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Hisanori Yamane, [Akihiro Nakanishi](https://orcid.org/0009-0001-1859-261X), [Shiro Funahashi](https://orcid.org/0000-0002-9381-3603), [Takayuki Nakanishi](https://orcid.org/0000-0003-3412-2842), [Kohsei Takahashi](https://orcid.org/0000-0002-6443-1534), [Naoto Hirosaki](https://orcid.org/0000-0001-9218-9557), [Takashi Takeda](https://orcid.org/0000-0003-2510-4562)

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[Preparation and photoluminescence of Eu<sup>2+</sup> and Eu<sup>3+</sup>-doped NaLuO<sub>2</sub>](https://mdr.nims.go.jp/datasets/1e95c8cd-1f32-4191-a887-8aec43d5ad89)

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Preparation and photoluminescence of Eu2+ and Eu3+-doped NaLuO2FULL PAPERPreparation and photoluminescence of Eu2+ and Eu3+-doped NaLuO2Hisanori Yamane1,³, Akihiro Nakanishi1, Shiro Funahashi1, Takayuki Nakanishi1,Kohsei Takahashi1, Naoto Hirosaki1 and Takashi Takeda1,‡1Advanced Phosphor Group, National Institute for Materials Science, Tsukuba, Ibaraki 305–0044, JapanSingle phase NaLuO2, which crystallizes in the ¡-NaFeO2-type structure (trigonal, space group R�3m), wassynthesized by heating a mixture of Na2O and Lu2O3 at 1150 °C. The crystal structure of this oxide was analyzedusing the Rietveld method for the powder X-ray diffraction pattern, and the oxygen atom coordinate [z =0.2397(3)] was refined. Mixtures of NaLuO2 and europium monoxide with a composition NaLuO2–xEuO (x =0.0, 0.01, 0.1, 1.0mol%) were heated at 900 °C for 12 h in a N2 atmosphere. The obtained samples were Eu2+ andEu3+-doped NaLuO2 with trace amounts of Lu2O3. The emission spectrum peak due to the 5d ¼ 4f transition ofEu2+ occurred at a wavelength of 617 nm under the excitation of a 442 nm blue light, and the full width at halfmaximum of the spectrum was 80 nm. Intense emission peaks due to the f-f transition of Eu3+ were observed atapproximately 590 nm when excited by a 230 nm ultraviolet light.Key-words : Solid state reaction, Ternary oxide, Powder X-ray diffraction, Rietveld analysis, Photoluminescence,Eu2+ and Eu3+ emission center[Received March 28, 2025; Accepted April 11, 2025]1. IntroductionConsidering potential applications for X-ray scintillatorsand white LED lighting, Jarý et al. have been activelyinvestigating the fluorescent properties of Eu2+-dopedtrinary sulfides (ALnS2, A = Na, K, Rb; Ln = La, Gd, Lu,Y) with the ¡-NaFeO2-type structure (trigonal, spacegroup R�3m).1–8) Ternary oxides with the ¡-NaFeO2-typestructure consisting of alkali metals and rare-earth ele-ments have also been synthesized,9–14) and the fluorescentproperties of Eu3+ or Bi3+-doped ones have been previ-ously reported.14–16) Additionally, research has also beenconducted on the magnetic properties of ALnO2 (A = Na;Ln = Er, Tm, Yb, Lu, A = K; Ln = Y, Nd, Sm–Lu).17,18)An emission peak of 0.05% Eu2+-doped ternary sulfideNaLuS2 with the ¡-NaFeO2-type structure was reported at641 nm under blue light (429 nm) excitation.3) No studieshave attempted to introduce the Eu2+ activator into ternaryoxides with the same structure type. In the present study,single phase NaLuO2 was synthesized from Na2O andLu2O3, and the preparation of Eu2+-doped NaLuO2 wasattempted through the reaction of NaLuO2 and EuO.2. ExperimentalThe starting materials used were Na2O (78.0–122.0%,Na2O2 ¯ 22.0%, Sigma-Aldrich), Lu2O3 (99.9%, HighPurity Chemicals), Eu2O3 (99.9%, Shin-Etsu ChemicalCo. Ltd.), and Eu (99.9%, Rare Metallic Co., Ltd.). Due toNa2O, Eu, and EuO reacting with water vapor and oxygenin the air, the handling of these materials was carried outin a glove box filled with N2 gas. For the synthesis ofNaLuO2, Na2O and Lu2O3 were weighed at a molar ratioof 1.1:1.0, mixed in an agate mortar, and then pressed intoa disk shape compact with a diameter of 5mm and athickness of ³1mm with a die and a hand pressing tool.The compact was placed in a boat made of thin Ni plateand sealed along with N2 gas in a stainless-steel container(SUS316L) composed of a tube (inner diameter 10.7mm,length 80mm) and caps. The container containing the sam-ple was heated in an electric furnace at 1150 °C for 0.5 h,then cooled at a rate of ¹100 °C/h until it reached 850 °C.Subsequently, the power supply to heater elements wasstopped and the sample was cooled in the furnace (furnacecooling).A mixture of Eu2O3 and Eu metal fragments was sealedin the stainless-steel container and heated at 800–900 °Cfor 24 h to prepare EuO.19) The obtained NaLuO2 and EuOwere ground in an agate mortar, weighed to give aNaLuO2-to-EuO molar ratio of 1:x% (x = 0.01, 0.1, 1.0),and mixed before being pressed into compact disks. TheNaLuO2–xEuO compacts were placed in the Ni boat,which was then sealed in the stainless-steel container andheated at 900 °C for 12 h. After furnace cooling, the con-tainer was opened in air, and the sample was pulverized inthe agate mortar.For the single-phase NaLuO2 sample, the X-ray dif-fraction (XRD) pattern was measured using a powder X-³ Corresponding author: H. Yamane; E-mail: YAMANE.Hisanori@nims.go.jp‡ Corresponding author: T. Takeda; E-mail: TAKEDA.Takashi@nims.go.jpJournal of the Ceramic Society of Japan 133 [7] 389-392 2025DOI https://doi.org/10.2109/jcersj2.25048 JCS-Japan©2025 The Ceramic Society of Japan 389This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by-nc/4.0/),which permits use, distribution, and reproduction in any medium for non-commercial purposes, provided the original work is properly cited.https://doi.org/10.2109/jcersj2.25048https://creativecommons.org/licenses/by-nc/4.0/ray diffractometer (RIGAKU, SmartLab) with a CuK¡1(45 kV, 200mA) radiation source, and Rietveld analysiswas performed using the PDXL software.20) The crystalstructure was drawn using VESTA.21) The crystallinephases were identified and the cell parameters of thetrigonal phase in the NaLuO2–xEuO samples were mea-sured using a powder X-ray diffractometer (RIGAKU,SmartLab) with parallel beam CuK¡ radiation (40 kV,30mA) for accurate cell parameters. Excitation and emis-sion spectra of the powdered samples were measured usinga fluorescence spectrofluorometer (JASCO, FP8600), andthe quantum efficiency of the emission was measuredusing a quantum efficiency measurement system (OtsukaElectronics Co., Ltd., QE-2100).3. Results and discussionIn the synthesis of NaLuO2, trace Na metal deposit wascharacterized in the stainless-steel container after heatingat 1150 °C. Excess Na2O above the NaLuO2 stoichiometriccomposition evaporated from the sample and was pyro-lyzed or reduced to Na metal by the stainless steel.Figure 1 shows the powder XRD pattern of the synthe-sized NaLuO2. The Rietveld analysis results with the¡-NaFeO2-type crystal structure model are summarizedin Table 1. All diffraction