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## Creator

[Akihiro Nakanishi](https://orcid.org/0009-0001-1859-261X), [Takayuki Nakanishi](https://orcid.org/0000-0003-3412-2842), [Naoto Hirosaki](https://orcid.org/0000-0001-9218-9557), [Koji Morita](https://orcid.org/0000-0001-6040-7054), [Takashi Takeda](https://orcid.org/0000-0003-2510-4562)

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[Photoluminescence of perovskite-type Cr<sup>3+</sup>-activated phosphors in the La–Al–Ti–O system](https://mdr.nims.go.jp/datasets/ba26a562-eac7-4931-897d-c4a593c187b8)

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Photoluminescence of perovskite-type Cr3+-activated phosphors in the La–Al–Ti–O systemFULL PAPERPhotoluminescence of perovskite-type Cr3+-activated phosphorsin the La–Al–Ti–O systemAkihiro Nakanishi1,³, Takayuki Nakanishi1, Naoto Hirosaki1, Koji Morita2 and Takashi Takeda1,‡1Advanced Phosphor Group, National Institute for Materials Science, Tsukuba, Ibaraki 305–0044, Japan2Polycrystalline Optical Material Group, National Institute for Materials Science, Tsukuba, Ibaraki 305–0044, JapanCr3+-activated near-infrared emitting phosphor has a potential application in plant growth and sensingtechnologies. Herein, we synthesized near-infrared emitting La1¹x/3Al1¹x¹yTixO3:yCr3+ phosphor by a solid-statereaction. La1¹x/3Al1¹x¹yTixO3:yCr3+ (x = 0.2, y = 0.003) showed the near-infrared emission at 751 nm with a fullwidth at half-maximum of 41 nm under 351 nm excitation. The internal and external quantum efficiencies were54.4 and 25.5%, respectively. The luminescence intensities at 150 °C of the La1¹x/3Al1¹x¹yTixO3:yCr3+ (x = 0.1,0.2, and 0.3, y = 0.003) were 32, 17, and 3% of those at room temperature, respectively. The thermal quenchingeffect strongly occurred with increasing the amount of Ti and it was related to the change in the absorbance edgeof the host material La1¹x/3Al1¹x¹yTixO3.Key-words : Cr3+-activated phosphor, Near-infrared emission, Oxide, Solid state reaction[Received November 26, 2025; Accepted December 23, 2025; Published online February 3, 2026]1. IntroductionWhite LEDs combined with blue or ultraviolet LED andphosphors are widely used because of high efficiency andlong life.1–3) Yellow- [Y3Al5O12:Ce3+ and Ca-¡-SiAlON(Cam/2Si12¹m¹nAlm+nOnN16¹n:Eu2+)], and red- [CaAlSiN3:Eu2+ and M2Si5N8:Eu2+ (M = Ca, Sr)] emitting phosphorsare commercially utilized for white LEDs.4–8)Near-infrared (NIR) light sources have been applied invarious agricultural technologies, including plant growthand sensing systems.9–11) The height and growth of plantsis controlled by adjusting the ratio of red to near-infraredlight irradiation, owing to the strong absorption of bio-logical pigments such as red-absorbing phytochrome (PR)and far-red-absorbing phytochrome (PFR).12) In addition,the reflectance difference between live and dead leaves iswidely utilized for assessing plant health and evaluatingvegetation. Plants exhibit high reflectance at ³750–850nm due to the presence of the “red edge,” and the reflec-tance near the red edge is strongly influenced by planthealth.13) For vegetation evaluation, the normalized differ-ence vegetation index (NDVI) is typically calculated usingthe reflectance of red and near-infrared region.14) Conse-quently, efficient near-infrared light sources have becomeincreasingly important in the agricultural field. Althoughhalogen lamps have been used as practical