ceramics weresmaller, whereas those of the (210) and (300) peaks were larger than those of the powder. Thisphenomenon was described by Watanabe et al. [19] as well as observed in our previous study[20]. During SPS, the uniaxial pressure is believed to align the c-axis of the crystal grainsperpendicular to the direction of the pressure, thereby promoting anisotropic grain growth, whichis reflected in the XRD pattern as changes in peak intensities (Fig. 2).The micrographs displayed in Fig. 3(a)–(d) show FE-SEM images of both powder and ceramics.The particles of both powders were either nearly spherical or elliptical in shape at 600°Ccalcination temperature. The microstructural images revealed that the fabricated ceramicsexhibited a dense and uniform microstructure. The average grain size d was calculated using theaverage cross-sectional area per grain as Sg = (1/6) πd2 [14,21]. The estimated average grainsizes d of these ceramics were 139 and 135 nm, respectively, which are lower than the wavelengthof visible light (380–780 nm).3.2. Optical propertiesThe in-line transmittance spectra of Ce:FAP ceramics are shown in Fig. 4. The thickness of the 1and 2 at.% Ce:FAP ceramics were 1.5 and 1.6 mm, respectively, but the in-line transmittance-- Compressed PDF version --Research Article Vol. 14, No. 9 / 1 Sep 2024 / Optical Materials Express 211730 40 5002462θ (degree)Intensity (a.u.)JCPDS No. 15−08761% Ce:FAP Ceramic2% Ce:FAP Ceramic210211202 3102% Ce:FAP Powder1% Ce:FAP Powder002112301300212102311 222213312113203Fig. 2. XRD patterns of 1 and 2 at.% Ce:FAP powders and ceramics.500 nm500 nm(a)(c)500 nm(d)500 nm(b)Fig. 3. FE-SEM images of (a) 1 at.% Ce:FAP powder, (b) 1 at.% Ce:FAP ceramics, (c) 2at.% Ce:FAP powder, and (d) 2 at.% Ce:FAP ceramics. The Ce-FAP powders calcinatedat 600 °C while the ceramics sintered at 950 °C temperature. The residual pores in theceramics are indicated by red circles.-- Compressed PDF version --Research Article Vol. 14, No. 9 / 1 Sep 2024 / Optical Materials Express 2118was calculated for the 1 mm-thick ceramic from Eq. (2) to compare with our previous study[20]. The dashed black line represents the theoretical FAP transmittance calculated using therefractive-index dispersion n(λ). The red line represents the in-line transmittance spectrum of theun-doped FAP ceramics sintered at 1000 °C measured using a thermopile obtained from Ref.[20], whose grain size is comparable with that of the Ce:FAP ceramics.decrease at approximately 330 nm is ascribed to Ce3+ ion absorption. At a wavelength of 500 nm, the in-line transmittance for the 1 and 2 at.% Ce:FAP ceramics are 67.5% and 64.4%, and the total loss coefficients are δ1 = 2.7 cm-1 and δ2 = 3.2 cm-1, respectively. The scattering coefficients due to grain boundaries were roughly determined using evaluated average grain size d and equation (1), resulting in γ1 = 0.91 cm-1 and γ2 = 0.89 cm-1 at the same wavelength. The grain-boundary scattering loss is lower for the 2 at.% ceramics owing to the smaller grain size. The discrepancy observed in the total loss coefficient could stem from Ce3+ absorption and the lower number of remaining pores identified in the FE-SEM image as shown in Fig. 3. At 1000 nm wavelength, the in-line transmittances for 1 and 2 at.% Ce:FAP ceramics are 85.8% and 84.7%, respectively, which are close to theoretical transmittance. A low-temperature hot-isostatic-pressing (HIP) treatment may be effective to eliminate residual pores and further increase the transmittance [23].Figure 4. In-line transmittance of (a) 1 at.