DOI 10.1007/s10812-017-0557-5

Journal of Applied Spectroscopy, Vol. 84, No. 5, November, 2017 (Russian Original Vol. 84, No. 5, September–October, 2017)

LUMINESCENCE OF Eu:Y3Al5O12, Eu:Lu3Al5O12, AND Eu:GdAlO3 NANOCRYSTALS SYNTHESIZED BY SOLUTION COMBUSTION E. V. Vilejshikova,a* A. A. Khort,b K. B. Podbolotov,b P. A. Loiko,c V. I. Shimanski,d S. N. Shashkov,e and K. V. Yumasheva

UDC 535.37;620.3

Nanocrystals of rare-earth garnets Y3 Al5O12 and Lu3 Al5O12 and perovskite GdAlO3 highly doped (10–20 at%) with Eu3+ are synthesized by the solution combustion technique and subsequent annealing in air at 800 and 1300 oC. Their structure, morphology, and phase composition are studied. These materials exhibit intense red luminescence under UV excitation. Eu:GdAlO3 luminescence has CIE 1931 color coordinates (0.632, 0.368); dominant wavelength, 599.6 nm; and color purity, >99%. Judd–Ofelt parameters, luminescence branching ratios, and lifetimes of the Eu3+ 5D0 state are determined. The luminescence quantum yield for Eu:GdAlO3 (10 at%) reaches 74% with a lifetime of 1.4 ms for the 5D0 state. The synthesized materials are promising for red ceramic phosphors. Keywords: nanocrystal, garnet, perovskite, solution combustion synthesis, europium ion, luminescence. Introduction. Rare-earth (RE) materials of the RE2O3–Al2O3 system, where RE = Gd, Y, or Lu, are attractive for activation by the RE ions (RE3+ ) and are utilized as crystalline laser media and bases for powder and ceramic luminophores. Cubic garnet crystals RE3Al5O12 (space group Ia 3 d ) and perovskite-like orthorhombic aluminate crystals REAlO3 (Pbnm) are notable among RE2O3–Al2O3 materials. Crystals of the RE2O3–Al2O3 system, e.g., Lu3Al5O12 = LuAG and YAlO3 = YAP, doped with Yb3+, Tm+3+, etc., have been scrutinized lately for near-IR lasers [1–5]. Crystals of RE3Al5O12 and REAlO3 possess good mechanical, thermal, and thermo-optical properties [6]. The high luminescence quantum yields and relatively long (for the oxides) lifetimes of RE3+ in upper laser levels are attractive for spectroscopy. Applications of RE3Al5O12 and REAlO3 with RE3+ as lasers are practically limitless. Micro- and nanocrystals of luminophores with Tb3+ (green) and Ce3+ (yellow) [7–9] are used in color displays, electron-beam tubes, and luminescent lamps. The structures of RE3Al5O12 crystals are typically stable over broad temperature ranges. Therefore, garnet powders with Dy3+, Sm3+, Eu3+, and Tb3+ are used in thermally sensitive luminophores [10] for high-temperature detectors based on the temperature dependences of metastable RE3+ excited-state lifetimes. Materials activated by Eu3+ are interesting because intense red and organish-red luminescence associated with 5 7 D0 → FJ transitions can be excited. The strongest luminescence is usually located at ~610 nm (5D0 → 7F2) and is used in red luminophores [11]. The 5D0 → 7F2 transition (~702 nm) was used for laser generation in crystals with Eu3+ [12]. Red luminophores with Eu3+, e.g., Eu3+:Y2O3, are characterized by high luminescence quantum yields for UV excitation and high color purity and are used in three-color displays and white-light lamps. Garnets and aluminates activated by Eu3+ were also studied as luminophores [13–16]. The energy-level structure of Eu3+ (electronic configuration [Xe]4f 6 ) features a metastable 5D0 state (lifetime of hundreds of microseconds to milliseconds) separated from a set of lower lying 7F1–7F6 excited states and the 7F0 ground state by an energy gap of ~12,000 cm–1. The Eu3+ electric-dipole (ED) transition 5D0 → 7F2 is hypersensitive to the local symmetry [17] and dominates over the magnetic-dipole (MD) transition 5D0 → 7F1 if the Eu3+ is positioned on an inversion center. The intensity ratio R = IED /IMD varies depending on the local symmetry and its distortion. Therefore, Eu3+ is used as a label sensitive to the material structure. _____________________ *