near-infraredlight sources, their applications are limited due to highpower consumption, large size, short operating time, andhigh operating temperature. Therefore, the development ofnear-infrared emitting phosphors has been promoted totake advantage of phosphor-converted LEDs.15,16)Cr3+ is widely used as an activator in near-infraredphosphors because of its low cost and tunable emissionbandwidth. Furthermore, its absorption bands extend fromthe ultraviolet (UV) to the blue region, enabling efficientexcitation by UV or blue LEDs for the fabrication ofphosphor-converted LEDs.17) It is probable that the Cr3+prefers to occupy the six-coordinated octahedral site.18,19)Perovskite-type structure is suitable host material becauseof the six-coordinated octahedral site in its structures.Some Cr3+-activated perovskite-type phosphor are re-ported such as LaAlO3:Cr3+,20) BaLaMgNbO6:Cr3+,21)Ca2AlNbO6:Cr3+,22) and Sr2AlTaO6:Cr3+.23) LaAlO3:Cr3+exhibits sharp red emission peak at ³730 nm, and poten-tial for applying plant-growth LEDs, bio-imaging, andmechanoluminescence material.20,24,25) Škapin et al., re-ported that the perovskite-type structure was maintained inthe solid solution of La1¹x/3Al1¹xTixO3.26) A-site deficien-cy of La3+ is introduced due to the charge compensationfor Ti4+ doping to Al3+ of the B-site in the perovskite-typestructure of LaAlO3. In this study, the solid solution ofLa1¹x/3Al1¹xTixO3 is focused as a host material. The lumi-nescence properties depend on the crystal field strengtharound the activator of Cr3+.17,21,22) In the perovskite-typeLa1¹x/3Al1¹xTixO3 structure, the cation arrangement nearthe Cr3+ is changed, leading to a modification of the crys-tal field and enabling modulation of the luminescenceproperties. Therefore, we synthesized Cr3+-activatedLa1¹x/3Al1¹xTixO3 phosphors and investigated the optimal³ Corresponding author: A. Nakanishi; E-mail: NAKANISHI.Akihiro@nims.go.jp‡ Corresponding author: T. Takeda; E-mail: TAKEDA.Takashi@nims.go.jpJournal of the Ceramic Society of Japan 134 [3] 128-132 2026DOI https://doi.org/10.2109/jcersj2.25160 JCS-Japan©2025 The Ceramic Society of Japan128This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/),which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.https://doi.org/10.2109/jcersj2.25160https://creativecommons.org/licenses/by/4.0/concentration of Cr3+, luminescence property, quantumefficiency, and temperature-dependent luminescence. Theeffect of Ti4+ amounts to the temperature-dependent lumi-nescence was revealed.2. ExperimentalLa1¹x/3Al1¹x¹yTixO3:yCr3+ (x = 0.1–0.7, ¦x = 0.1, y =0.0015–0.05) phosphors were synthesized by a solid-statereaction. Stoichiometric amounts of La2O3 (99.99%,Kojundo Chemical, Japan), Al2O3 (99.99%, TAIMEICHEMICALS Co., LTD., Japan), TiO2 (99.9%, KojundoChemical, Japan), and Cr2O3 (99.9%, Kojundo Chemical,Japan) were mixed thoroughly in an alumina mortar.La2O3 was pre-heated at 800 °C for 10 h. After mixing, theprecursor powder in the alumina crucible was calcined at1500 or 1600 °C for 4 h in air.The crystal phases of the synthesized powders wereinvestigated at room temperature using powder X-raydiffraction (XRD, SmartLab X-ray Diffractometer, Rigaku,Japan) with Cu-K¡1 radiation at 45 kV, 200mA, and 2ªin the range of 20–100°. The luminescence properties ofpowder samples were measured with a fluorescence spec-trometer (FP-8600 spectrofluorometer, JASCO, Japan).The internal quantum efficiency (IQE) and