% and (b) 2 at.% Ce:FAP transparent ceramics.3.3 Fluorescent propertiesThe Ce3+ ion possessing a 4f1 site can function as an effective emission center due to the parity and spin-allowed 4f-5d transitions. The PLE/PL of the 1 and 2 at.% Ce:FAP ceramics, including their decay curves, are shown in Fig. 5. The excitation spectra in Fig. 5(a) consist of a broad wavelength range of 240–400 nm, obtained by monitoring the emission at 413 nm. The strongest band was detected with a maximum peak at 310 nm accompanied by a less intense shoulder band at 292 nm. The excitation peaks represent the transitions from the ground state 2F5/2 and 2F7/2 of Ce3+ to the excited 5d states [24–25]. In Fig. 5(a), the ceramic emission spectra encompass the 350–700 nm range with an intense emission centered at 413 nm under 310 nm excitation wavelength. The emission is induced by the inter-configurational 5d→4f transition from the excited 5d state to the ground state of Ce3+ ions [26]. The inset images correspond to the luminescence of the ceramics under UV light excitation (370 nm) observed in the dark. The images confirm the emission of light from both ceramics.The Ce3+ ions exhibited a broad emission spectrum, indicating emission via multiple transitions. FAP hosts contain two calcium cations referred to as Ca2+(I) and Ca2+(II). Gaft et al. revealed that Ce3+ ions could replace both the Ca2+(I) and Ca2+(II) sites [27], and Yang et al. demonstrated the presence of an excitation band at approximately 296 nm [28]. The PL properties of Ce3+-doped FAP at various concentrations have been extensively discussed by Zeng et al. [12]. In addition, they indicated that Ce3+ ions occupy Ca2+(II) and Ca2+(I) sites at 292 and 313 nm excitation wavelengths, respectively. This result suggests that the Ce3+ ions Fig. 4. In-line transmittance of (a) 1 at.% and (b) 2 at.% Ce:FAP transparent ceramics.The Ce:FAP ceramics exhibited high transparency in the near-infrared and visible wavelengthregions; however, it showed absorption bands owing to the presence of Ce3+ ions [22]. Figure 4shows the in-line transmittance (> 37%) across the visible spectrum; the sudden decrease atapproximately 330 nm is ascribed to Ce3+ ion absorption. At a wavelength of 500 nm, the in-linetransmittance for the 1 and 2 at.% Ce:FAP ceramics are 67.5% and 64.4%, and the total losscoefficients are δ1= 2.7 cm−1 and δ2= 3.2 cm−1, respectively. The scattering coefficients dueto grain boundaries were roughly determined using evaluated average grain size d and Eq. (1),resulting in γ1= 0.91 cm−1 and γ2= 0.89 cm−1 at the same wavelength. The grain-boundaryscattering loss is lower for the 2 at.% ceramics owing to the smaller grain size. The discrepancyobserved in the total loss coefficient could stem from Ce3+ absorption and the lower number ofremaining pores identified in the FE-SEM image as shown in Fig. 3.At 1000 nm wavelength, the in-line transmittances for 1 and 2 at.% Ce:FAP ceramics are85.8% and 84.7%, respectively, which are close to theoretical transmittance. A low-temperaturehot-isostatic-pressing (HIP) treatment may be effective to eliminate residual pores and furtherincrease the transmittance [23].3.3. Fluorescent propertiesThe Ce3+ ion possessing a 4f 1 site can function as an effective emission center due to the parityand spin-allowed 4f -5d transitions. The PLE/PL of the 1 and 2 at.