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Article

Influence of Ga Substitution on the Local Structure and Luminescent Properties of Eu-Doped CaYAlO4 Phosphors

by
Ju Hyun Oh
1,
Hyunwoo Kim
1,
Mijeong Kang
2 and
Seunghun Lee
1,*
1
Department of Physics, Pukyong National University, Busan 48513, Republic of Korea
2
Department of Optics and Mechatronics Engineering, Pusan National University, Busan 46241, Republic of Korea
*
Author to whom correspondence should be addressed.
Inorganics 2023, 11(8), 329; https://doi.org/10.3390/inorganics11080329
Submission received: 29 June 2023 / Revised: 26 July 2023 / Accepted: 4 August 2023 / Published: 6 August 2023
(This article belongs to the Section Inorganic Solid-State Chemistry)
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Abstract

:
Understanding the local environment of luminescent centers in phosphors serves as a blueprint for designing the luminescent properties of phosphors. Chemical substitution is a general strategy for engineering the local structure around luminescent center ions. In this study, we systematically investigate the luminescent properties of Ga-substituted Eu-doped CaYAlO4 (CYAGO:Eu) phosphors and the local structure of the Eu ions. The Ga substitution at the Al sites leads to a significant enhancement in the electric dipole transition of Eu3+ (5D0 → 7F2). The Judd–Ofelt analysis reveals that Eu3+ ions are substituted for Ca/Y, and the Ga substitution increases the asymmetricity of the local structure around the Eu ions because of the different ionic radii and electronegativities of Al and Ga. In addition, Eu2+ emission is missing regardless of the Ga substitution and post-hydrogen treatments. The present work provides deeper insight into the role of chemical substitution in oxide phosphors.

1. Introduction

Phosphor-converted white-light-emitting diodes (pc-wLEDs), as alternatives to conventional incandescent or fluorescent lamps, have distinct advantages, such as low power consumption (~80%) and long working lifetime (~25 times) [1,2,3]. pc-wLEDs have been leading to changes in the light industry but still require improvements [4,5]. The most commonly used white LED devices utilize blue LEDs as a first-level light source and yellow phosphors to achieve white light [6,7]. While this approach is straightforward for obtaining white light, it has some drawbacks, such as a low color rendering index (CRI < 80) and high correlated color temperature (CCT > 7000 K) owing to the absence of red color components [8,9,10]. To address these limitations, integrating red, green, and blue phosphors has been employed to create white-light LEDs. However, this approach also has some issues, as it requires further consideration of balancing different phosphors, taking different aging and temperature responses of the phosphors into account. Therefore, single-component white phosphors have been studied intensively [11,12,13].
CaYAlO4 (CYAO) has a tetragonal structure (space group: I4/mmm) and is a promising candidate for luminescent host materials because of its inexpensive raw materials, good chemical and thermal stability, and low environmental toxicity [14,15]. CYAO is a typical ABCO4-structured compound, where A is an alkaline-earth cation, B is a trivalent rare-earth element including yttrium and scandium, and C is a transition metal element, aluminum, or gallium. These crystals share a structure similar to that of K2NiF4-type perovskite oxides with layered structures. In the CYAO unit cell, Al3+ ions occupy the site of octahedral symmetry, whereas Ca2+ and Y3+ ions are randomly distributed in the sites of C4v symmetry [16,17]. Because CYAO has both 2+ and 3+ cations, it has been regarded as a possible material for hosting multivalent rare-earth activators with different emission wavelengths for single-component white phosphors. Eu doping to the CYAO host has thus been expected to be a straightforward way to achieve single-component white phosphors since Eu has 2+ and 3+ oxidation states, which exhibit different emission bands; the Eu3+ ion is a representative red-emitting activator, whereas the Eu2+ ion has different emission wavelengths depending on host materials but shorter emission wavelengths than the Eu3+ ion in general [18,19]. However, obtaining the Eu2+ state in CYAO is difficult, and it has been suggested to be due to its larger ion radius than Ca2+ and Y3+, which are compactly surrounded by AlO6 octahedrons. To resolve this, substituting Si4+ for Al3+ has been suggested to provide a larger space at the Ca/Y sites to accommodate Eu2+ ions [20]. It has also been reported that changing the ratio of Y3+ to Al3+ leads to the activation of Eu2+ emission [21], whose idea is based on the Eu2+ emission hosted by CaAl2O4 [22,23]. Such composition engineering, including chemical substitution, is a basic approach to designing the luminescence properties of phosphors via adjusting the local environment around luminescent centers; thus, it has been studied in a myriad of phosphors [24,25,26,27,28]. However, systematic studies on the local environment changes or luminescence properties due to chemical substitution in the CYAO host are still lacking; in particular, the effect of Ga substitution has not been investigated yet. Considering the ionic radius of Al, possible substitutional elements to manipulate the electrochemical environment around the Eu ions in the CYAO host are Si or Ga, both of which are the nearest neighbor elements in the periodic table. Ga substitution is expected to be the counterpart to Si substitution, based on their ionic radii (Si4+: 0.4 Å and Ga3+: 0.62 Å) and bonding strength (i.e., dissociation energy; Si–O: ~799 kJ/mol and Ga–O: ~353 kJ/mol). We can thus expect that the study on the effect of Ga substitution provides an opportunity to re-examine the previous studies on the effect of Si substitution. As it is representative of the oxides containing multiple cations with different oxidation states, a better understanding of this material can provide more insight into designing the luminescence properties of oxide phosphors.
In this study, we systematically investigate the local structure and photoluminescence properties of Ga-substituted Eu-doped CYAO (CYAGO:Eu) phosphors and compare them with those of pure CYAO:Eu. Specific X-ray diffraction (XRD) peaks corresponding to the CYAO phase in the CYAGO:Eu samples shift to a lower angle via Ga substitution due to its larger ion radius than Al3+ (Ga3+: 0.62 Å and Al3+: 0.535 Å), indicating that Ga is substituted for Al successfully. Both the intense electric dipole transition of Eu3+ and its decay time constant follow a single-exponential model, indicating that Eu3+ ions occupy the Ca/Y sites. The Judd–Ofelt analysis reveals that the asymmetricity of Eu–O bonds increases with the Ga substitution. The absence of Eu2+ emission in the CYAO cannot be overcome by the Ga substitution and post-hydrogen treatment. We suggest that the overall bond dissociation energy correlates with the activation of Eu2+ emission via hydrogen incorporation in the CYAO host.