external quan-tum efficiency (EQE) were measured by QE-2100 system(Otsuka Electronics, Japan). BaSO4 was used for refer-ences. The temperature-dependent luminescence proper-ties were measured by a QE-2100 system combined witha temperature control stage (10002L, Rinkam ScientificInstruments, UK). The diffuse reflectance was measured inthe range of 200–750 nm using UV–VIS–NIR spectropho-tometer (UV-3600i Plus, SHIMADZU, Japan).3. Results and discussionIn order to optimize the Ti amount in the La1¹x/3-Al1¹x¹yTixO3 system, La1¹x/3Al1¹x¹yTixO3:yCr3+ phos-phors (x = 0.1–0.7, ¦x = 0.1, y = 0.01) were synthesizedat 1500 °C for the preliminary experiment at first. La1¹x/3-Al1¹x¹yTixO3 (x = 0.2) was selected as the host materialbecause the emission intensity peak around 750 nm wasthe highest under 365 nm excitation as shown in Fig. 1.In addition, the emission intensity of La1¹x/3Al1¹x¹yTixO3(x = 0.2, y = 0.01) calcined at 1600 °C was higher thanthat of the sample calcined at 1500 °C. Based on the resultsof the preliminary experiment, La1¹x/3Al1¹x¹yTixO3:yCr3+(x = 0.2, y = 0–0.05) phosphors were synthesized at1600 °C for 4 h. The XRD patterns of the La1¹x/3Al1¹x¹y-TixO3:yCr3+ (x = 0.2, y = 0–0.05) phosphors calcined at1600 °C for 4 h are shown in Fig. 2. Diffraction patterns ofall samples are consistent with the International Centre forDiffraction Data (ICDD) card peak patterns of LaAlO3 withthe trigonal system (No. 00-030-1144),27) confirming theformation of single-phase phosphors. The cell volume ofLa1¹x/3Al1¹x¹yTixO3:yCr3+ (x = 0.2, y = 0, 0.01, 0.03, and0.05) is shown in Fig. 3. With increasing the amount ofCr3+, the cell volume expanded because the larger cation ofCr3+ (rVI = 0.615¡) replaced the Al3+ (rVI = 0.535¡) ofthe B-site in the perovskite-type structure.Fig. 1. Emission intensity under 365 nm excitation of La1¹x/3-Al1¹x¹yTixO3:yCr3+ (x = 0.1–0.7,¦x = 0.1, y = 0.01) phosphors.Fig. 2. XRD patterns of the La1¹x/3Al1¹x¹yTixO3:yCr3+ (x =0.2, y = 0–0.05) phosphors calcined at 1600 °C for 4 h.Fig. 3. Cell volume of La1¹x/3Al1¹x¹yTixO3:yCr3+ (x = 0.2, y =0, 0.01, 0.03, and 0.05) phosphors calcined at 1600 °C for 4 h.Journal of the Ceramic Society of Japan 134 [3] 128-132 2026 JCS-Japan129The excitation and emission spectra of the La1¹x/3-Al1¹x¹yTixO3:yCr3+ (x = 0.2, y = 0–0.05) phosphors cal-cined at 1600 °C for 4 h are shown in Figs. 4(a) and 4(b).The sample with y = 0.003 exhibited the highest emissionintensity under 351 nm excitation. The emission intensitywas rapidly decreasing after y = 0.01 sample. The excita-tion spectra showed the absorption band peak around 350and 570 nm, while the emission spectrum showed emis-sion peak at 751 nm with a full width at half maximum(FWHM) of 41 nm in the sample with y = 0.003, originat-ing from the d-d transition of Cr3+. It is possible that thecrystal field variation at the B-site of the perovskite-typeLa1¹x/3Al1¹xTixO3 (x = 0.2) affects the emission profile ofCr3+.28)The IQE, EQE, and absorbance of the La1¹x/3Al1¹x¹y-TixO3:yCr3+ (x = 0.1–0.3, y = 0.003) phosphors are sum-marized in Fig. 5. The excitation wavelengths for thesamples with x = 0.1, 0.2, and 0.3 were set to 342, 351,and 354 nm, respectively, corresponding to their excitationpeak. The IQE, EQE and absorbance of the sample withx = 0.1, 0.2, and 0.3 were 62.4, 21.3, and 34.1% forx = 0.1; 54.4, 25.5, and 47.0% for x = 0.2; and 38.9, 24.1,and 62.0% for x = 0.3, respectively. The highest value ofthe