To whom correspondence should be addressed. a

Center for Optical Materials and Technologies, Belarusian National Technical University, 65/17 Nezavisimost′ Ave., Minsk, 220013, Belarus; e-mail: [email protected]; bBelarusian State Technological University, Minsk, 220006, Belarus; cITMO University, St. Petersburg, 197101, Russia; dBelarusian State University, Minsk, 220030, Belarus; eSOL Instruments, Minsk, 220005, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 84, No. 5, pp. 810–820, September–October, 2017. Original article submitted November 20, 2016. 866

0021-9037/17/8405-0866 ©2017 Springer Science+Business Media New York

Luminophores based on RE3Al5O12 and REAlO3 are traditionally synthesized by solid-state sintering followed by grinding. The reagents are the oxides Al2O3 and RE2O3. The drawbacks of this method are the need for lengthy hightemperature treatment and the low dispersion of the final material that requires lengthy grinding and is accompanied by contamination of the luminophore with uncontrolled impurities. Precipitation from solutions [18] and sol-gel [19] and hydrothermal syntheses [20] were also used. As a rule, the first two methods are lengthy and complicated. The last required costly equipment to achieve the high temperatures and pressures for the synthesis. Solution combustion has several advantages such as energy efficiency, rapidity, high specific surface area and dispersion, and uniform final material [21, 22]. Herein, the structures and luminescence properties of Eu:Y3Al5O12, Eu:Lu3Al5O12, and Eu:GdAlO3 nanopowders synthesized by solution combustion are reported. Materials Synthesis and Study Methods. Nanomaterials of Eu2O3–RE2O3–Al2O3, where RE = Y, Gd, or Lu, were prepared by solution combustion synthesis (SCS) of mixtures of the metal nitrates and reductants: 2.1RE(NO3)3 + 0.9Eu( NO3)3 + 5Al(NO3)3 + 5C5H5NO2 + 12.5CH4N2O → → (RE0.7Eu0.3)3Al5O12 + 22.5CO2 + 27N2 + 37.5H2O . The reductant was a mixture of urea (U) and glycine (G) in a 2.5:1 U:G mole ratio. Stoichiometric amounts of metal nitrate and reductant giving a reductant/oxidant ratio φ = 1.25 were dissolved in a small volume of hot distilled H2O. The resulting solution was stirred constantly and treated slowly with a solution of ammonia to pH 6.5–7.0. The resulting gel was quickly dehydrated in a microwave oven to produce a foam. Then, a heat-resistant beaker with the precursor was placed for 10–15 s into a muffle furnace heated previously to 600oC to initiate an exothermic combustion accompanied by the release of a large volume of gaseous reaction products. The combustion formed a light loose powder. The synthesized powder was ground and divided into three portions for further investigation. Two portions were calcined in air for 60 min at 800 and 1300oC. Samples were pressed into pellets 1 cm in diameter for spectroscopic studies. X-ray phase analysis (XPA) established the phase composition and crystal structure of the samples. X-ray diffraction patterns were recorded on a Bruker D8 Advance diffractometer. Phases were identified using the Powder Diffraction Standards 2003 database and the DIFFRACPlus program (Bruker). Diffraction patterns were processed using the Rietveld method. The average diameter of the nanocrystals was calculated using the Scherrer formula d = K λ / β cosθ, where K = 1; λ = 0.15418 nm (CuKα-radiation); β, reflection width at half maximum; and θ, diffraction angle. Phase transitions were studied by differential scanning calorimetry (DSC) on a Netzsch DSC 404 F3 Pegasus calorimeter. SEM images of powders were obtained on a LEO 1455VP scanning electron microscope at accelerating potential 20 kV in reflected- and secondary-electron modes. Elemental composition was determined by energy-dispersive x-ray microanalysis (EDX) using an Oxford Instruments X-MaxN detector operating together with the electron microscope. Luminescence spectra of Eu3+ were recorded with continuous excitation by a blue GaN-diode laser at λex = 400 nm using a compact SOLAR S100 spectrometer. Spectra were corrected for the spectral sensitivity of the CCD array (Toshiba TCD1205D) and the transmission spectrum of the optical fiber (Z-light, low-OH Si multimode fiber) in the spectrometer. The excitation source for studying the luminescence kinetics was a pulsed (~18 ns) third-harmonic YAG:Nd laser (LOTIS TII LS-2137, λex = 355 nm). The signal from a Hamamatsu C5460 photomultiplier tube (PMT) was recorded by a Tektronix TDS3052B rapid digital oscilloscope. A ZhS-11 light filter was used to avoid saturation of the PMT and CCD array by the exciting radiation. The recording wavelength (605 nm) was determined by an MDR-12 monochromator. Raman spectra were measured using a Confotec MR150 scanning laser Raman microscope. Spectra were recorded at excitation wavelength 785 ± 0.2 nm in the range 100–1000 cm–1 with resolution ~1 cm–1 (Echelle grating, 75 lines/mm). The exciting radiation was focused to a spot <1 μm in diameter. The Raman signal was recorded by a Peltier-cooled CCD-array. A micro-objective with 40× magnification and numerical aperture NA = 0.75 was used in the confocal microscope. Results and Discussion. Structural properties. The main crystalline phases of the obtained powders according to XSA (Fig. 1) were gadolinium aluminate GdAlO3 [orthorhombic system, Pbnm, a = 5.250(7), b = 5.293(6), c = 7.44(1) Å for T = 1300oC] and Y3Al5O12 and Lu3Al5O12 [cubic system, Ia 3 d , a = 12.021(4) and 11.931(1) Å for T = 1300oC]. The reaction product for Gd was the aluminate despite the identical synthesis conditions: 9.8Gd(NO3)3 + 4.2Eu(NO3)3 + 14Al(NO3)3 + 10C5H5NO2 + 35CH4N2O → → 14Gd0.7Eu0.3AlO3 + 85CO2 + 82N2 + 95H2O . This was probably due to characteristic destabilization of the Gd3Al5O12 garnet cubic structure through the joint action of the high synthesis temperatures and activation by Eu3+. Table 1 presents the unit-cell constants and nanocrystal sizes. 867