2. Results and Discussion

Figure 1 shows the X-ray diffraction (XRD) patterns of the CYAGO:Eu phosphors synthesized at different temperatures. The XRD patterns of the samples synthesized at 630 and 730 °C exhibit mainly the Y2O3 and Ga2O3 crystalline phases, indicating that the CYAO phase is not formed. A change in the XRD patterns is observed in the sample sintered at 860 °C. A new diffraction peak, which is not seen at the lower synthesis temperatures, is observed near 33.3° and corresponds to the (103) peak of the CYAO phase. The CYAO phase becomes dominant in the samples synthesized at ≥950 °C. As observed elsewhere, the Y2O3 phase remains as a minor secondary phase. Some of the peaks (marked by the asterisks) could not be definitively identified—among the possible combinations of the elements (i.e., Ca, Y, Al, Ga, Eu, and O), we have speculated that the peaks correspond to Ca3Al2O6 and/or monoclinic Y2O3 phases (see Figure S2 in the Supplementary Information for details). It is notable that some XRD peaks gradually shift to a lower angle with increasing the synthesis temperature, and the full width at half maximum (FWHM) of the XRD peaks becomes narrower. Figure 1b shows the enlarged XRD patterns of the CYAGO:Eu samples with the CYAO phase. The four peaks located at approximately 33.3°, 34.7°, 46.5°, and 49.9° correspond to the (103), (110), (114), and (200) peaks of the CYAO phase, respectively. The peak positions of the (110) and (200) peaks change remarkably with respect to the synthesis temperatures, while those of the (103) and (114) peaks do not, which indeed implies a change in lattice parameters. We calculated the lattice constants of CYAGO:Eu as shown in Table 1, including the lattice parameters of Eu-doped pure CaYAlO4 (CYAO:Eu) prepared at a synthesis temperature of 1190 °C for comparison. The lattice parameters of CYAO:Eu are calculated to be a = b = 3.640(2) Å and c = 11.87(9) Å. However, the Ga substitution leads to an increase in both a and c lattice parameters manifestly. The increase is attributed to the difference in ionic radius between Ga and Al ions; the ionic radius of the Ga3+ ion (r = 0.62 Å for 6-coordination) is larger than that of the Al3+ ion (r = 0.54 Å for 6-coordination) [29]. With increasing the synthesis temperatures, the lattice parameters gradually increase and become saturated, which indicates that the Ga substitution is fully completed at the synthesis temperature of >1080 °C, but the crystallinity continues to improve.
To investigate the luminescence properties of the CYAGO:Eu samples correlated with the structural change, we measured the PL excitation and emission spectra of the samples synthesized at different temperatures. Figure 2a shows the excitation spectra of the CYAGO:Eu samples. The excitation spectra were recorded at 622 nm (5D0 → 7F2 transition of Eu3+). The broad excitation band in the range of 225–350 nm is attributed to the overlapped charge-transfer (CT) states from the 2p orbital of O2− and the 4f orbital of Eu3+ [20,30,31]. The four excitation bands observed in the range of 350–425 nm correspond to the 4f-4f transitions of Eu3+ ion: 363 nm (7F0 → 5D4), 381 nm (7F0 → 5G2), 396 nm (7F0 → 5L6), and 415 nm (7F0 → 5D3) [20,30,31].
Figure 2b shows the emission spectra of the CYAGO:Eu samples, measured at an excitation wavelength of 279 nm. For the CYAGO:Eu samples synthesized above 950 °C, seven emission bands are observed at 581 nm, 591 nm, 599 nm, 622 nm, 657 nm, 693 nm, and 702 nm. These emission bands are associated with the transitions of 5D0 → 7F0 (581 nm), 5D0 → 7F1 (591 and 599 nm), 5D0 → 7F2 (622 nm), 5D0 → 7F3 (657 nm), and 5D0 → 7F4 (693 and 703 nm), respectively [20,30,31]. The PL intensities gradually increase with increasing the synthesis temperature, which is attributed to the improvement in crystallinity and/or the change in the local environment around Eu ions following Ga substitution. According to Judd–Ofelt theory [32,33,34], the 5D0 → 7F1 transition of the Eu3+ ion is a magnetic dipole transition, which is permitted by the selection rule. Conversely, the 5D0 → 7F2 transition, a forced electric dipole transition, is only allowed for Eu ions at a lattice site without inversion symmetry. The emission spectra of the CYAGO:Eu demonstrate that the emission due to 5D0 → 7F2 is more prominent than that from 5D0 → 7F1. Such a dominant electric dipole transition indicates that the Eu3+ ions occupy the Ca/Y sites rather than the Al sites because the Al site has an AlO6 octahedral structure with an inversion center [35]. For the samples synthesized at temperatures below 860 °C, the main emission band is observed at 612 nm. Before the formation of the CaYAlO4 phase, the Eu3+ ions appear to be dominantly hosted by the Y2O3 phase or Eu2O3 itself under the detection limit of XRD—the Eu3+ 5D0 → 7F2 transition in Y2O3 and Eu2O3 has been observed near 610 nm [36,37,38,39].