peaks could be indexed withthe cell parameters of the hexagonal system [a =3.33373(7)¡, c = 16.5494(4)¡]. These parameters areclose to those reported by Blasse et al. (a = 3.32¡,c = 16.52¡),9) Zaitsev et al. (a = 3.34¡, c = 16.48¡),13)Spitsyn et al. (a = 3.222¡, c = 16.495¡),11) Murav’evaet al. [a = 3.322(1)¡, c = 16.495(5)¡],12) Hashimotoet al. [a = 3.3518(6)¡, c = 16.5300(12)¡],17) and GuOet al. (a = 3.335¡, c = 16.545¡).15) The only O site zcoordinate to be refined, was 0.2397(3). This coordinatewas consistent with the z of ¡-NaFeO2-type oxidesNaErO2 [0.236(4)]17) and KLnO2 (Ln = Y, Nd, Sm–Lu)[0.2248(5)–0.2337(9)],18) and the z of S site for the sul-fides ALnS2 (A = K, Rb; Ln = La, Gd, Lu, Y) [0.2303(2)–0.23718(11)].22,23) The crystal structure of NaLuO2 refinedby Rietveld analysis is shown in Fig. 2. The Na–O andLu–O interatomic distances shown in the analysis were2.471(4) and 2.273(3)¡, respectively. The bond valencesums calculated using the parameters presented by Breseand O’Keeffe24) were 0.979 and 2.654 for the Na and Lusites, respectively, which are close to the Na and Lu oxi-dation numbers I and III, respectively.The powder XRD pattern of the NaLuO2–xEuO sampleswith the parallel beam geometry are shown in Fig. 3.Trace Lu2O3 XRD peaks of 222 and 400 were observed at2ª = 29.69 and 34.43°, respectively, in the samples of x =0.01, 0.1, and 1.0%. Figure 4 shows the cell parameters aand c and volume Vof the trigonal phase normalized by thex = 0% values. Cell parameters and volumes were slightlysmaller for the x = 0.01 and 0.1% samples compared withthose of the x = 0% sample but exceeded those for the x =1.0% sample. A trace Na metal deposition was identifiedin the stainless-steel container in which the NaLuO2–xEuOsamples were synthesized. The Lu2O3 XRD peak intensityfrom the x = 1.0% sample was the highest, suggesting theNa2O component was reduced in the reaction betweenNaLuO2 and EuO, becoming Na vapor and being releasedfrom the sample. Some of the Eu2+ was oxidized andincorporated into NaLuO2 as Eu3+, and the excess Lu2O3,which became redundant owing to the loss of Na2O andEu3+ substitution, was expelled.Fig. 1. Observed (dots) and calculated (solid) X-ray diffractionprofiles for NaLuO2. Tick marks below the diffraction patternrepresent the allowed Bragg reflections. The difference profile islocated at the bottom of the figure.Table 1. Crystallographic data and structure refinement forNaLuO2Formula NaLuO2Formula weight 229.956Space group trigonal, R�3m H (166)Radiation wavelength 1.540593¡ (CuK¡1)Cell parameters a = 3.33373(7)¡,c = 16.5494(4)¡Cell volume V = 159.285(6)¡3Density (calculated) 7.19 g/cm3R indexes Rwp = 9.70%, Rp = 7.64%,Rexp = 4.65%Goodness of fit S = 2.0855Atomic coordinates and isotropic displacement parametersAtom Site x y z Uiso (¡2)Na 3a 0 0 0 0.0097(12)Lu 3b 0 0 1/2 0.0018(2)O 6c 0 0 0.2397(3) 0.0015(13) Fig. 2. Crystal structure of NaLuO2.Yamane et al.: Preparation and photoluminescence of Eu2© and Eu3©-doped NaLuO2JCS-Japan390Figure 5 shows the excitation and emission spectra ofthe x = 0.1% sample, in which 442 nm blue light exci-tation produced red emission from the 5d ¼ 4f transitionof Eu2+ with a peak at 617 nm. The external and internalquantum efficiencies of this luminescence were 4.9 and15.8%, respectively. Compared to the emission peak at641 nm and the excitation peak at 429 nm reported