TABLE 1. Unit-Cell Constants (a, b, c), Unit-Cell Volume V, and Average Nanocrystal Size d Calculated Using the Scherrer Formula for Initial and Calcined Eu:GdAlO3, Eu:Y3Al5O12, and Eu:Lu3Al5O12 Sample

a, Å

b, Å

c, Å

V, Å3

d, nm

5.251(1)

5.307(8)

7.45(8)

207.6

29

5.265(3)

5.288(5)

7.46(6)

207.3

35

5.250(7)

5.293(6)

7.44(1)

206.9

47

12.143(3)

–

–

1791.0

20

12.093(3)

–

–

1769.0

27

12.021(4)

–

–

1737.1

38

11.930(0)

–

–

1697.9

26

11.922(1)

–

–

1694.5

29

11.931(1)

–

–

1698.4

39

Structure (sp. gp.)

Eu:GdAlO3 init.

Orthorhombic (Pbnm)

o

Eu:GdAlO3 800 С o

Eu:GdAlO3 1300 С Eu:Y3Al5O12 init. o

Eu:Y3Al5O12 800 С o

Eu:Y3Al5O12 1300 С Eu:Lu3Al5O12 init. o

Eu:Lu3Al5O12 800 С o

Eu:Lu3Al5O12 1300 С

Cubic (Ia-3d) Cubic (Ia-3d)

TABLE 2. Compositions of Eu:GdAlO3, Eu:Y3Al5O12, and Eu:Lu3Al5O12 Samples by EDX (Calcination Temperature 1300oC) Element

