Modification of Ce3+ luminescence in Li+ and Mg2+ codoped GAGG:Ce scintillators: A comparative study using synchrotron radiation

The luminescence characteristics of Gd3Al2Ga3O12:Ce3+ garnet scintillator crystals codoped with Li+ and Mg2+ ions were investigated. The excitation and emission spectra were obtained within the energy range of 3.6-21.6 eV, at both liquid helium and room temperature conditions to examine the effects of Li+ and Mg2+ codoping on cerium valence states, gadolinium-cerium energy transfer, and defect formation. Codoping with Mg2+ was found to efficiently convert Ce3+ to Ce4+, evidenced by significant changes in the shapes of the excitation spectra profile for Ce3+ luminescence in the exciton range, while Li+ had little impact. The presence of the high concentration of Ce4+ ions hampered the Gd3+→Ce4+ energy transfer. Redshifts in Ce3+ emission peaks indicated codopant-induced perturbations to the crystal field environment. The significant changes in the location of the Ce3+ excitation bands of Ce3+ luminescence in the exciton range further suggested alterations in the elemental distributions by the creation of complex defect clusters, particularly with Mg2+ codoping. The results demonstrate that Li+ and Mg2+ uniquely impact cerium valence, energy transfer processes, and structural properties in GAGG:Ce crystals.


Introduction
To develop new functional materials and deepen our understanding of physics, investigating the structure-property relationship is essential.Fine-tuning material properties and designing novel luminescent materials, and crystal engineering strategies form the basis of emerging application innovations [1].Through structure, composition, and synthesis method links, RE3M5O12 garnets form an enigmatic family of inorganic composites [1][2][3][4][5][6].With their compositional flexibility, RE3M5O12 From medical imaging to white LED applications and afterglow materials, Ce 3+ doped garnets hold great potential, capturing much attention.A breakthrough emerged with the development of the Gd3Al2Ga3O12:Ce with an impressive light yield (LY) value approaching the theoretical limit of 60000 photons per 1 MeV along with a fast scintillation response of roughly 100 ns [7].According to [8,9], the GAGG matrix contains a disordered structure that significantly diminishes scintillation efficiency in medical imaging applications, such as long rise time and slow decay time, accompanied by afterglow.The impact of Li⁺ and Mg²⁺ codoping on the luminescence properties and defect formations of gadolinium aluminum gallium oxide single crystals doped with cerium (III) was investigated.Li + and Mg 2+ codoped Gd₃Al₂Ga₃O₁₂:Ce 3+ single crystals were synthesized using the micro-pulling down technique (μ-PD), with codoping concentrations of 1 at.%Ce³⁺ and 3000 ppm of Li⁺ and Mg²⁺ in the melt.Synchrotron radiation at both cryogenic and ambient temperatures, as well as photoluminescence decay kinetics curves at ambient temperature, were employed to analyze the crystals.

Synthesis of single crystals
Ce 3+ doped and Li + and Mg 2+ codoped gadolinium aluminum gallium oxide single crystals were grown using the μ-PD method.The MgCO3, Ga2O3, Li2CO3, Al2O3, CeO2, and Gd2O3, raw materials were stoichiometrically mixed and placed into an iridium crucible with a 2.5 mm diameter [10,11].The concentrations of Ce 3+ Li + and Mg 2+ ions in the melt were 1.0 at.% and 3000 ppm, respectively.The crystal growth was performed in the presence of a gas mixture consisting of 2% O2 and Ar.A seed crystal of GAGG with a <100> orientation was utilized for this purpose.The pull rate of 0.1 millimeters per minute.The incorporation of an additional 1% by weight of Ga2O3 facilitated the offsetting of gallium evaporation.

XRD measurement
To determine the crystal structure, a portion of the crystals was finely ground using a mortar and pestle.The resulting powder was examined using X-ray diffraction (XRD) on a Bruker D8 diffractometer, covering a 2θ range of 15-65°.The instrument was equipped with a Cu Kα radiation source (wavelength 1.54 Å, 8.04 keV).The diffraction patterns were collected and compared to the standard pattern to confirm the phase purity of the sample.

Photoluminescence decay times and spectra of excitation and emission
Both emission as well as emission spectra were recorded at the Superlumi experimental station located at the P66 beamline line at the PETRA III storage ring, in DESY, Hamburg.Spectra were recorded at liquid helium (LHe) and room temperatures (RT).Each excitation spectrum was corrected for spectral distortion, while the emission spectra were not.The photoluminescence decay times were measured as described in Ref. [2].

