Eu³⁺-doped gadolinium oxyhydroxide Gd_1−xEu_xOOH crystals were synthesized by the flux method. The X-ray diffraction data for the crystals were well refined assuming a monoclinic structure with the P2₁/m space group. Gd_1−xEu_xOOH (x ≤ 0.2) crystals showed strong red emission, and the highest fluorescence quantum yield (Φ_f) was 0.27, obtained for x = 0.10. Φ_f decreased rapidly as the Eu³⁺ content x increased above 0.2, owing to concentration quenching. Analysis with a percolation model indicated three-dimensional energy transfer between the Eu³⁺ ions.

Keywords:Gadolinium Oxyhydroxide; Fluorescence Quantum Yield; Percolation Model

Some rare-earth compounds have excellent optical properties and hence are widely used in high-performance luminescence devices and as catalyst supports. Trivalent rare-earth ions such as Eu³⁺, Tb³⁺, and Tm³⁺-doped in a suitable host material show strong emission based on electron transition between the 4f orbitals. The luminescence properties depend strongly on the chemical composition and crystal structure of the host material. One of the Gd(III) compounds, gadolinium oxyhydroxide (GdOOH), which has a monoclinic structure with the P2₁/m space group, can be obtained as a stable phase by the thermal dehydration of hydroxide Gd(OH)₃ [1] -[4] . Although this compound can function as a host material for phosphors, there are very few reports on its photoluminescence properties [5] [6] . Fluorescence quantum yield, Φ_f, which is defined as the ratio of the number of emitted photons to the number of absorbed photons, is a fundamental parameter that decides the photoluminescence efficiency of a compound/phosphor [7] . Hence, evaluation of the activator content dependence of Φ_f using high-quality single-crystal specimens is important.

In this study, we synthesized Gd_1−xEu_xOOH crystals and evaluated their luminescence properties, including Φ_f. In addition, we evaluated the critical Eu³⁺ concentration in the Gd_1−xEu_xOOH system by using a percolation model.

Crystals of Eu³⁺-doped gadolinium oxyhydroxide, Gd_1−xEu_xOOH (x = 0, 0.02, 0.04, 0.06, 0.08, 0.1, 0.15, 0.2, 0.3, 0.4, 0.5, 0.6, 0.8, 1), were synthesized by the flux method. Appropriate amounts of Gd₂O₃ powder (99.9%) and Eu₂O₃ powder (99.9%) were mixed in an agate mortar for 1 h. Then, the Gd₂O₃/Eu₂O₃ mixture was placed in a 20-mL Zr crucible and covered by a layer of a NaOH (15 g)/KOH (5 g) mixture. The crucible was placed in an alumina container, covered with an alumina lid, and heated in an electric furnace at 330˚C for 72 h in ambient atmosphere. Subsequently, the flux was dissolved in large excess of water; the obtained crystals were washed thoroughly in distilled water to remove any residual flux and dried on a hot plate at 80˚C for 1 h.

The chemical composition of the crystals was measured using an inductively coupled plasma optical emission spectrometer (ICP-OES; Seiko Instruments Inc., SPS1700HVR) and an energy-dispersive X-ray spectrometer (EDS; JEOL, EX-23000 BU). The crystal structures were identified by powder X-ray diffraction (XRD; Rigaku, RINT2000). The diffraction data were acquired using CuKα radiation generated at 40 kV and 20 mA, in the 2θ range 10~80˚ at room temperature, and refined by the Rietveld method [8] . Thermogravimetric analysis (TG) and differential thermal analysis (DTA) were carried out in air, at temperatures between 30 and 785˚C at a heating rate of 10˚C /min (Seiko Instruments Inc., TG/DTA6300). The fluorescence excitation and emission spectra were measured at room temperature using a fluorescence spectrophotometer (Hitachi, F-7000). The excitation spectra were corrected for the spectral distribution of the lamp intensity by the Rhodamine B method, and the emission spectra were corrected for the spectral response of the instrument by using a substandard light source. The Φ_f values were measured using an absolute photoluminescence quantum yield measurement system (Hamamatsu, C9920-02), at room temperature [7] .