Eu:GdAlO3

Eu:Y3Al5O12

Eu:Lu3Al5O12

O, at%

54.71

52.22

61.78

Al, at%

25.28

30.70

21.64

RE, at%

18.25

14.27

14.88

Eu, at%

1.76

2.81

1.70

Eu/Re, %

9.6

19.7

11.4

KEu

0.32

0.66

0.38

The elemental compositions of the samples were determined using EDX. Table 2 presents the results for samples calcined at 1300oC. The Eu3+ in all samples replaced passive RE3+ according to the stoichiometric formulas (Gd1–xEux)AlO3, (Y1–xEux)3Al5O12, and (Lu1–xEux)3Al5O12 with x = 0.096, 0.197, and 0.114, respectively. Thus, the Eu3+ segregation coefficient in the nanocrystals (KEu = Ncr /Ncharge < 1) was greatest for Eu:Y3Al5O12 (KEu = 0.66) and least for Eu:GdAlO3 (KEu = 0.32). Diffraction patterns of uncalcined samples (Fig. 1a–c) showed peaks for the main phases, weak peaks for impurity phases, and a broad halo, the intensity of which decreased up to complete disappearance as the calcination temperature T was increased. The unit-cell volume (V ) for GdAlO3 and Y3Al5O12 decreased by ~3% with increasing T because the amorphous phase crystallized, microstrains relaxed, and the number of oxygen vacancies formed during the exothermal synthesis decreased. As noted above, Eu3+ (ionic radius 1.066 Å for coordination number VIII) replaced RE3+ (ionic radius 1.053 Å for Gd, 1.019 Å for Y3+, and 0.977 Å for Lu3+ ) in GdAlO3, Y3Al5O12, and Lu3Al5O12. The ionic radii suggested that V should increase if Eu3+ was incorporated into the crystal lattice. The volume V decreased after calcination for samples containing GdAlO3 and Y3Al5O12, indicating that the Eu3+ content in the crystal lattice changed little. The volume V also decreased after calcination of Lu3Al5O12 at 800oC. However, V increased on going to T = 1300oC. The decrease of V in the first step for GdAlO3 and Y3Al5O12 was probably related to crystallization, relaxation of structural microstrains, and reduction of the number of oxygen vacancies. In turn, the increase of V was related to thermodynamic activation of the replacement of Lu3+ by Eu3+. Because the ionic radius of Eu3+ was greater than that of Lu3+ (and much greater than for the pairs Eu3+–Gd3+ and Eu3+–Y3+), the thermodynamic potential during the rapid exothermal synthesis turned out to be insufficient for complete incorporation of Eu3+ into Lu3Al5O12. As a result, a certain amount of quasi-free Eu3+ remained in the Lu3Al5O12 powder and persisted during the low-temperature treatment. The thermodynamic potential increased during high-temperature calcination (1300oC) so that Eu3+ was incorporated more completely into the Lu3Al5O12 lattice. 868