Measuring luminescence lifetime provides an insight into the local environment of a luminescence activator. Figure 3 shows the decay curves of the Eu3+ 5D0 → 7F2 transition (622 nm) of the CYAGO:Eu samples upon the excitation at 279 nm. The decay curve of the samples synthesized at 950 °C is described by a double-exponential decay model: I(t) = I1exp(−t/τ1) + I2exp(−t/τ2), where I(t) is the emission intensity at time t; I1 and I2 are two constants; and τ1 and τ2 are decay time constants, respectively (see Figure S4 in the Supplementary Information for detail). In contrast, the decay behaviors of the samples synthesized at 1080 and 1190 °C follow a single-exponential decay model: I(t) = I0exp(−t/τ). The result is consistent with the consequence of the lattice parameter change. The double-exponential luminescence decay suggests the possible presence of two different luminescent centers of Eu3+ [40,41]. We attribute the double-exponential decay to an incomplete solid-state reaction; another Eu3+ emission is likely to be hosted by Y2O3:Eu3+ or Eu2O3. Although the samples synthesized at >950 °C contain the Y2O3 phase, the strong Eu3+ emission is speculated to be mainly governed by the CYAGO:Eu phase.
When Eu3+ ions are embedded in a host matrix with an inversion symmetry, an electric dipole transition is strictly forbidden by the parity-selection rule. In other words, the emission from the electric dipole transition (i.e., 5D0 → 7F2) is allowed for Eu3+ ions with no inversion symmetry. However, the magnetic dipole transition obeys the selection rule and is thus independent of local symmetry. An asymmetric ratio (R) factor is described by R = IED/IMD, where IED and IMD are the PL intensity of 5D0 → 7F2 and 5D0 → 7F1 transitions, respectively, thus providing an understanding of the site symmetry of the crystal field surrounding the Eu3+ ion [42,43]. To investigate the luminescence properties of the CYAGO:Eu samples, possibly correlated with their local structures, we calculated the R factors of the CYAGO:Eu samples (Figure 4). The R-factor of the CYAO:Eu sample is also included for comparison (see Figure S5 in the Supplementary Information for detail). Except for the sample synthesized at 950 °C, the R factors of the CYAGO:Eu samples are found to be ≈ 2.6, which is higher than that of CYAO:Eu (≈2.4). Several reports have suggested that particle size and/or morphology affect the luminescent properties of phosphors [44,45], but we were not able to find a significant difference in both the particle size and morphology of CYAO:Eu and CYAGO:Eu samples (see Figure S6 in Supplementary Information for the SEM images). The R factors of CYAGO:Eu and CYAO:Eu indicate that Eu3+ sites become more asymmetric as a result of Ga substitution. The higher R factor of the CYAGO:Eu sample synthesized at 950 °C is speculated to be due to the incomplete solid-state reaction as discussed above.
To gain further insight into the local environment of Eu3+ ions, we performed theoretical calculations of optical transition strength parameters Ωλ (λ = 2 and 4) from the emission spectra. According to Judd–Ofelt theory, [32,34] the 5D0 → 7F1 transition (magnetic dipole) is independent of the crystal environment. In contrast, the 5D0 → 7FJ (J = 2, 4, and 6) transition (electric dipole) depends on the Ωλ (λ = 2, 4, and 6) parameter. The Ω2 parameter depends on the local crystal environment of rare-earth ion sites. The Ω4 and Ω6 are related to the viscosity and rigidity of a host matrix [46]. The optical transition strength parameters (Ωλ) can be obtained from the integrated intensity ratio of an electric dipole transition to a magnetic dipole transition as follows [47,48]:
A J A M D = ∫ I J d v ∫ I M D d v = e 2 S M D v J 3 v M D 3 n n 2 + 2 2 9 n 3 Ω J ( Ψ J U J Ψ ′ J ′ ) 2