Gd_1−xEu_xOOH crystallized at the bottom of the crucible. As shown in Figure 1, the as-grown GdOOH crystals were transparent and had a plate-like shape with flat surfaces, which could be attributed to the growth in a liquid phase. The maximum length of the crystal was about 1.2 mm. The external surface of the crystals remained largely unchanged through the entire composition range. No trace of Zr, Na, and K impurities was detected by ICP-OES measurements, although these elements were expected to be incorporated into the crystals because of the use of the Zr crucible and NaOH/KOH mixture. The Gd/Eu ratio in the grown crystals, which was measured by EDS, was in good agreement with the molar ratios of Gd₂O₃ and Eu₂O₃, indicating that Gd_1−xEu_xOOH crystals with different compositions could be obtained by simply changing the molar ratio of the starting materials.

Figure 2 shows the powder XRD profiles of Gd_1−xEu_xOOH and the result of data refinement by the Rietveld method. The black squares show the experimental data, and the red and blue solid lines show the calculated profile and the difference between the experimental and calculated profiles, respectively. All the diffraction data were well refined assuming a monoclinic structure with the P2₁/m space group, and no structural change was observed over the entire composition range considered. The lattice parameters for GdOOH (a = 0.4339 nm, b = 0.3717 nm, c = 0.6072 nm, β = 108.79˚) and EuOOH (a = 0.4346 nm, b = 0.3744 nm, c = 0.6107 nm, β = 108.62˚) agreed with those reported in previous studies [9] [10] . When Gd³⁺ (ionic radius: 0.0935 nm) in GdOOH was replaced by Eu³⁺ (ionic radius: 0.0947 nm), the lattice constants showed a slight increase, in accordance with Vegard’ law. Gd_1−xEu_xOOH had low rare-earth site symmetry and a layered structure in which the rare earth layers were sandwiched between an O layer and an OH layer [11] [12] .

The TG curve of the as-grown crystals indicated no apparent weight loss in the temperature range 30~400˚C, indicating that the as-grown crystals did not include Gd(OH)₃, which normally decomposes at 270˚C [4] . In the TG profile, the first large weight loss was observed at 445˚C, with a corresponding endothermic effect, attributable to the removal of water molecules from GdOOH crystals. The experimental weight loss of 4.5% agreed well with the theoretical value of 4.7% calculated assuming the following thermal decomposition: 2GdOOH → Gd₂O₃ + H₂O. The XRD profiles of the specimens subjected to TG/DTA measurements showed a single phase, Gd₂O₃. These results indicated that the as-grown crystals comprised a single phase of GdOOH and did not contain Gd₂O₃ and Gd(OH)₃ at all.

Figure 3 shows the excitation spectra of the Gd_1−xEu_xOOH crystals; spectral measurements were performed at 617 nm, which is the maximum emission wavelength of Eu³⁺ in this system (⁵D₀→⁷F₂ transition). The excitation spectra showed sharp peaks between 300 nm and 550 nm, which were attributed to the direct intraconfigurational 4f-4f transitions from the ⁷F₀ and ⁷F₁ levels of Eu³⁺. The intensity of these peaks increased as the Eu³⁺ content increased up to x = 0.1. The wide peak between 200 nm and 300 nm, which was attributed to the overlapping of

the O²⁻ to Eu³⁺ charge transfer band (CTB) and the band due to energy transfer from Gd³⁺ to Eu³⁺ [13] , indicated energy transfer from the oxyhydroxide host to Eu³⁺. The intensity of the sharp peaks appearing at wavelengths longer than 300 nm changed with the Eu³⁺ content, but that of the wide peak at wavelengths below 300 nm did not change much.

Figure 4 shows the emission spectrum of the Gd_0.9Eu_0.1OOH crystals under 243 nm illumination. Strong red luminescence from Eu³⁺ was observed in the visible range. These peaks could be attributed to the transition from the lowest exited level ⁵D₀ to the ⁷F₀₋₄ levels of the ground term. The most intense transition was ⁵D₀→⁷F₂, the allowed electric dipole transition, at 617 nm. The other electric dipole transition, ⁵D₀→⁷F₄, was observed near 710 nm. In addition to electric dipole transitions, a strong magnetic dipole transition, ⁵D₀→⁷F₁, was observed near 595 nm. A ⁵D₀→⁷F₀ transition, which is normally forbidden by the free-ion selection rules, occurred at around 580 nm, as the selection rules are broken by the low symmetry of the rare-earth sites.

