Photoluminescence Spectra Correlations with Structural Distortion in Eu³⁺- and Ce³⁺-Doped Y₃Al_5-2x(Mg,Ge)_xO₁₂ (x = 0, 1, 2) Garnet Phosphors

Abstract

Garnet-type materials consisting of Y₃Al_5-2x(Mg,Ge)_xO₁₂ (x = 0, 1, 2), combined with Eu³⁺ or Ce³⁺ activator ions, were prepared by a solid-state method to determine the structural and optical correlations. The structure of Y₃Al_5-2x(Mg,Ge)_xO₁₂ (x = 1, 2) was determined to be a cubic unit cell (Ia-3d), which contains an 8-coordinated Y³⁺ site with octahedral (Mg,Al)O₆ and tetrahedral (Al,Ge)O₄ polyhedra, using synchrotron powder X-ray diffraction. When Eu³⁺ or Ce³⁺ ions were substituted for the Y³⁺ site in the Y₃Al_5-2x(Mg,Ge)_xO₁₂ host lattices, the emission spectra showed a decrease in the magnetic dipole f-f Eu³⁺ transition and a redshift of the d-f Ce³⁺ transition, related to centrosymmetry and crystal field splitting, respectively. These changes were monitored according to the increase in Mg²⁺ and Ge⁴⁺ contents. The dodecahedral and octahedral edge sharing was identified as a key distortion factor for the structure-correlated luminescence in the Eu³⁺/Ce³⁺-doped Y₃Al_5-2x(Mg,Ge)_xO₁₂ garnet phosphors.

1. Introduction

Ce³⁺-doped Y₃Al₅O₁₂ (YAG) phosphor has been widely utilized as a smart light source in conjunction with blue LED chips [1,2,3]. Initially developed in 1967 by G. Blasse and A. Bril, the yellow Ce³⁺-activated YAG phosphor was prepared for use in flying-spot cathode-ray tubes for color television, emitting intense yellow light via the 5d-4f transition of Ce³⁺ ions within the cubic garnet YAG structure [4]. The garnet mineral belongs to the nesosilicate subclass, characterized by isolated tetragonal polyhedra [5]. The YAG garnet structure, a cubic crystal system (Ia-3d), consists of dodecahedral YO₈, octahedral AlO₆, and tetrahedral AlO₄ units. The local dodecahedral YO₈ polyhedra within the garnet structure exhibit edge-sharing with YO₈ and AlO₆ polyhedra, as well as vertex-sharing with isolated AlO₄ tetrahedra [6,7,8,9]. In garnet host lattices, Ce³⁺ activator ions can occupy the dodecahedral site, influencing crystal field splitting and resulting in a shift of the d–f transition [10,11,12]. Researchers J. Ueda and S. Tanabe have explored the effects of crystal and electronic structures on Ce³⁺-doped YAG, particularly regarding the crystal field splitting of the lowest 5d levels and a new distortion parameter estimated by the ratio of dodecahedral edges [6]. Similarly, Eu³⁺ activator ions provide insight into site-resolved luminescence, distinguishing between centrosymmetric and non-centrosymmetric sites in garnet-type phosphors using the magnetic dipole and electric dipole transitions observed in emission spectra [13,14,15]. In this study, the structure of single-phase Y₃Al_5-2x(Mg,Ge)_xO₁₂ (x = 1, 2) garnet materials was determined using synchrotron X-ray analysis, revealing a cubic unit cell with Ia$\overline{3}$d symmetry. The cell parameters, volume, and distances within the host lattices were discussed. By substituting Ce³⁺ or Eu³⁺ activator ions into Y₃Al_5-2x(Mg,Ge)_xO₁₂ (x = 0, 1, 2), correlations between emission spectra, dipole transitions, and crystal field splitting were investigated in relation to structural distortion parameters.