where e is the electronic charge (4.803 × 10−10 esu); SMD is the intensity of the 5D0 → 7F1 transition (magnetic dipole); vMD and vJ are the center wavenumbers for the magnetic dipole transition and the electric dipole transitions (5D0 → 7FJ (J = 2, 4, and 6)), respectively; n is the index of refraction of the host (here, n = 1.9 [49,50,51]); and (ΨJ∥UJ∥Ψ′J′)2 is the square reduction matrix factor for the electric dipole transitions—0.0032 for 5D0 → 7F2, 0.0023 for 5D0 → 7F4, and 0.0002 for 5D0 → 7F6 [52]. The calculated values of Ω2 and Ω4 for different synthesis temperatures are listed in Table 2. Note that the Ω6 parameter was not considered here because the 5D0 → 7F6 transition was out of the measured range.
According to Kumar et al., the comparison of Ω2 and Ω4 can be used to estimate the covalence of the bonds between a Eu3+ ion and a ligand anion, and the symmetry around Eu3+ ions [53]. For all CYAGO:Eu in this study (Table 2), Ω2 is higher than Ω4 (i.e., Ω2 > Ω4), which dictates (1) the asymmetricity of the Eu3+ sites and/or (2) the covalency of the Eu–O bonds [53,54]. From the difference between the parameters (Ω2) of the CYAGO:Eu and CYAO:Eu samples, we can further discuss the effect of Ga substitution on the local structure of the Eu ions. The value of Ω2 for the Ga-substituted CYAO:Eu (3.806) is larger than that for pure CYAO (3.630), and the larger value implies a higher covalency of Eu–O bonds and/or more distorted Eu3+ sites due to Ga substitution [55,56]. However, Ga substitution is expected to make Eu–O bonds more ionic. The bonding character of Eu–O would be modified by the next-neighboring ion being substituted [57], and in the bond structure of Eu–O–X (X = Al or Ga), the covalency of O–Ga is stronger than O–Al since Ga3+ (1.81) has a larger electronegativity than Al3+ (1.61) [58]. Hence, fewer electrons are shared in the Eu–O bond, which leads to a less-covalent Eu–O bond [57,59,60]. We thus conclude that the increased asymmetricity of the Eu–O bond is responsible for the higher value of Ω2 due to Ga substitution. In other words, substituting Ga3+ for Al3+ further enhances the asymmetricity around the Eu ions and reduces their covalent nature. Ω4 is not directly related to the local structure of rare-earth ions and indicates the bulk properties such as rigidity and viscosity [46,61]. Ga substitution increases the rigidity (or viscosity) of the CYAO host matrix. Note that the values of Ω2 and Ω4 for the samples synthesized at 950 and 1020 °C appear much larger than those of the others—it is due to the incomplete solid-state reaction and is consistent with the results above.
As aforementioned, CYAO is a possible host material to accommodate multivalent Eu ions, but we were unable to observe any emission of Eu2+ ions for CYAGO:Eu. It has been reported that the Eu2+ emission is activated by hydrogen mediation, in which defect passivation by Si substitution is necessary [31]. Hydrogen can passivate surface dangling bonds and vacancy defects in oxides, thus stabilizing the CYAO host and activating Eu2+ emission [31]. Interstitial hydrogen can also provide a donor in oxides, which may lead to the partial conversion of Eu3+ to Eu2+ in the CYAO host. For such hydrogen incorporation into CYAO, the formation of oxygen vacancies ( H 2 g a s + O l a t t i c e 2 − → H 2 O g a s + V ¨ o ) needs to be suppressed during the post-hydrogen treatment. Si substitution increases the overall bonding strength, thus suppressing the formation of oxygen vacancy against the post-hydrogen treatment, which enables hydrogen incorporation in the CYAO host. Figure 5 shows the PLE and PL spectra of the CYAGO:Eu sample after the post-hydrogen treatment. No significant change is seen in either spectra tracking Eu2+ (top) and Eu3+ (bottom) emissions. Owing to the lower bonding dissociation energy of a Ga–O bond (~353 kJ/mol) than that of an Al–O bond (~511 kJ/mol), the overall bonding strength thus becomes weaker throughe Ga substitution, which in turn likely hinders the hydrogen incorporation into the CYAO host in contrast to the Si substitution (see Figure S7 in the Supplementary Information for details) [31]. The Ga substitution in the CaYAlO4 host seems less effective in manipulating the luminescence properties of the Eu-doped CaYAlO4, but the absence of Eu2+ emission after the post-hydrogen treatment demonstrated in this work further supports the hydrogen-mediated activation of Eu2+ emission via defect passivation. Our experimental approach and analysis presented here can serve as a guide for compositional variation in phosphors.