forNaLuS2: 0.05%Eu,3) the emission peak wavelength was24 nm shorter and the excitation peak wavelength was 13nm longer for the NaLuO2–xEuO samples. This is due tothe difference in chemical bonding between the Eu–O andEu–S in the crystals with the same structure type. The fullwidth at half maximum (FWHM) of the emission peakestimated from the reported spectrum of NaLuS2: 0.05%Eu was 79 nm (0.24 eV),3) which agreed with theFWHM of 80 nm (0.25 eV) for NaLuO2–xEuO within theresolution of the spectrofluorometer. The sample was leftin air for more than 2 weeks, but no change in the lumi-nescence intensity or XRD pattern was observed.Under UV light excitation with a 230 nm wavelength,an emission spectrum with the highest intensity peak at596 nm caused by the 4f–4f transition (5D0 ¼ 7F1) of Eu3+was observed. This emission spectrum was also over-lapped by a broad peak due to the 5d ¼ 4f transition ofEu2+. At the excitation spectrum measured at 596 nm, anexcitation band of Eu2+ with a peak at 442 nm appearedin addition to the charge transfer band at approximately230 nm. Excluding the broad emission peak of Eu2+, theEu3+ emission spectrum was consistent with that ofNaLuO2:Eu3+ synthesized from Eu2O3.14,15,25)The emission peak intensity of Eu2+ was highest at x =0.1% and decreased at x = 1% as shown in Fig. 6. Incontrast, the intensity of Eu3+ at 596 nm increased withincreasing x. In the isotypic Eu2+-doped sulfide KLuS2, asignificant decrease in luminescence intensity (concentra-tion quenching) was reported at Eu optimum concentra-tions from 0.01 to 0.5% and above.1) The x = 1% sampleof NaLuO2–xEuO showed a decrease in intensity from theemission of Eu2+, and the emission of Eu3+ was alreadydominant at x = 0.1%. This suggests that there is a limit tothe amount of Eu2+ introduced into NaLuO2, which couldbe estimated to be less than 0.1%.Electron spin resonance (ESR) showed that Eu2+ wasintroduced at three centers, two of which were the K+ andLu3+ sites of KLuS2.2) In the case of NaLuO2, the cellparameters and volumes of samples with x = 0.01 and0.1% were slightly smaller than those with x = 0%. Theeffective ionic radii for six-fold configuration are 1.02,0.861, 1.17, and 0.947¡ for Na+, Lu3+, Eu2+, and Eu3+,respectively.26) If Eu2+ and Eu3+ occupy the Na+ site andLu3+ site, respectively, the cell parameters will increase.The decrease in cell parameters could be explained by theFig. 3. X-ray diffraction patterns of the NaLuO2–xEuO sam-ples, (a) x = 0, (b) 0.01, (c) 0.1, and (d) 1.0. The peaks fromLu2O3 are indicated with “+”.Fig. 4. Normalized hexagonal cell parameters (a, c) and vol-umes (V ) for the trigonal phase in the NaLuO2–xEuO samples(x = 0, 0.01, 0.1, and 1.0).Fig. 5. Excitation and emission spectra measured for theNaLuO2–0.1EuO sample at 25 °C.Journal of the Ceramic Society of Japan 133 [7] 389-392 2025 JCS-Japan391occupation of the Na+ site by both Eu2+/Eu3+. Thus, forthe samples with x = 0.01 and 0.1%, Eu2+ and Eu3+might been introduced mainly at the Na+ site, while for thex = 1% sample, substitution of Eu3+ for Lu3+ at the Lusite proceeded, which increased the cell parameters andvolume, leading to an increase in the Eu3+ emissionintensity.4. SummarySingle-phase NaLuO2 with the ¡-NaFeO2-type structurewas synthesized by heating a mixture of Na2O and Lu2O3sealed with N2 gas in a stainless-steel container at 1150 °C.Eu2+ doping of NaLuO2 