Figure 5 shows the Eu³⁺ content dependence of Φ_f for the Gd_1−xEu_xOOH crystals under excitation at 243, 258, and 394 nm (Eu³⁺: ⁷F₀→⁵L₆), with the former two wavelengths being close to the peak maxima in the excitation spectra. The highest Φ_f of 0.27 was obtained when the Eu³⁺ content was around 0.1. The decrease in Φ_f for x < 0.1 was attributed to the small amount of Eu³⁺, which is the photoactivator, in the system. At all excitation wavelengths, Φ_f decreased rapidly as the Eu³⁺ content increased beyond 0.2, because of concentration quenching caused by the transfer of excitation energy between the Eu³⁺ ions.

The critical Eu³⁺ concentration was evaluated by using the percolation model, which has been used to explain energy transfer in the solid state [14] -[16] . In this study, the aforesaid model was applied to the system under the following assumptions: energy transfer between the Eu³⁺ ions occurs only among the neighbor sites in the rare-earth sublattice. When the Eu³⁺ concentration was below the critical value, the Eu³⁺ ions showed emission individually, and the Φ_f values were high. When the Eu³⁺ concentration exceeded the critical value P_c, Φ_f decreased rapidly because of energy transfer from the percolating cluster to the killer center, which consists of vacancies and impurities. V. A. Vyssotsky et al. suggested the following expression for calculating the critical concentration in the percolation model:

where P_c, z, and d are the critical concentration, number of neighbor sites, and number of dimensions, respectively [16] . When energy transfer between the Eu³⁺ ions occurs only in the inner layers of the rare-earth sublattice, the number of dimensions is considered to be 2 (pseudo-two-dimensional energy transfer). When energy transfer occurs in the interlayers, the number of dimensions is considered to be 3 (three-dimensional energy transfer). Figure 6 shows the percolation model and the P_c value calculated using Equation (1). In this figure, only the rare-earth sublattice is depicted, and the Eu³⁺ ions between which energy transfer occurs are bonded by gray cylinders. Energy transfer between the Eu³⁺ ions is assumed to occur within 0.363 nm for (a), 0.373 nm for (b), 0.404 nm for (c), and 0.435 nm for (d). The experimental result agrees well with the calculated value of P_c (0.19) estimated for Eu³⁺-Eu³⁺ interactions within 0.404 nm (c). In this case, the Eu³⁺ ions have eight neighbors

with three-dimensional energy transfer. This result indicates three-dimensional energy transfer occurs between the Eu³⁺ ions in this system.

To improve the luminescence properties of the crystals, it is necessary to increase the critical concentration of Eu³⁺. In general, compounds with a low-dimensional arrangement of rare-earth ions show a high critical concentration. On the basis of the results obtained using the percolation model in this study, we state that two-dimensional models have a higher critical concentration (0.50 and 0.33) than do three-dimensional models. When the increase in the distance between the rare-earth layers restrains interlayer energy transfer, the critical concentration of Eu³⁺ in this system can increase. Therefore, excellent luminescence properties can be obtained by co-doping larger ions in the rare-earth sublattice with a high level of Eu³⁺ doping. The high alkali durability of the Gd_1−xEu_xOOH crystals is confirmed by the fact that they are stable in NaOH/KOH solution at 330˚C.

Gd_1−xEu_xOOH crystals with a maximum length of 1.2 mm were synthesized by the flux method. The Gd_1−xEu_xOOH (x ≤ 0.2) crystals showed strong red emission, with the maximum Φ_f of 0.27 for x = 0.10. The critical Eu³⁺ content was 0.2, because Φ_f decreased rapidly beyond this level owing to concentration quenching. This critical content was the closest to that estimated by a percolation model for Eu³⁺-Eu³⁺ interactions within 0.404 nm, wherein Eu³⁺ had eight neighbors, indicating three-dimensional energy transfer between the Eu³⁺ ions in the rare-earth sublattice. The synthesized crystals showed excellent alkali durability and hence may be used as a high-efficiency host material in unique photomedical applications such as photochemical internalization and photodynamic therapy.

This study was supported by JSPS KAKENHI Grant Number 21560696, 24560827.