3. Materials and Methods

Sample preparation: Samples with a composition of Ca0.95YAl0.8Ga0.2O4:Eu0.05 (CYAGO:Eu) were prepared through a conventional solid-state method with stoichiometric amounts of Al2O3 (extra pure, Hayashi Pure Chemical Ltd., Japan), CaCO3 (99.5%, Junsei Chemical Co., Ltd., Japan), Y2O3 (99.99%, Sigma Aldrich, USA), Ga2O3 (99.99%, Alfa Aesar, USA), and Eu2O3 (99.99%, Alfa Aesar, USA). The mixture of the starting materials was subjected to a planetary ball milling process using zirconium oxide balls. In thermogravimetry and differential scanning calorimetry (TG/DSC) measurements for a temperature range of 25–1200 °C, we were able to observe only a significant weight loss and an endothermic peak corresponding to the decomposition of CaCO3 at around 740 °C (see Figure S1 in the Supplementary Information for details). Because the synthesis temperature could not be optimized and specified easily from the TG/DSC curve, the systematic sample preparation in this study was indeed necessary to confirm that the Ga substitution was achieved successfully, thereby tracing the local structural change as a result of Ga substitution. In the synthesis processes, a natural thermal gradient in a tube furnace was employed to apply different temperatures to each sample in dehydrated air (flow rate: 0.15 L/min). Such a combinatorial approach excludes unintentional parameter variations, except for the temperature. Each sample was synthesized at 630 to 1190 °C. After the synthesis process, the obtained powders were ground in an agate mortar with a pestle for 10 min to remove lumps and ensure homogeneity. In energy-dispersive spectroscopy (EDS) measurements, any impurity elements were not detected within the resolution limit of the equipment (Oxford INCA system equipped in JEOL JSM-6700).
Characterizations: X-ray diffraction (XRD) measurements were carried out using an X’pert-MPD system (Panalytical, Netherlands) with CuKα1 = 1.5406 Å. The diffraction patterns were collected in the 2θ range of 10–90° with a step size of 0.013°. Photoluminescence (PL) and PL excitation (PLE) measurements were performed using a Photon Technology International (PTI) spectrophotometer equipped with a 60 W Xe-arc lamp. Luminescence decay time curves were collected using a Fluorolog-QM (Horiba, Japan) spectrometer with a 450 W arc xenon lamp.