EQE was 25.5% in the La1¹x/3Al1¹x¹yTixO3:yCr3+(x = 0.2, y = 0.003) phosphor. With increasing the valueof x, the IQE was decreasing, while the absorbance wasincreasing.The temperature-dependent luminescence intensity nor-malized by the room temperature value of the La1¹x/3-Al1¹x¹yTixO3:yCr3+ (x = 0.1–0.3, y = 0.003) phosphorsare shown in Fig. 6(a). The excitation wavelengths for thesamples with x = 0.1, 0.2, and 0.3 were set to 342, 351,and 354 nm, respectively, corresponding to their excita-tion peak. The photoluminescence intensity gradually de-creased with increasing the temperature in all samples dueto the thermal quenching effect. The normalized intensitiesat 150 °C of the sample with x = 0.1, 0.2, and 0.3 are 32,17, and 3%, respectively. The thermal quenching effectstrongly occurred with increasing the amount of Ti. Ticontent plays a crucial role in controlling the luminescenceproperties of La1¹x/3Al1¹x¹yTixO3:yCr3+ (x = 0.1–0.3, y =0.003) phosphors. Both IQE and the temperature at whichthe intensity drops 50% of its room temperature intensity(T50) were also decreasing with increasing the amount ofTi. The diffuse reflectance of La1¹x/3Al1¹xTixO3 (x = 0.1–0.3) without Cr3+ doping is shown in Fig. 6(b). In all sam-ples, the absorbance edge was in the range of ³300 nm.With increasing the value of x, the absorption edge was redshifted, indicating that the bandgap decreased.It is probable that the thermal ionization effect occursseriously in the more Ti-containing sample. Li et al. havereported LaAlO3 phosphors activated with Mn4+, whichhas the same electron configuration as Cr3+. LaAlO3 andTi4+-doped LaAlO3 were used as host materials. The con-duction band of Ti4+-doped LaAlO3 is shifted down com-pared with that of LaAlO3, resulting in the enhancement ofquenching due to the dissipation of excited electrons ofMn4+ into the conduction band.29) From the UV–vis dif-fuse reflectance measurement, it is confirmed that the bandFig. 4. (a) Excitation and (b) emission spectra of the La1¹x/3-Al1¹x¹yTixO3:yCr3+ (x = 0.2, y = 0–0.05) phosphors calcined at1600 °C for 4 h.Fig. 5. IQE, EQE, and absorbance of the La1¹x/3Al1¹x¹yTixO3:yCr3+ (x = 0.1–0.3, y = 0.003) phosphors calcined at 1600 °C for4 h.Nakanishi et al.: Photoluminescence of perovskite-type Cr3©-activated phosphors in the La–Al–Ti–O systemJCS-Japan130gap of the host material La1¹x/3Al1¹xTixO3 (x = 0.1–0.3)decreased with increasing the amount of Ti, indicating thatthe thermal quenching is caused due to the thermal ioni-zation effect.30) On the other hand, the absorbance (asshown in Fig. 5) was lineally increasing with increasingthe amount of Ti although the amount of Cr doping wasfixed at y = 0.003. It is possible that the local coordinationenvironment around Cr3+ is modulated by the Ti4+ doping,resulting in the absorption enhancement. Liang et al. havereported that the emission intensity of Mg14Ge5O24:Mn4+phosphor was enhanced by Ti4+ doping to the host mate-rial.31) Lingling et al. investigated the effect of Ti4+ dop-ing to SrGe4O9:Mn4+ phosphor. Ti4+ doping improved theemission intensity because of the modification of the coor-dination environment around Mn4+.32) However, the ab-sorption enhancement and thermal quenching behaviorexhibit a trade-off relationship in the La1¹x/3Al1¹x¹yTixO3:yCr3+ (x = 0.1–0.3, y = 0.003) phosphors. Therefore,fine-tuning the Ti content within the range of x = 0.1–0.3 is necessary to optimize the luminescence