2. Materials and Methods

Garnet materials doped with Eu³⁺- or Ce³⁺-doped Y₃Al_5-2x(Mg,Ge)_xO₁₂ (x = 0, 1, 2) compounds were synthesized by mixing appropriate stoichiometric amounts of powdered Y₂O₃ (Alfa, 99.9%), MgO (Alfa, 99.95%), Al₂O₃ (Alfa, 99.95%), GeO₂ (Alfa, 99.999%), Eu₂O₃ (Alfa, 99.9%), and CeO₂ (Aldrich, 99.9%), along with 5 wt% LiF (Alfa, 99.98%) or Li₂CO₃ (Alfa, 99%) flux. The powdered precursors were mixed in an agate mortar and pestle and then heated at 950 °C and 1400 °C for 6 h in air in a box furnace. The obtained powder samples containing the CeO₂ precursor underwent additional reheating at 1000 °C for 12 h in a 5% H₂/95% N₂ atmosphere. Phase identification was conducted using a powder diffractometer (Cu Kα radiation; Shimadzu XRD-6000, Kyoto, Japan), and structural analysis of the obtained garnet materials was performed using synchrotron powder X-ray diffraction (λ = 0.65303 Å). The new structural data were collected at the PLS-II 6D UNIST-PAL beamline of the Pohang Accelerator Laboratory (PAL) [16]. The garnet Y₃Al_5-2x(Mg,Ge)_xO₁₂ (x = 1 and 2) structures were refined using the Rietveld refinement program FullProf Suite [17,18]. Excitation and emission photoluminescence spectra of the powdered phosphors were obtained via UV spectroscopy using spectrofluorometers (FluoroMate FS-2, Scinco, Seoul, Korea).

3. Results and Discussion

Figure 1a,b illustrate the Rietveld refinement fitting of the powdered X-ray diffraction (XRD) data for Y₃Al_5-2x(Mg,Ge)_xO₁₂ (x = 1 and 2). The summarized structural data are provided in Table 1, Table 2 and Table 3. A single phase of the garnet structure, determined to be a cubic unit cell (Ia$\overline{3}$d), was obtained through a solid-state reaction method. This cubic phase of garnet, including Y₃Al₅O₁₂ (YAG, ICSD 170157) and Y₃Al_5-2x(Mg,Ge)_xO₁₂ (x = 1 and 2) structures, comprises 8-, 6-, and 4-coordinated Y³⁺, Al³⁺(1)-Mg²⁺, and Al³⁺(2)-Ge⁴⁺ ions, respectively, occupying 24c, 16a, and 24d Wyckoff sites. The 6-coordinated Mg²⁺ (with a radius of 0.72 Å for 6 coordination number (CN)), Al³⁺(1) (with a radius of 0.535 Å for 6 CN), and 4-coordinated Al³⁺ (with a radius of 0.39 Å for 4 CN) and Ge⁴⁺ (with a radius of 0.39 Å for 4 CN) sites are suitable for substitutions in the garnet structure [19]. Therefore, the formula for the garnet-structured Y₃Al_5-2x(Mg,Ge)_xO₁₂ (x = 0, 1, 2) can be expressed as Y₃Al(1)_2-xMg_xAl(2)_3-xGe_xO₁₂ based on the ionic radii of the cations in the unit cell, as depicted in Figure 1c. Similar to the isolated AlO₄ tetrahedra in the YAG structure, the tetrahedral (Al,Ge)O₄ polyhedron in the Y₃Al_5-2x(Mg,Ge)_xO₁₂ structures is also isolated, with no sharing of O atoms. There are two different bond distances between Y³⁺ and O²⁻ ions in the YO₈ polyhedron, whereas a single bond distance is observed in the (Mg,Al)O₆ and (Al,Ge)O₄ polyhedra within the host lattices. The YO₈ polyhedron shares edges with nearby YO₈ polyhedra and (Mg,Al)O₆ octahedra. YAG exhibits lattice parameters and volume, such as a = 11.9900(14) Å and V = 1723.68 ų (as shown in Table 1). The cell parameters and volumes of Y₃Al_5-2x(Mg,Ge)_xO₁₂ (x = 1 and 2) are larger than those of YAG compounds, such as Y₃MgAl₃GeO₁₂ (a = 12.1479(2) Å and V = 1796.82(17) ų) and Y₃Mg₂AlGe₂O₁₂ (a = 12.2628(1) Å and V = 1844.027(14) ų). The Y–O bond distances of an 8-coordinated Y (with a radius of 1.019 Å) comprise four long distances (2.433 Å) and four short distances (2.303 Å) in the YAG structure. The Y–O bond distances in Y₃MgAl₃GeO₁₂ and Y₃Mg₂AlGe₂O₁₂ host lattices remain consistent with those of the YAG structure, as shown in Figure 1c. However, the bond distances of a 6-coordinated (Mg,Al)-O₆ and Mg-O₆ exhibit distinct increases from 1.921 Å (for Al-O in YAG) to 1.997 Å and 2.094 Å, respectively, representing 4% and 9% differences. Conversely, the bond distances of Al–O₄ and (Al,Ge)-O₄ tetrahedra in the garnet structures remain similar, ranging from 1.766 to 1.785 Å and 1.760 Å. Interestingly, with the increase in the cell parameter and volume, the bond distance increase is observed primarily in the octahedral polyhedron in the Y₃Al_5-2x(Mg,Ge)_xO₁₂ structure when x = 0, 1, and 2.