% Ce:FAP ceramics, includingtheir decay curves, are shown in Fig. 5. The excitation spectra in Fig. 5(a) consist of a broadwavelength range of 240–400 nm, obtained by monitoring the emission at 413 nm. The strongestband was detected with a maximum peak at 310 nm accompanied by a less intense shoulder bandat 292 nm. The excitation peaks represent the transitions from the ground state 2F5/2 and 2F7/2of Ce3+ to the excited 5d states [24,25].In Fig. 5(a), the ceramic emission spectra encompass the 350–700 nm range with an intenseemission centered at 413 nm under 310 nm excitation wavelength. The emission is induced by the-- Compressed PDF version --Research Article Vol. 14, No. 9 / 1 Sep 2024 / Optical Materials Express 2119are primarily located at the Ca2+(I) sites in our ceramic samples because only the 310 nm peak is visible in the excitation spectrum shown in Fig. 5(a). The luminescence decay curves of the 1 and 2 at.% Ce:FAP ceramics were obtained at wavelength of 430 nm under laser excitation at 375 nm (Fig. 5b). These curves were fitted to a double exponential function as I(t) = A1exp(-t/τ1)+A2exp(-t/τ2). The average lifetimes of the 1 and 2 at.% Ce:FAP ceramics were determined to be 58 and 51 ns, respectively, using the following equation [29]:   2 21 1 2 2 1 1 2 2( ) / ( )A A A A                                               (3)The short lifetime of the 1 at.% and 2 at.% Ce:FAP ceramic can be attributed to the allowed character of the Ce3+ ion 5d–4f transition. Additionally, an increased Ce doping concentration decreased the ceramic’s lifetime. Such behavior is consistent with previous studies on Ce-doped apatite [30].  To investigate the feasibility of using this material as a scintillator, we will try to measure its fluorescence properties under X-ray irradiation in the future. Although no significant difference has been observed between the fluorescence properties for the 1 and 2 at.% Ce concentrations in this study, detailed concentration-dependent analyses will also be conducted at higher concentrations to gain further insights into the Ce-occupied sites and fluorescent mechanisms. Figure 5. PLE and PL spectra of (a) 1 and 2 at.% Ce:FAP ceramics (λex = 310 nm, λem = 413 nm) and (b) temporal decay curve of 1 and 2 at.% Ce:FAP ceramic samples. Inset images in Fig.5 (a) correspond to luminescence of the ceramics under UV light excitation (370 nm) observed in the dark.4. ConclusionWe successfully fabricated 1 and 2 at.% Ce:FAP transparent ceramics via liquid-phase synthesis and spark plasma sintering. The low SPS temperature of 950 °C effectively suppressed grain growth and grain boundary scattering, and the corresponding treated ceramics exhibited an optical transparency > 64% at a wavelength of 500 nm. The excitation spectrum of Ce-doped FAP exhibited a broad peak centered at approximately 310 nm, whereas the emission spectrum displayed a broad band covering the entire blue emission spectrum. Overall, the Ce:FAP transparent ceramics exhibit a high transparency at visible wavelengths, and owing Fig. 5. PLE and PL spectra of (a) 1 and 2 at.% Ce:FAP ceramics (λex = 310 nm, λem= 413 nm)and (b) temporal decay curve of 1 and 2 at.% Ce:FAP ceramic samples. Inset images inFig. 