was attempted by heating theresulting NaLuO2 with a variation of 0.01, 0.1, and 1.0mol% EuO at 900 °C. The reaction of NaLuO2 with EuOreleased Na and Lu2O3 and a part of Eu2+ was oxidized toEu3+. The amount of Eu2+ introduced into NaLuO2 wasestimated to be less than 0.1mol%, and the peak emissionof the 5d ¼ 4f transition of Eu2+ was observed at 617 nmunder blue light excitation at 442 nm. The spectrum withthe maximum intensity peak of Eu3+ emission at 596 nmunder UV light excitation at 230 nm was consistent withthat reported for Eu3+ doped NaLuO2.Acknowledgments This work was supported by JST, theCore Research for Evolutional Science and Technology (grantnumber JPMJCR19J2). It was also financially supported bythe Innovative Science and Technology Initiative for Security(Grant Number JPJ004596, ATLA, Japan).References1) V. Jarý, L. Havlák, J. Bárta, E. Mihóková and M. Nikl,Chem. Phys. Lett. 574, 61 (2013).2) V. Laguta, M. Buryi, L. Havlák, J. Bárta, V. Jarý and M.Nikl, Phys. Status Solidi-R. 08, 801 (2014).3) V. Jarý, L. Havlák, J. Bárta, M. Buryi, E. Mihóková, M.Rejman, V. Laguta and M. Nikl, Materials 8, 6978(2015).4) V. Jarý, L. Havlák, J. Bárta, E. Mihóková, M. Buryi andM. Nikl, J. Lumin. 170, 718 (2016).5) L. Havlak, V. Jary, J. Barta, M. Buryi, M. Rejman, V.Laguta and M. Nikl, Mater. Design 106, 363 (2016).6) V. Jarý, L. Havlák, J. Bárta, M. Rejman, A. Bystřický,C. Dujardin, G. Ledoux and M. Nikl, ECS J. Solid StateSc. 7, R3182 (2018).7) V. Jarý, L. Havlák, J. Bárta, M. Buryi, M. Rejman, M.Pokorný, C. Dujardin, G. Ledoux and M. Nikl, ECS J.Solid State Sc. 9, 016007 (2020).8) M. G. Brik, V. Jarý, L. Havlák, J. Bárta and M. Nikl,Chem. Eng. J. 418, 129380 (2021).9) G. Blasse, J. Inorg. Nucl. Chem. 28, 2444 (1966).10) R. Hoppe and H. Sabrowsky, Z. Anorg. Allg. Chem.339, 144 (1965).11) V. I. Spitsyn, I. A. Murav’eva, L. M. Kovba and I. I.Korchak, Zh. Neorg. Khim. 14, 1451 (1969).12) I. A. Murav’eva, L. M. Kovba and V. I. Spitsyn, Dokl.Akad. Nauk SSSR 172, 1380 (1967).13) B. E. Zaitsev, B. N. Ivanov-Émin, V. M. Akimov, E. N.Siforova and V. Miliado Campos, J. Struct. Chem. 11,634 (1971).14) G. Blasse and A. Bril, J. Chem. Phys. 45, 3327 (1966).15) R.-W. Guo, C.-X. Guo and D. Wu, Acta Phys.-Chim.Sin. 57, 607 (2008).16) A. C. Van der Steen, J. J. A. Van Hesteren and A. P.Slok, J. Electrochem. Soc. 128, 1327 (1981).17) Y. Hashimoto, M. Wakeshima and Y. Hinatsu, J. SolidState Chem. 176, 266 (2003).18) B. Dong, Y. Doi and Y. Hinatsu, J. Alloy. Compd. 453,282 (2008).19) K. Hirose, Y. Doi and Y. Hinatsu, J. Solid State Chem.182, 1624 (2009).20) PDXL Version 2.8.4.0 (2017), Integrated X-ray PowderDiffraction Software. Rigaku Corporation, Tokyo 196-8666, Japan.21) K. Momma and F. Izumi, J. Appl. Crystallogr. 44, 1272(2011).22) J. Fabry, L. Havlak, M. Dusek, P. Vanek, J. Drahokoupiland K. Jurek, Acta Crystallogr. B 70, 360 (2014).23) W. Bronger, J. Eyck, K. Kruse and D. Schmitz, Eur. J.Sol. State Inor. 33, 213 (1996).24) N. E. Brese and M. O’Keeffe, Acta Crystallogr. B 47,192 (1991).25) G. Blasse and B. C. Grabmaier, “Luminescent Materi-als”, Springer-Verlag, Berlin (1994) p. 43.26) R. Shannon, Acta Crystallogr. A 32, 751 (1976).Fig. 6. Eu2+ and Eu3+ emission peak intensities observed forthe spectra of the NaLuO2–xEuO samples (x = 0, 0.01, 0.1, and1.0).Yamane et al.: Preparation and photoluminescence of Eu2© and Eu3©-doped NaLuO2JCS-Japan392