Figure 1. Synchrotron XRD patterns of Y₃Al_5-2x(Mg,Ge)_xO₁₂ (a) x = 1 and (b) x = 2 and (c) the garnet Y₃Al_5-2x(Mg,Ge)_xO₁₂ (x = 0, 1, 2) structure with cell parameters and cell volumes.

Table 1. Rietveld refinement and crystal data for Y₃Al_5-2x(Mg,Ge)_xO₁₂ (x = 0, 1, 2).

Chemical Formula	Y3Al5O12 (ICSD 170157)	Y3MgAl3GeO12	Y3Mg2AlGe2O12
Radiation type, λ (Å)		Synchrotron (6D-BM), 0.65303
2θ range (deg)		4–36
Crystal system	Cubic	Cubic
Space group	Ia-3d	Ia-3d
Lattice parameter (Å)	a = 11.9900 (14)	a = 12.1479 (2)	a = 12.2628 (1)
Volume (Å3)	V = 1723.68	V = 1792.682 (17)	V = 1844.027 (14)
Density (g/cm3)	4.57	4.888	4.816
Rp (%)		0.717	0.728
Rwp (%)		1.22	1.47
Rexp (%)		1.26	1.88
S		0.96	0.78
χ2		0.931	0.608

Table 2. Refined atomic coordinates of Y₃Al_5-2x(Mg,Ge)_xO₁₂ (x = 0, 1, 2).

Y3Al5O12 (ICSD 170157)
Atom	Wyckoff position	x	y	z	Biso	SOF
Y	24c	0.125	0	0.25	0.00365 (12)	1
Al(1)	16a	0	0	0	0.0030 (3)	1
Al(2)	24d	0.375	0	0.25	0.0011 (3)	1
O	96h	0.28023 (17)	0.10110 (16)	0.19922 (17)	0.0036 (4)	1
Y3MgAl3GeO12
Atom	Wyckoff position	x	y	z	Biso	SOF
Y	24c	0	0.25	0.125	0.33 (8)	1
Mg	16a	0	0	0	0.49	0.5
Al(1)	16a	0	0	0	0.49	0.5
Al(2)	24d	0	0.25	0.375	0.57	0.6667
Ge	24d	0	0.25	0.375	0.57	0.3333
O	96h	−0.0305 (5)	0.0551 (6)	0.1518 (6)	0.4 (2)	1
Y3Mg2AlGe2O12
Atom	Wyckoff position	x	y	z	Biso	SOF
Y	24c	0	0.25	0.125	0.2 (2)	1
Mg	16a	0	0	0	1.0 (2)	1
Al(2)	24d	0	0.25	0.375	0.1 (2)	0.3333
Ge	24d	0	0.25	0.375	0.1 (2)	0.6667
O	96h	−0.0315 (7)	0.0576 (6)	0.1576 (6)	0.2 (2)	1

Table 3. Selected interatomic distances for Y₃Al_5-2x(Mg,Ge)_xO₁₂ (x = 0, 1, 2).

Y3Al5O12 (ICSD 170157)		Y3MgAl3GeO12		Y3Mg2AlGe2O12
Atom	Distance (Å)	Atom	Distance (Å)	Atom	Distance (Å)
Y-O (×4)	2.433	Y-O (×4)	2.419 (2)	Y-O (×4)	2.424 (4)
Y-O (×4)	2.303	Y-O (×4)	2.332 (2)	Y-O (×4)	2.338 (4)
Al-O (×6)	1.921	(Mg/Al1)-O (×6)	1.997 (2)	Mg-O (×6)	2.094 (4)
Al-O (×4)	1.766	(Al2/Ge)-O (×4)	1.785 (2)	Ge/Al-O (×4)	1.760 (4)