Phase analysis using the XRD technique
In figure 1 the XRD patterns for GAGG:Ce, GAGG:Ce,Li + , and GAGG:Ce,Mg 2+ garnet crystals are shown.All samples exhibit a garnet phase, confirming that both Li + and Mg 2+ ions in high concentration can enter the garnet structure without affecting its thermodynamic stability.Figure 2 compares the excitation spectra recorded at LHe (figure 2a) and RT (figure 2b) Ce 3+ 5d1→4f interconfigurational transition centered at 550 nm in GAGG:Ce 3+ uncodoped and Li + and Mg 2+ codoped crystals recorded at LHe and RT.Depending on the codoping ions, different profiles can be seen in the spectra.Both excitation spectra for Ce 3+ 5d1→4f interconfigurational transition centered at 550 nm in GAGG:Ce and GAGG:Ce,Li crystals measured at LHe and RT are similar.The observed spectra exhibit distinct absorption line patterns within the wavelength range of 276-282 nm and 310-315 nm.These absorption lines belong to the 8 S7/2→ 6 IJ and 8 S7/2→ 6 P transitions within Gd 3+ ions, respectively [12], and confirm the Gd 3+ →Ce 3+ transfer of the excitation energy which is efficient at both temperatures [13].The 4f→5d3 inetrconfigurational transition of Ce 3+ ions is centered at 230 nm [14].A strong excitation band below 210 nm arises from exciton creation energy at the absorption edge of the host garnet lattice [15].The excitation spectra (LHe and RT) for Ce 3+ 5d1→4f energy transfer [12].Furthermore, the intense CT transition of stable Ce 4+ ions under 350 nm coincides with the energy levels of Gd 3+ ions 6 I (275 nm) and 6 P (310 nm).This may allow for improved transfer of energy between Gd 3+ ions and Ce 4+ ions.The move of the host lattice edge in the direction to higher energy in Mg 2+ and Li + codoped GAGG:Ce crystals comparing GAGG:Ce crystal shows that Li + and Mg 2+ codoping creates specific complex defects, which change the radial distribution on the atoms [3,18,19,20,21].The Li + has a rather little impact on the creation of complex defect clusters in GAGG:Ce crystals, thus, changing the segregation forces for atom distributions.Therefore, Li + co-doping has a smaller impact on the position of the exciton creation energy in GAGG:Ce,Li crystal, as shown in figures 2a, 2b. Figure 3 compares normalized emission spectra for uncodoped and Li + and Mg 2+ codoped Gd3Al2Ga3O12:Ce samples excited at 200 nm and recorded at LHe and RT.The spectra measured at LHe show narrow and low-intensity Gd 3+ 6 P7/2→ 8 S7/2 intraconfigurational transitions located at 313 nm [12].The Gd 3+ emission intensity is comparable in uncodoped and Li + codoped samples.In the Mg 2+ codoped crystal, the Gd 3+ emission intensity is at the noise level.This observation supports the hypothesis on the Gd 3+ →Ce 4+ energy transfer process and that Mg 2+ more efficiently changes the cerium charge from +3 to +4 [3].The bright luminescence centered at 550 nm belongs to the 5d1→4f interconfigurational transition in Ce 3+ ions.Interestingly, Li + and Mg 2+ codoping slightly broadens the emission band shapes and redshifts the maximum emission energy.This indicates that codoping with Li + ions and Mg 2+ ions perturb the local surroundings of Ce 3+ ions in the Gd3Al2Ga3O12:Ce crystal lattice.The perturbation is stronger for Mg 2+ codoping.It is important to note that the irregularities in the low-energy side of the Ce 3+ emission bands recorded at both LHe and RT are likely due to the spectral distortion of the low-resolution CCD camera.The RT emission spectra for uncodoped and Li + and Mg 2+ codoped GAGG:Ce crystals show the lack of the Gd 3+ luminescence.The observed phenomenon can be attributed to the effective transfer of energy from the gadolinium sublattice to the cerium ions in this system [12].Moreover, the maxima of Ce 3+ 5d1→4f emission bands are shifted towards lower energy.The emission spectra recorded both at LHe and RT are consistent with the excitation spectra (figure 2).The observed alterations in the host lattice edge and the peak emission wavelength of Ce 3+ luminescence, manifesting as shifts towards higher and lower energies, respectively, within Li + and Mg 2+ codoped crystals, indicate a discernible perturbation in the local elemental arrangement induced by the incompatible codoping.A comprehensive elucidation of this mechanism can be found in References 3, 4, 16, 19, and 20.The influence of co-dopants Li + and Mg 2+ on the energy levels of Ce 3+ can be observed through the decay times of Ce 3+ emission, as depicted in figure 4. The experimental data was fitted using multiexponential functions, as described by Equation 1 in Ref. [2].The decay curves were measured for Ce 3+ 4f→5d1 interconfigurational excitation at 450 nm and recording Ce 3+ luminescence at 550 nm.