5(a) correspond to luminescence of the ceramics under UV light excitation (370 nm)observed in the dark.inter-configurational 5d→4f transition from the excited 5d state to the ground state of Ce3+ ions[26]. The inset images correspond to the luminescence of the ceramics under UV light excitation(370 nm) observed in the dark. The images confirm the emission of light from both ceramics.The Ce3+ ions exhibited a broad emission spectrum, indicating emission via multiple transitions.FAP hosts contain two calcium cations referred to as Ca2+ (I) and Ca2+ (II). Gaft et al. revealed thatCe3+ ions could replace both the Ca2+ (I) and Ca2+ (II) sites [27], and Yang et al. demonstratedthe presence of an excitation band at approximately 296 nm [28]. The PL properties of Ce3+-dopedFAP at various concentrations have been extensively discussed by Zeng et al. [12]. In addition,they indicated that Ce3+ ions occupy Ca2+ (II) and Ca2+ (I) sites at 292 and 313 nm excitationwavelengths, respectively. This result suggests that the Ce3+ ions are primarily located at theCa2+ (I) sites in our ceramic samples because only the 310 nm peak is visible in the excitationspectrum shown in Fig. 5(a).The luminescence decay curves of the 1 and 2 at.% Ce:FAP ceramics were obtained atwavelength of 430 nm under laser excitation at 375 nm (Fig. 5(b)). These curves were fittedto a double exponential function as I(t)=A1exp(-t/τ1)+A2exp(-t/τ2). The average lifetimes ofthe 1 and 2 at.% Ce:FAP ceramics were determined to be 58 and 51 ns, respectively, using thefollowing equation [29]:τ = (A1τ21 + A2τ22 )/(A1τ1 + A2τ2) (3)The short lifetime of the 1 at.% and 2 at.% Ce:FAP ceramic can be attributed to the allowedcharacter of the Ce3+ ion 5d–4f transition. Additionally, an increased Ce doping concentrationdecreased the ceramic’s lifetime. Such behavior is consistent with previous studies on Ce-dopedapatite [30].To investigate the feasibility of using this material as a scintillator, we will try to measureits fluorescence properties under X-ray irradiation in the future. Although no significantdifference has been observed between the fluorescence properties for the 1 and 2 at.% Ceconcentrations in this study, detailed concentration-dependent analyses will also be conductedat higher concentrations to gain further insights into the Ce-occupied sites and fluorescentmechanisms.-- Compressed PDF version --Research Article Vol. 14, No. 9 / 1 Sep 2024 / Optical Materials Express 21204. ConclusionWe successfully fabricated 1 and 2 at.% Ce:FAP transparent ceramics via liquid-phase synthesisand spark plasma sintering. The low SPS temperature of 950 °C effectively suppressed graingrowth and grain boundary scattering, and the corresponding treated ceramics exhibited anoptical transparency> 64% at a wavelength of 500 nm. The excitation spectrum of Ce-dopedFAP exhibited a broad peak centered at approximately 310 nm, whereas the emission spectrumdisplayed a broad band covering the entire blue emission spectrum. Overall, the Ce:FAPtransparent ceramics exhibit a high transparency at visible wavelengths, and owing to theiremission properties and relatively short lifetimes, these materials has the possibility to utilize inphotonic applications.Funding. Fusion Oriented REsearch for disruptive Science and Technology (JPMJFR203S); Japan Society for thePromotion of Science (21H01611, 23K21035).Acknowledgment. Portions of this work were presented at Laser Congress 2023 (ASSL, LAC), Technical DigestSeries (Optica Publishing Group), paper JW2A.2Disclosures. The authors declare no conflicts of interest.Data availability. The data underlying the results presented in this paper are not publicly available at this time butmay be obtained from the authors upon reasonable request.References1. S. Nishiura, S. Tanabe, K. Fujioka, et al., “Properties of transparent Ce:YAG ceramic phosphors for white LED,” Opt.Mater. 