In Figure 2a,b, a distinct shift in the apparent peaks to lower angles, particularly those at 2θ = 32–34°, was observed as Al³⁺ ions were gradually replaced by Mg²⁺ and Ge⁴⁺ ions in the Eu³⁺- and Ce³⁺-doped Y₃Al_5-2x(Mg,Ge)_xO₁₂ (x = 0, 1, 2) phosphors, respectively. A single phase of the garnet structure with a cubic crystal system (Ia$\overline{3}$d) was obtained, free from any apparent impurities. The cell volumes of both Y_2.5Eu_0.5Al_5-2x(Mg,Ge)_xO₁₂ and Y_2.95Ce_0.05Al_5-2x(Mg,Ge)_xO₁₂ (x = 0, 1, 2) phosphors increased as the content of Mg²⁺ and Ge⁴⁺ increased. Figure 3a displays the emission photoluminescence (PL) spectra of Y_2.5Eu_0.5Al_5-2x(Mg,Ge)_xO₁₂ (x = 0, 1, 2) phosphors. The electronic f-f transitions of Eu³⁺ ions in the host lattices are assigned as ⁵D₀–⁷F₁, ⁵D₀–⁷F₂, ⁵D₀–⁷F₃, and ⁵D₀–⁷F₄ within the range of 550 and 750 nm [13,14,15]. It is known that when Eu³⁺ ions are located at the centrosymmetric site in a crystal structure, the magnetic dipole transition (⁵D₀–⁷F₁) dominates, whereas, in the absence of inversion Eu³⁺ ions in the host lattice, the electric dipole transition (⁵D₀–⁷F₂) dominates [13,14,15]. The centrosymmetric symmetry of the local-environment-center Eu³⁺ ions in the Y₃Al_5-2x(Mg,Ge)_xO₁₂ structure was inferred from the normalized intensity ratio of the magnetic dipole transition, as depicted in Figure 3a, Figures S1 and S2. All intensity values in the emission spectra were normalized by dividing them by the maximum intensity value of the electric dipole transition peak. As Eu³⁺ ions occupy the 8-coordinated Y³⁺ site in the Y₃Al_5-2x(Mg,Ge)_xO₁₂ (x = 0, 1, 2) structures, the dominant magnetic dipole transition around 590 nm was noticeably decreased up to x = 0, 1, and 2. This indicates that the ideal cubic field, characterized by a centrosymmetric Y³⁺ center (D_4h point group), was gradually distorted to a dodecahedral field with the substitution of Mg²⁺ and Ge⁴⁺ ions. Figure 3b presents the emission spectra of Y_2.95Ce_0.05Al_5-2x(Mg,Ge)_xO₁₂ (x = 0, 1, 2) phosphors (S1 and 2) and relative energy diagrams of cubic and distorted cubic polyhedrons, showcasing the 8-coordinated site geometry for the 5d Ce³⁺ orbital energy levels in the host materials. Upon substituting Mg²⁺ and Ge⁴⁺ ions for Al³⁺ ions in the structure, the distorted 8-coordinated Ce³⁺ ions in the YAG exhibit a significant redshift of the e_g orbital splitting from a normal cubic polyhedron, resulting in emissions from the red-shifted 5d energy level caused by the high-crystal-field effect [10,11,12].

Figure 2. The calculated XRD pattern of YAG with Miller indices and obtained XRD patterns: (a) Eu³⁺-doped and (b) Ce³⁺-doped Y₃Al_5-2x(Mg,Ge)_xO₁₂ (x = 0, 1, 2) phosphors.

Figure 3. The emission spectra of (a) Eu³⁺-doped and (b) Ce³⁺-doped Y₃Al_5-2x(Mg,Ge)_xO₁₂ (x = 0, 1, 2) phosphors with relative centrosymmetric intensity and energy-level diagram.