Ce 3+ luminescence decay kinetic
The decay time values varied significantly depending on the codoping ion.Table 1 presents the decay constants and total fraction intensities.The Ce 3+ decay time in uncodoped Gd3Al2Ga3O12 sample showed a mono-exponential character with a decay time value of τ1=59 ns, typical for Ce 3+ ions in garnets [1,3,5].The Ce 3+ decays in Li + and Mg 2+ -codoped Gd3Al2Ga3O12:Ce 3+ samples showed double exponential profiles.The Ce 3+ decay times in Gd3Al2Ga3O12:Ce 3+ ,Li + crystal, are observed to be a combination of two exponential components.These components were characterized by decay times of τ1=11 ns and τ2=47 ns.The Ce 3+ decay time in GAGG:Ce 3+ ,Mg 2+ crystals also showed two components with decay times of τ1=3 ns and τ2=38 ns.In Li + and Mg 2+ codoped GAGG:Ce 3+ crystals, both decay components were significantly accelerated, but this effect was more pronounced in the Mg 2+ codoped sample.This observation supports the hypothesis that Li + and Mg 2+ ions impose a formation of complex defect clusters.A detailed description is provided in References 3,17, 16.

Conclusions
The spectroscopic investigation of Li + and Mg 2+ codoped GAGG:Ce crystals revealed distinct effects on Ce 3+ /Ce 4+ ratios, Gd 3+ →Ce 3+ energy transfer, elemental non-homogeneity, and defect formation.Mg 2+ codoping in the concentration of 3000 ppm efficiently induced Ce 4+ ions through a charge compensation mechanism.The energy transfer from the Gd 3+ sublattice to Ce 3+ ions in Gd3Al2Ga3O12:Ce 3+ ,Mg 2+ sample is hampered due to the higher concentration of stable Ce 4+ ions.In contrast, Li + codoping in concentration 3000 ppm had a lesser impact on Ce 3+ and Gd 3+ luminescence spectral features.Codoping also caused small redshifts in Ce 3+ emission due to crystal lattice perturbations which caused the formation of Ce 3+ multicenters.The changes in the positions and shapes of the onsets of the fundamental absorption edge of host lattice and Ce 3+ emission bands indicated modifications of the local surroundings around Ce 3+ ions, the radial atom distribution, and the creation of complex defect clusters, especially in the Mg 2+ -codoped crystal.The results demonstrated that Li + and Mg 2+ codopants could uniquely modify the luminescent and structural properties of GAGG:Ce scintillators through changes in the Ce valence state, the formation of Ce 3+perturbed centers, and increasing radial inhomogeneity.The introduction of Li + and Mg 2+ ions through codoping induced alterations near stable Ce 3+ ions, resulting in the emergence of supplementary pathways for the quenching of Ce 3+ luminescence.As a consequence, there was a notable increase in the rate at which the photoluminescence decay times occurred.

Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Figure 3 .
Figure 3. LHe (a) and RT (b) Ce 3+ ions photoluminescence emission spectra in the uncodoped and Li + and Mg 2+ codoped GGAG:Ce exited at 200 nm.The spectra are normalized at the peak of the Ce 3+ emission band.
[16,17]nfigurational transition centered at 550 nm in Mg 2+ codoped GAGG:Ce crystal are different.The Gd³⁺ absorption lines at 276-282 nm, associated with the 8 S7/2→ 6 IJ excitation transitions, are very weak at both LHe and room temperatures.Similarly, the 8 S7/2→ 6 PJ intraconfigurational excitation transition at 310-315 nm is barely above the noise level.The 4f→5d3 inetrconfigurational transition of Ce 3+ ions centered at 230 nm also shows low intensity.These observations indicate that Li⁺ and Mg²⁺ codoping has a significant impact on the defect formation processes and changes the oxidation state of Ce 3+ →Ce 4+[16,17].The charge compensation mechanism and crystal chemistry can account for the low intensity of Ce 3+ excitation bands and Gd 3+ excitation lines in the Mg 2+ codoped sample.Namely, Mg 2+ cooping efficiently transforms Ce 3+ into Ce 4+ valence state.As a result, the amount of Ce 3+ ions decreases significantly.This weakens the Ce 3+ 4f→5d3 absorption transition and impedes Gd 3+ →Ce 3+

Table 1 .
Decay time constants for the Ce 3+ emission originating from the 5d1→4f transition in Ce 3+ ; Ce 3+ ,Li + and Ce 3+ ,Mg 2+ doped GAGG crystals, calculated with a single and double-component exponential equation.The abbreviation frac.tot.int.represents the fraction of the total intensity and was calculated using equation 3, while mean decay time was calculated using equation 2 from Ref.4.