Fig. 1. Diffraction patterns of initial Eu:GdAlO3 (a), Eu:Y3Al5O12 (b), and Eu:Lu3Al5O12 samples (c) and those calcined at 800 and 1300oC; indices (h, k, l) are shown for the corresponding reflections; temperature dependence of specific heat capacities (Cp) of uncalcined Eu:GdAlO3 (1), Eu:Y3Al5O12 (2), and Eu:Lu3Al5O12 samples (3) (d). Figure 1d shows temperature dependences of the specific heat capacities Cp for samples of all compositions. A local minimum of the heat capacity corresponding to an exothermal effect was observed in the range 880–950oC. This could be interpreted as completion of the thermal decomposition (combustion) of residual organic components from the initial solutions. Repeated exothermal maxima that could be due to relaxation processes and crystallization of amorphous phases were found as the temperature increased further to 996, 1009, and 1071oC for Y3Al5O12, GdAlO3, and Lu3Al5O12, respectively. The average nanocrystal size d that was calculated using the Scherrer formula increased after calcination in the ranges 29–47 nm (for Eu:GdAlO3), 20–38 nm (Eu:Y3Al5O12), and 26–39 nm (Eu:Lu3Al5O12) because of thermal recrystallization (enlargement) of the nanocrystals. The morphologies of the synthesizes powders were studied using SEM images (Fig. 2) and were characteristic of materials prepared by solution combustion. Samples were highly porous agglomerates. The calcination led to enlargement of the crystallites without sintering them because RE3Al5O12 and REAlO3 crystals typically have high initial melting points (>1600oC). Raman spectra of samples calcined at 1300oC (Fig. 3) contained bands attributed to characteristic vibrations in the garnet RE3Al5O12 [23] and perovskite REAlO3 structures [24]. The phonon energy was greatest for Eu:GdAlO3 at hνmax ~ 620 cm–1; for Eu:Y3Al5O12 and Eu:Lu3Al5O12, ~810 and ~830 cm–1. Weak nonradiative relaxation through a multiphonon mechanism could be expected in the studied materials because of the high energy gaps between the 5D0 and 7F6 levels (~12,000 cm–1). Luminescence properties. Luminescence spectra of initial Eu:GdAlO3 powders and those calcined at 800 and 1300oC excited at 400 nm (Fig. 4a) exhibited bands at 575 (Eu3+ 5D0 → 7F0 transition), 587 (5D0 → 7F1), 611 (5D0 → 7F2), 650 (5D0 → 7F3), 690 (5D0 → 7F4), and 810 nm (5D0 → 7F6). The spectral positions and shapes of these bands remained unchanged after heat treatment. However, spectra of calcined samples were missing broad components for luminescence of extraneous phases and impurities. Crystals of GdAlO3 had an orthorhombic GdFeO3-like perovskite structure. 869

Fig. 2. SEM images of Eu:GdAlO3 (a), Eu:Y3Al5O12 (b), and Eu:Lu3Al5O12 samples (c); calcination temperature 1300oC.

Fig. 3. Raman spectra of Eu:GdAlO3, Eu:Y3Al5O12, and Eu:Lu3Al5O12 samples calcined at 1300oC; λex = 785 nm; bands attributed to vibrations in the RE3Al5O12 (●) and REAlO3 structures (♦) are noted.

Fig. 4. Luminescence spectra of initial Eu:GdAlO3 (a), Eu:Y3Al5O12 (b), and Eu:Lu3Al5O12 samples (c) (1) and those calcined at 800 (2) and 1300oC (3); λex = 400 nm. The Gd3+ and substituting Eu3+ were coordinated in positions of low symmetry (C1h) without an inversion center. This amplified the hypersensitivity to the local symmetry of the Eu3+ ED 5D0 →7F2 transition, which corresponded to the luminescence band with a maximum at 611 nm, dominated the spectrum, and determined the luminescence color. 870

The following asymmetry parameter was used to analyze the luminescence spectra of Eu3+ [25]:

R = IED( 5D0 → 7F2)/IMD( 5D0 → 7F1) .

(1)