4. Conclusions

CaYAlO4 is ostensibly capable of hosting the multivalent state of Eu ions but merely allows emission attributed to the Eu3+ state. Ga substitution for the Al sites in Eu-doped CaYAlO4 leads to a local structural change around the Eu ions owing to the larger ionic radius of Ga3+ compared to Al3+, which is evinced by the Judd–Ofelt analysis of the photoluminescence spectra of Ga-substituted Eu-doped CaYAlO4. Eu2+ emission is absent in Ga-substituted Eu-doped CaYAlO4 even after post-hydrogen treatment, which indicates that Ga substitution does not facilitate hydrogen incorporation in the CaYAlO4 host. These results provide a better understanding of the luminescence properties of Eu-doped CaYAlO4 hosts.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/inorganics11080329/s1, Figure S1: Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) curves of Ga-Substituted Eu-doped CaYAlO4 (CYAGO:Eu) sample; Figure S2: X-ray diffraction (XRD) patterns of Eu-doped CaYAlO4 (CYAO:Eu) and Ga-substituted Eu-doped CaYAlO4 (CYAGO:Eu) samples synthesized at 1190 °C; Figure S3: International Commission on Illumination (CIE) chromaticity diagram of the Ga-substituted Eu-doped CaYAlO4 (CYAGO:Eu) samples synthesized at different temperatures under 277 nm excitation; Table S1: International Commission on Illumination (CIE) chromaticity coordinates (x, y) of the Ga-substituted Eu-doped CaYAlO4 (CYAGO:Eu) samples synthesized at different temperatures under 277 nm excitation; Figure S4: Decay curve of a Ga-substituted Eu-doped CaYAlO4 (CYAGO:Eu) sample synthesized at 950 °C; Figure S5: Photoluminescence (PL) and PL excitation (PLE) spectra of an Eu-doped CaYAlO4 (CYAO:Eu) sample synthesized at 1190 °C; Figure S6: Field emission scanning electron microscopy (FE-SEM) images of Ga-substituted CaYAlO4:Eu samples; Figure S7: Photoluminescence (PL) spectra under 365 nm excitation of a Ga-substituted Eu-doped CaYAlO4 (CYAGO:Eu), pure Eu-doped CaYAlO4 (CYAO:Eu), and Si-substituted Eu-doped CaYAlO4 (CYASO:Eu) samples after post-hydrogen treatment at 800 °C. Additional information on the sample preparation of Si-substituted CaYAlO4:Eu (CYASO:Eu) is included, and References [20,31,62,63,64,65] are cited in the Supplementary Materials.

Author Contributions

Conceptualization, J.H.O. and S.L.; methodology, J.H.O.; validation, J.H.O. and M.K.; investigation, J.H.O. and H.K.; writing—original draft preparation, J.H.O. and S.L.; writing—review and editing, M.K.; visualization, J.H.O.; supervision, S.L. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (NRF-2021R1C1C1009863, NRF-2020R1A4A4078780). M. Kang acknowledges support from National Research Foundation of Korea Grant funded by the Korean Government (NRF-2021R1C1C1010213).