33(5), 688–691 (2011).2. V. Tucureanu, A. Matei, and A. M. Avram, “Synthesis and characterization of YAG:Ce phosphors for white LEDs,”Opto-Electron. Rev. 23(4), 239–251 (2015).3. B. Liu, X. Duan, and Y. Yi, “Improved light extraction efficiency of cerium-doped biomedical imaging scintillator bymonolayers of periodic arrays of polystyrene spheres,” Mater. Res. Soc. Symp. Proc. 1627(1), mrsf13-1627-l09-98(2014).4. W. Huang, Z. Mao, L. Chen, et al., “Synthesis and characterisation of fluorescent and biocompatible hydroxyapatitenanoparticles with cerium doping,” Micro Nano Lett. 13(5), 699–703 (2018).5. M. Mori, J. Xu, G. Okada, et al., “Comparative study of optical and scintillation properties of Ce:YAGG, Ce:GAGGand Ce:LuAGG transparent ceramics,” J. Ceram. Soc. Jpn. 124(5), 569–573 (2016).6. Y. Takebuchi, K. Watanabe, D. Nakauchi, et al., “Scintillation properties of Ce-doped LiAlO2 for neutron detection,”J. Ceram. Soc. Jpn. 129(7), 20233397 (2021).7. A. Lempicki and J. Glodo, “Ce-doped scintillators: LSO and LuAP,” Nucl. Instr. and Meth. in Phys. Res. A 416(2-3),333–344 (1998).8. S. A. Payne, L. D. Deloach, L. K. Smith, et al., “Ytterbium-doped apatite-structure crystals: A new class of lasermaterials,” J. Appl. Phys. 76(1), 497–503 (1994).9. A. V. Paduraru, A. M. Musuc, O. C. Oprea, et al., “Synthesis and characterization of photoluminescent Ce(III) andCe(IV) substituted hydroxyapatite nanomaterials by co-precipitation method: Cytotoxicity and biocompatibilityevaluation,” Nanomaterials 11(8), 1911 (2021).10. D. V. Milojkov, O. S. Silvestre, S. D. Vojislav, et al., “Fabrication and characterization of luminescent Pr3+ dopedfluorapatite nanocrystals as bioimaging contrast agents,” J. Lumin. 217, 116757 (2020).11. B. Demir and E. Ayas, “Effects of sintering temperature and doping content on luminescence properties of rare earth(Sm3+, Eu3+, and Dy3+) doped natural fluorapatite,” J. Solid State Chem. 306, 122783 (2022).12. Q. Zeng, H. Liang, G. Zhang, et al., “Luminescence of Ce3+ activated fluoro-apatites M5 (PO4)3F (M ≤ Ca, Sr, Ba)under VUV-UV and X-ray excitation,” J. Phys.: Condens. Matter 18(42), 9549–9560 (2006).13. C. Zhang, S. Huang, D. Yang, et al., “Tunable luminescence in Ce3+, Mn2+ -codoped calcium fluorapatite throughcombining emissions and modulation of excitation: A novel strategy to white light emission,” J. Mater. Chem. 20(32),6674–6680 (2010).14. R. Apetz and M. P. B. Van Bruggen, “Transparent alumina : a light-scattering model,” J. Am. Ceram. Soc. 86(3),480–486 (2003).15. H. Furuse, N. Horiuchi, and B. N. Kim, “Transparent non-cubic laser ceramics with fine microstructure,” Sci. Rep.9(1), 10300 (2019).16. H. Furuse, T. Okabe, H. Shirato, et al., “High-optical-quality non-cubic Yb3+ -doped Ca10(PO4)6F2 (Yb:FAP) laserceramics,” Opt. Mater. Express 11(6), 1756 (2021).17. H. Furuse, Y. Mochizuki, D. Kato, et al., “Strontium fluorapatite (S-FAP) nano-grained laser ceramics,” Scr. Mater.241, 115881 (2024).18. P. Becker, E. Libowitzky, R. Kleinschrodt, et al., “Linear optical properties and Raman spectroscopy of naturalfluorapatite,” Cryst. Res. Technol. 51(4), 282–289 (2016).