For Eu:GdAlO3 powder, R ~ 2. This agreed with symmetry representations because the expected symmetry of the Eu3+ coordination environment did not have an inversion center. It was shown earlier that R quickly fell from ~11 to 2 as the Eu concentration was increased from 2 to 10 at% in GdAlO3 nanocrystals [26]. Luminescence spectra of calcined Eu:Y3Al5O12 powders exhibited clearly discernible changes (Fig. 4b). The spectrum of the uncalcined sample contained a set of strong bands located in the range 580–640 nm. The shapes and positions of bands with maxima at 587 and 592 nm (5D0 → 7F1 ), 604 (5D0 → 7F2 ), 626 (5D0 → 7F3), 705 (5D0 → 7F4 ), and 822 nm (5D0 → 7F6 ) did not change after heat treatment. Bands with maxima at 611 and 691 nm disappeared as the calcination temperature was increased and could be attributed to Eu3+ transitions in an extraneous phase that disappeared during the heat treatment. It was concluded that the local environment of Eu3+ in the extraneous phase did not have an inversion center because bands attributed to ED transitions 5D0 → 7F2 (611 nm) and 5D0 → 7F4 (691 nm) contributed most to its luminescence spectrum intensity. Bands with maxima at 587, 592, 604, 626, 705, and 822 nm that corresponded to a single-center Eu3+ spectrum in the cubic crystalline Y3Al5O12 matrix were observed in the luminescence of the sample calcined at 1300oC. Parameter R = 0.36 < 1 agreed with symmetry concepts regarding coordination of Eu3+ in positions with D2 symmetry that were slightly distorted centrosymmetric D2h positions. Inhomogeneously broadened components that were observed in luminescence spectra of the initial Eu:Lu3Al5O12 powder also disappeared in spectra of calcined samples (Fig. 4c). They belonged to luminescence of an extraneous amorphous phase containing Eu3+. Band maxima attributed to Eu3+ transitions in cubic Lu3Al5O12 were located at 574 (5D0 → 7F0), 586 (5D0 → 7F1), 605 (5D0 → 7F2), 626 (5D0 → 7F3), and 705 nm (5D0 → 7F4). In this instance, R = 0.46 < 1, which was slightly greater than for the isostructural Eu:Y3Al5O12 because of the greater distortion of the local symmetry of the Eu3+ environment due to the larger difference in the ionic radii of Eu3+ and Lu3+ (see above). It is noteworthy that the 5D0 → 7F0 transition is forbidden by selection rules for ED and MD transitions [17] and is usually not observed in spectra of crystals with an ordered structure. However, it can appear for amorphous [27] or highly defective materials because of a certain randomness in the local environment or crystal-field effects (J-mixing) [17]. The intensity of the corresponding band in the luminescence spectra of the samples fell rapidly as the calcination temperature increased. This confirmed that the fraction of the amorphous phase decreased. Luminescence decay curves of Eu3+ (Fig. 5) changed considerably after calcination. The characteristic decay time 3+ of Eu red luminescence corresponded to the lifetime of these ions in the 5D0 state. The luminescence decay curve for initial Eu:GdAlO3 was clearly not monoexponential and included a fast component (τ1 ~ 0.23 ms) that was attributed to Eu3+ in an amorphous phase and defective crystals and a slow component (τ2 ~ 1.17 ms) that was attributed to Eu:GdAlO3 nanocrystals. The fast component disappeared and the decay became monoexponential with τex = 1.36 ms as the calcination temperature was increased to 800oC and then to 1300oC. This was indicative of a weakening of the nonradiative relaxation caused by fewer crystal defects and a smaller fraction of ions situated near the surface as the size increased and the fraction of the amorphous phase decreased. Analogous changes were observed for garnets Eu:Y3Al5O12 and Eu:Lu3Al5O12. The fast component of the initial samples had τ1 < 10 μs; the slow component, τ2 ~ 0.65 ms. The luminescence decay time for samples calcined at 1300oC reached τexp = 0.7 ms. The luminescence decay curves were still not monoexponential, probably because of concentrational quenching. Judd–Ofelt (J–O) parameters Ωk, where k = 2, 4, 6, are usually determined from luminescence spectra Ilum(ν), where ν is the light frequency, when examining the radiation characteristics of Eu3+ in luminophores because absorption spectra cannot be measured directly. Luminescence branching coefficients BJJ ′ were calculated using the formula: BJJ ′ = ∫IJJ′(ν)dν/∫Ilum(ν)dν .

(2)

The transition line strength is considered to be a linear combination of the squares of matrix elements U (k) [28, 29]: (k ) S JJ ′ = ∑ Ω kU JJ ′. k = 2,4,6

(3)

The transition probability is related to the line strength:

AJED J′

=

64π4 e 2 3h〈λ〉 3JJ ′

2

⎛ n2 + 2 ⎞ n ⎜⎜ ⎟⎟ S JJ ′ . ⎝ 3 ⎠

(4)

871

Fig. 5. Luminescence decay curves of initial Eu:GdAlO3 (a), Eu:Y3Al5O12 (b), and Eu:Lu3Al5O12 samples (c) (1) and those calcined at 800 (2) and 1300oC (3); λex = 355 nm. Here 〈λJJ ′〉 is the average transition wavelength; h, Planck′s constant; e, electron charge; n, material index of refraction. MD transition 5D0 → 7F1 of Eu3+ depends weakly on the local symmetry of the environment. The probabilities of other transitions AJJ′ and the radiative lifetime of the 5D0 state τrad can be estimated by using the known probability of this transition AMD = AMD(vac)n3 (AMD(vac) = 14.5 s–1) [30] and experimental luminescence branching coefficients BJJ ′: AJJ ′ = n3AMD(vac) BJJ ′/B01,

τrad = 1/∑J ′AJJ ′.