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Changes in the crystalline phase of Ga-substituted Eu-doped CaYAlO4 (CYAGO:Eu) samples prepared at different synthesis temperatures: (a) X-ray diffraction (XRD) patterns for CYAGO:Eu samples synthesized at different temperatures (from 630 to 1190 °C). The bars indicate the expected XRD peak positions of CaYAlO4, Y2O3, and Ga2O3 (JCPDS No. 81-0742, 41-1105, and 06-0503, respectively). The peaks marked by asterisks could not be definitively identified but are speculated to correspond to Ca3Al2O6 and/or monoclinic Y2O3 phases. (b) The magnified XRD patterns for selected diffraction peaks of the CaYAlO4 phase. The dashed lines indicate the diffraction peak position corresponding to the (103), (110), (114), and (200) planes of the CaYAlO4 phase. The inset shows the full width at half maximum (FWHM) values for main peaks at 33.3° (i.e., (103) peak).
Figure 1. Changes in the crystalline phase of Ga-substituted Eu-doped CaYAlO4 (CYAGO:Eu) samples prepared at different synthesis temperatures: (a) X-ray diffraction (XRD) patterns for CYAGO:Eu samples synthesized at different temperatures (from 630 to 1190 °C). The bars indicate the expected XRD peak positions of CaYAlO4, Y2O3, and Ga2O3 (JCPDS No. 81-0742, 41-1105, and 06-0503, respectively). The peaks marked by asterisks could not be definitively identified but are speculated to correspond to Ca3Al2O6 and/or monoclinic Y2O3 phases. (b) The magnified XRD patterns for selected diffraction peaks of the CaYAlO4 phase. The dashed lines indicate the diffraction peak position corresponding to the (103), (110), (114), and (200) planes of the CaYAlO4 phase. The inset shows the full width at half maximum (FWHM) values for main peaks at 33.3° (i.e., (103) peak).
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Figure 2. Photoluminescence (PL) properties of Ga-substituted Eu-doped CaYAlO4 (CYAGO:Eu) samples synthesized at different temperatures: (a) Photoluminescence excitation (PLE) spectra monitored at 622 nm. (b) PL emission spectra under 279 nm excitation. The inset is a 2D contour plot of the emission spectra.
Figure 2. Photoluminescence (PL) properties of Ga-substituted Eu-doped CaYAlO4 (CYAGO:Eu) samples synthesized at different temperatures: (a) Photoluminescence excitation (PLE) spectra monitored at 622 nm. (b) PL emission spectra under 279 nm excitation. The inset is a 2D contour plot of the emission spectra.
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Figure 3. Decay curves of Eu3+ 5D0 → 7F2 transition (622 nm) for Ga-substituted Eu-doped CaYAlO4 (CYAGO:Eu) samples synthesized at different temperatures. The decay curves were measured under the excitation of 279 nm. The red lines are the best fits to each decay curve using a single- (with a time constant, τ) or double- (with two time constants: τ1 and τ2) exponential decay model. For the double-exponential decay model, the averaged decay time constants (τavg) were obtained from the equation, τavg = (I1τ12 + I2τ22)/(I1τ1 + I2τ2) (see the main text for details).
Figure 3. Decay curves of Eu3+ 5D0 → 7F2 transition (622 nm) for Ga-substituted Eu-doped CaYAlO4 (CYAGO:Eu) samples synthesized at different temperatures. The decay curves were measured under the excitation of 279 nm. The red lines are the best fits to each decay curve using a single- (with a time constant, τ) or double- (with two time constants: τ1 and τ2) exponential decay model. For the double-exponential decay model, the averaged decay time constants (τavg) were obtained from the equation, τavg = (I1τ12 + I2τ22)/(I1τ1 + I2τ2) (see the main text for details).
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Figure 4. Asymmetric ratio (R factors) of Ga-substituted Eu-doped CaYAlO4 (CYAGO:Eu) samples synthesized at different temperatures. The dashed line represents the R factor of the Eu-doped CaYAlO4 (CYAO:Eu) for comparison.