In Figure 4a, the unit cell and local structure of the dodecahedral site in Y₃Al_5-2x(Mg,Ge)_xO₁₂ are depicted. Ce³⁺ ions engaged in the Y³⁺ site of garnet structures can emit light due to the distortion factor expressed by the ratio of dodecahedral edges. The four short and long Y(Ce)-O bond distances in the garnet structures can be influenced by the deviation of O-O bonds shared with adjacent dodecahedra (d₈₈) and two non-shared tetrahedra (d₈₁) [6]. This deviation in the local structural arrangement around the Ce³⁺ ions within the garnet lattice can have an impact on the energy levels of the Ce³⁺ ions [6]. As distortion increases due to the compression on the cube, the maximum emission shifts to longer wavelengths according to the lower excited state of energy level in the cubic crystal field splitting. Figure 4b summarizes distortion parameters including d₈₈, d₆₈, d₈₁, d₈₈/d₈₁, and d₆₈/d₈₁ for Ce³⁺-doped Y₃Al_5-2x(Mg,Ge)_xO₁₂ (x = 0, 1, 2) phosphors. The edge-sharing distance of d₈₈ and distortion parameter of d₈₈/d₈₁ gradually decreased by substituting Mg-Ge ions in the structures, indicating that the compression of the cube was released by the decrease in distortion. However, the maximum emission wavelength of the Ce³⁺ d-f transition shifts to longer wavelengths, as shown in Figure 3. Furthermore, the centrosymmetric Eu³⁺ emission decreased when x = 0 to 1 and 2 in Eu³⁺-doped Y₃Al_5-2x(Mg,Ge)_xO₁₂ garnet phosphors. When Mg²⁺-Ge⁴⁺ ions were doped into the Y₃Al₅O₁₂ structure, the bond length of Mg-O among Y-O, Al-O, and Ge-O bonds from the structure analysis solely increased with increasing cell volumes, as shown in Figure 1c. For the distorted Eu³⁺ and Ce³⁺ cubic sites in the Y₃Al_5-2x(Mg,Ge)_xO₁₂ garnet structures, the parameter of dodecahedral and octahedral edge-sharing should be estimated as a distortion factor. Figure 4b illustrates the steady increase in edge-sharing distance d₆₈ and distortion ratio of d₆₈/d₈₁ in terms of key distortion parameters, resulting in the non-centrosymmetric manners and the redshift of the maximum emission wavelength.

Figure 4. (a) Unit cell and local structure of the dodecahedral site in Y₃Al_5-2x(Mg,Ge)_xO₁₂ (b) distortion parameters of d₈₈, d₆₈, d₈₁, d₈₈/d₈₁, and d₆₈/d₈₁ for Ce³⁺-doped Y₃Al_5-2x(Mg,Ge)_xO₁₂ (x = 0, 1, 2) phosphors.

4. Conclusions

Eu³⁺/Ce³⁺-doped Y₃Al_5-2x(Mg,Ge)_xO₁₂ (x = 0, 1, 2) phosphors were synthesized using a solid-state reaction method assisted by excess LiF or Li₂CO₃ flux at high temperature. Synchrotron powder X-ray diffraction analysis confirmed that the single-phase of Y₃Al_5-2x(Mg,Ge)_xO₁₂ (x = 1 and 2) garnet materials possessed a cubic unit cell (Ia$\overline{3}$d) with lattice parameters of a = 12.1479(2) Å and a = 12.2628(1) Å, respectively. These garnet structures consisted of an 8-coordination environment for Y³⁺ with (Mg²⁺,Al³⁺(1))O₆ octahedra and (Al³⁺(2),Ge⁴⁺)O₄ tetrahedra. Interestingly, the distance value of the Mg,Al(1)O₆ octahedra only increased with the doping of Mg²⁺ and Ge⁴⁺ ions into the YAG host lattice. Upon doping Eu³⁺ or Ce³⁺ ions into the Y₃Al_5-2x(Mg,Ge)_xO₁₂ (x = 0, 1, 2) garnet structure, the resulting phosphors exhibited increases in non-centrosymmetric and strong crystal field splitting manners with increasing contents of Mg²⁺ and Ge⁴⁺ ions. This indicated that the cubic polyhedra were compressed by the increase in distortion. The distortion factor of dodecahedral–dodecahedral edge sharing/non-edge sharing (d₈₈/d₈₁) decreased with the substitution of Mg²⁺-Ge⁴⁺ ions, while the distortion factor of dodecahedral–octahedral edge sharing/non-edge sharing (d₆₈/d₈₁) gradually increased. In Y₃Al_5-2x(Mg,Ge)_xO₁₂ (x = 0, 1, 2) garnet structures, when substituting ions in octahedral and tetrahedral sites, the dodecahedral–octahedral edge sharing/non-edge sharing (d₆₈/d₈₁) should be considered as the most crucial distortion parameter.