properties infuture studies.Table 1 shows the luminescence properties of Cr3+-activated phosphor with the perovskite-type struc-ture.22,23,33,34) The excitation peak of the x = 0.2 sampleis 351 nm, which is the closest to UV 365 nm LEDs. TheIQE and EQE are also comparable to others, while thephotoluminescence intensity at 150 °C is the lowest. Thetemperature-dependence luminescence property should beimproved by the adjustment of the Ti content of host mate-rial La1¹x/3Al1¹xTixO3. In summary, the La1¹x/3Al1¹x¹y-TixO3:yCr3+ phosphor exhibits near-infrared emissionunder UV excitation and thus has potential applicationsas a near-infrared phosphor in the plant-growth or sensingtechnologies.4. ConclusionNear-infrared emitting phosphor La1¹x/3Al1¹x¹yTixO3:yCr3+ were successfully synthesized by a solid-state reac-tion for use in the plant-growth or remote sensing appli-cation. La1¹x/3Al1¹x¹yTixO3:yCr3+ (x = 0.2, y = 0.003)phosphor exhibited near-infrared emission at 751 nm witha FWHM of 41 nm under UV excitation, indicating thepossibility of applying for the near-infrared emitting LEDwith UV LED chip. The IQE and EQE were 54.4 and25.5%, respectively. This infrared phosphor targeting theplant leaf red-edge region (³750 nm) was developed byTi4+ doping of a LaAlO3 host material. The emission peakis red-shifted relative to that of LaAlO3:Cr3+ phosphor as aresult of tuning the local coordination environment aroundthe Cr3+ activator. Notably, the IQE under UVexcitation isthe highest among the Al-containing Cr3+-activated infra-red phosphors, such as Ca2AlNbO6:Cr3+ and Sr2AlNbO6:Cr3+. The temperature-dependence luminescence and thediffuse reflectance measurement revealed that Ti amountplays a crucial role in controlling the thermal quenchingbehavior of La1¹x/3Al1¹x¹yTixO3:yCr3+ (x = 0.1–0.3, y =0.003) phosphors. This study provides a development strat-egy for Cr3+-activated near-infrared emitting phosphors.Acknowledgments This work was supported by Inno-vative Science and Technology Initiative for Security (GrantNumber JPJ004596, ATLA, Japan).References1) E. F. Schubert and J. K. Kim, Science 308, 1274 (2005).2) C. J. Humphreys, MRS Bull. 33, 459 (2008).Fig. 6. (a) Emission intensity normalized by the room temper-ature value of the La1¹x/3Al1¹x¹yTixO3:yCr3+ (x = 0.1–0.3, y =0.003) phosphors. (b) Diffuse reflectance spectra of La1¹x/3-Al1¹xTixO3 (x = 0.1–0.3) without Cr3+ doping.Table 1. Luminescence properties of Cr3+-activated phosphorswith the perovskite-type structureHost­ex(nm)­em(nm)FWHM(nm)IQE(%)EQE(%)I150°C(%)Ref.Ca2AlNbO6 337 744 49 32.6 54 22)Sr2AlTaO6 303 704 36 23)Sr2AlNbO6 323 760 34* 66.1 28.4 52 33)La2MgTiO6 349 766 59* 21.6 41.7 34)La–Al–Ti–O(x = 0.2)351 751 41 54.4 25.5 17Thisstudy*As the value was not shown in the paper, it was read from the emissionspectrum.Journal of the Ceramic Society of Japan 134 [3] 128-132 2026 JCS-Japan1313) C. C. Lin and R.-S. Liu, J. Phys. Chem. Lett. 2, 1268(2011).4) Z. Xia and A. Meijerink, Chem. Soc. Rev. 46, 275(2017).5) R.-J. Xie, N. Hirosaki, K. Sakuma, Y. Yamamoto andM. Mitomo, Appl. Phys. Lett. 84, 5404 (2004).6) K.-B. Kim, Y.-I. Kim, H.-G. Chun, T.-Y. Cho, J.-S. Jungand J.-G. Kang, Chem. Mater. 14, 5045 (2002).7) K. Uheda, N. Hirosaki, Y. Yamamoto, A. Naito, T.Nakajima and H. Yamamoto, Electrochem. Solid St. 9,H22 (2006).8) Y. Q. Li, J. E. J. van Steen, J. W. H. van Krevel, G.Botty, A. C. A. Delsing, F. J. 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