Figure 4. Asymmetric ratio (R factors) of Ga-substituted Eu-doped CaYAlO4 (CYAGO:Eu) samples synthesized at different temperatures. The dashed line represents the R factor of the Eu-doped CaYAlO4 (CYAO:Eu) for comparison.
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Figure 5. Photoluminescence (PL) and PL excitation (PLE) spectra of a Ga-substituted Eu-doped CaYAlO4 (CYAGO:Eu) sample after post-hydrogen treatment. (Top) PLE (blue dashed line, λem = 503 nm) and PL (blue solid line, λexc = 340 nm) spectra. (Bottom) PLE (pink dashed line, λem = 622 nm) and PL (pink solid line, λexc = 279 nm) spectra. The emission and excitation wavelengths for tracking Eu2+ emission (top) were chosen based on the previous reports in which Eu2+ emission has been observed [31]. The post-hydrogen treatment was performed for the CYAGO:Eu sample synthesized at 1190 °C under H2/Ar (2:8) (flow rate: 0.15 L/min) for 3 h at 800 °C.
Figure 5. Photoluminescence (PL) and PL excitation (PLE) spectra of a Ga-substituted Eu-doped CaYAlO4 (CYAGO:Eu) sample after post-hydrogen treatment. (Top) PLE (blue dashed line, λem = 503 nm) and PL (blue solid line, λexc = 340 nm) spectra. (Bottom) PLE (pink dashed line, λem = 622 nm) and PL (pink solid line, λexc = 279 nm) spectra. The emission and excitation wavelengths for tracking Eu2+ emission (top) were chosen based on the previous reports in which Eu2+ emission has been observed [31]. The post-hydrogen treatment was performed for the CYAGO:Eu sample synthesized at 1190 °C under H2/Ar (2:8) (flow rate: 0.15 L/min) for 3 h at 800 °C.
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Table 1. Lattice constants, unit cell volumes, and grain size of Ga-substituted Eu-doped CaYAlO4 (CYAGO:Eu) samples prepared at different synthesis temperatures. The lattice constants of Eu-doped CaYAlO4 (CYAO:Eu) are included for comparison. The grain sizes were calculated from the full width at half maximum (FWHM) values of the (103) peaks using Scherrer equation.
Table 1. Lattice constants, unit cell volumes, and grain size of Ga-substituted Eu-doped CaYAlO4 (CYAGO:Eu) samples prepared at different synthesis temperatures. The lattice constants of Eu-doped CaYAlO4 (CYAO:Eu) are included for comparison. The grain sizes were calculated from the full width at half maximum (FWHM) values of the (103) peaks using Scherrer equation.
SamplesSyn. Temp.
(°C)		a = b
(Å)		c
(Å)		Volume
(Å3)		Grain Size
(nm)
CYAO:Eu11903.640(2)11.87(9)157.4(1)53.86
CYAGO:Eu9503.648(2)11.92(1)158.6(6)39.49
10203.651(6)11.90(3)158.7(1)44.82
10803.654(0)11.89(8)158.8(5)51.82
11603.654(8)11.89(2)158.8(5)53.84
11903.654(6)11.89(1)158.8(1)56.79
Table 2. Optical transition strength parameters of CYAGO:Eu samples synthesized at different temperatures. The parameters of CYAO:Eu are included for comparison.
Table 2. Optical transition strength parameters of CYAGO:Eu samples synthesized at different temperatures. The parameters of CYAO:Eu are included for comparison.
SamplesSyn. Temp.
(℃)		Ω2
(10−20 cm2)		Ω4
(10−20 cm2)
CYAO:Eu11903.6302.512
CYAGO:Eu9504.2582.780
10203.8472.536
10803.8242.580
11603.7982.604
11903.8062.619
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Oh, J.H.; Kim, H.; Kang, M.; Lee, S. Influence of Ga Substitution on the Local Structure and Luminescent Properties of Eu-Doped CaYAlO4 Phosphors. Inorganics 2023, 11, 329. https://doi.org/10.3390/inorganics11080329

AMA Style

Oh JH, Kim H, Kang M, Lee S. Influence of Ga Substitution on the Local Structure and Luminescent Properties of Eu-Doped CaYAlO4 Phosphors. Inorganics. 2023; 11(8):329. https://doi.org/10.3390/inorganics11080329

Chicago/Turabian Style

Oh, Ju Hyun, Hyunwoo Kim, Mijeong Kang, and Seunghun Lee. 2023. "Influence of Ga Substitution on the Local Structure and Luminescent Properties of Eu-Doped CaYAlO4 Phosphors" Inorganics 11, no. 8: 329. https://doi.org/10.3390/inorganics11080329

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