-- Compressed PDF version --https://doi.org/10.1016/j.optmat.2010.06.005https://doi.org/10.1016/j.optmat.2010.06.005https://doi.org/10.1515/oere-2015-0038https://doi.org/10.1557/opl.2014.13https://doi.org/10.1049/mnl.2017.0729https://doi.org/10.2109/jcersj2.15239https://doi.org/10.2109/jcersj2.20233https://doi.org/10.1016/S0168-9002(98)00689-5https://doi.org/10.1063/1.357101https://doi.org/10.3390/nano11081911https://doi.org/10.1016/j.jlumin.2019.116757https://doi.org/10.1016/j.jssc.2021.122783https://doi.org/10.1088/0953-8984/18/42/002https://doi.org/10.1039/c0jm01036ghttps://doi.org/10.1111/j.1151-2916.2003.tb03325.xhttps://doi.org/10.1038/s41598-019-46616-8https://doi.org/10.1364/OME.426701https://doi.org/10.1016/j.scriptamat.2023.115881https://doi.org/10.1002/crat.201500341Research Article Vol. 14, No. 9 / 1 Sep 2024 / Optical Materials Express 212119. Y. Watanabe, T. Ikoma, A. Monkawa, et al., “Fabrication of transparent hydroxyapatite sintered body with highcrystal orientation by pulse electric current sintering,” J. Am. Ceram. Soc. 88(1), 243–245 (2005).20. H. Furuse, D. Kato, K. Morita, et al., “Characterization of transparent fluorapatite ceramics fabricated by sparkplasma sintering,” Materials 15(22), 8157 (2022).21. H. Furuse, S. Nakasawa, H. Yoshida, et al., “Transparent ultrafine Yb3+:Y2O3 laser ceramics fabricated by sparkplasma sintering,” J. Am. Ceram. Soc. 101(2), 694–702 (2018).22. A. Yousuf, Y. Mochizuki, S. Hirai, et al., “Fluorescence and optical properties of Er and Ce doped fluorapatite (FAP)transparent ceramics,” in Laser Congress 2023 (ASSL, LAC), Technical Digest Series (Optica Publishing Group),paper JW2A.2 (2023).23. S. Cohen, B. Ratzker, M. Solol, et al., “Polycrystalline transparent magnesium aluminate spinel processed by acombination of spark plasma sintering (SPS) and hot isostatic pressing (HIP),” J. Eur. Ceram. Soc. 38(15), 5153–5159(2018).24. Z. L. Wang, Z. W. Quan, P. Y. Jia, et al., “A facile synthesis and photoluminescent properties of redispersible CeF3,CeF3:Tb3+, and CeF3:Tb3+/LaF3 (core/shell) nanoparticles,” Chem. Mater. 18(8), 2030–2037 (2006).25. A. Gektin, N. Shiran, V. Nesterkina, et al., “Luminescence of heavily Ce-doped alkaline-earth fluorides,” J. Lumin.129(12), 1538–1541 (2009).26. M. Xie, Y. Huang, Y. Tao, et al., “Improving the BAM VUV-Irradiation Degradation with a UV-Blue EmittingPhosphor CLPF-Tm,” J. Electrochem. Soc. 157(11), J401–J404 (2010).27. M. Gaft, G. Panczer, R. Reisfeld, et al., “Laser-induced time-resolved luminescence as a tool for rare-earth elementidentification in minerals,” Phys. Chem. Miner. 28(5), 347–363 (2001).28. P. Yang, G. Q. Yao, and J. H. Lin, “Photoluminescence of Ce3+ in haloapatites Ca5(PO4)3 X,” Inorg. Chem. Commun.7(2), 302–304 (2004).29. A. Sillen and Y. Engelborghs, “The correct use of ‘average’ fluorescence parameters,” Photochem. Photobiol. 67(5),475–486 (1998).30. I. E. Kolesnikov, A. M. Nikolaev, E. Lähderanta, et al., “Structural and luminescence properties of Ce3+ -dopedhydroxyapatite nanocrystalline powders,” Opt. Mater. 99, 109550 (2020).-- Compressed PDF version --https://doi.org/10.1111/j.1551-2916.2004.00041.xhttps://doi.org/10.3390/ma15228157https://doi.org/10.1111/jace.15232https://doi.org/10.1016/j.jeurceramsoc.2018.07.024https://doi.org/10.1021/cm052360xhttps://doi.org/10.1016/j.jlumin.2009.04.046https://doi.org/10.1149/1.3491358https://doi.org/10.1007/s002690100163https://doi.org/10.1016/j.inoche.2003.12.001https://doi.org/10.1111/j.1751-1097.1998.tb09082.xhttps://doi.org/10.1016/j.optmat.2019.109550