Luminescence branching coefficients for the set of MD transition 5D0 → 7F1 and ED transitions 5D0 → 7F2, D0 → F4 and 5D0 → 7F6 must be known to determine the J–O parameters in this manner. As a rule, the last typically has small branching coefficients B06 and is rarely observed in experimental luminescence spectra [17]. This is also related to the low sensitivity of available PMTs in the spectral range 800–850 nm that hinders experimental estimation of B06. Therefore, B06 is often omitted when examining J–O parameters. However, parameter Ω6 can turn out to be substantial for small branching coefficients of the 5D0 → 7F6 transition because the square of matrix element U06 for it is an order of magnitude less than those of U02 and U04. Therefore, neglect of this transition is the main source of error when estimating the whole set of J–O parameters for Eu3+. Weak luminescence belonging to the Eu3+ 5D0 → 7F6 transition at 800–850 nm was observed in Eu:GdAlO3 (Fig. 4a) and Eu:Y3Al5O12 (Fig. 4b). Structured luminescence bands that could be interpreted as the 5D0 → 7F6 transition were not observed in this region for Eu:Lu3Al5O12. Therefore, parameter Ω6 was not estimated. Table 3 presents J–O parameters Ω2, Ω4, and Ω6 that were determined in this manner for samples calcined at o 1300 C; experimentally determined luminescence branching coefficients BJJ ′; calculated probabilities of radiative transitions AJJ ′; the total probability of transitions from the 5D0 state Asum = ΣJ ′AJJ ′; and the radiative lifetime of the 5 D0 state τrad. The quantity τrad = 1.79 ms for Eu:GdAlO3. The luminescence quantum yield η = τexp /τrad = 76%. The lifetime was shortened because of concentrational quenching, which was substantial for 9.6 at% Eu3+, and nonradiative relaxation. The quantity τrad = 4.21 and 5.1 ms for garnets Eu:Y3Al5O12 and Eu:Lu3Al5O12. The luminescence quantum yield was ≤15%, mainly because of concentrational effects and luminophore structural defects that enhanced nonradiative relaxation. The quantity η for the garnet samples could be increased by optimizing the synthesis conditions (calcination temperature and composition). The luminescence color characteristics were determined according to the CIE 1931 system (Commission internationale de l′eclairage). Table 4 presents the color coordinates (x, y) and dominant wavelength λ d. The luminescence was red with high color purity ( p > 99%) for all samples. The Eu:GdAlO3 sample (x = 0.632, y = 0.368) had λ d = 599.6 nm. Garnets Eu:Y3Al5O12 and Eu:Lu3Al5O12 had luminescence color coordinates x = 0.611, y = 0.388, and λ d = 595.2 nm and x = 0.588, y = 0.411, and λ d = 590.8 nm (T = 1300oC for all samples). 5

872

7

TABLE 3. Probabilities of Radiative Transitions of Eu3+ from the 5D0 State for Eu:GdAlO3, Eu:Y3Al5O12, and Eu:Lu3Al5O12 Samples Calcined at 1300oC Transition

AJJ ′, s–1

〈λ〉, nm

Asum, s–1

BJJ ′, %

τrad, ms

Eu:GdAlO3 (Ω2 = 2.42·10–20 cm2, Ω4 = 2.90·10–20 cm2, Ω6 = 3.32·10–20 cm2) 5 5

D0→7F1 7

D0→ F2

5 5

7

587

134MD

24

611

233

ED

45

167

ED

24

ED

3.1

700

D0→ F4 7

810

D0→ F6

17.3

558

1.79

Eu:Y3Al5O12 (Ω2 = 0.66·10–20 cm2, Ω4 = 1.35·10–20 cm2, Ω6 = 9.7510–20 cm2) 5 5 5 5

D0→7F1 7

D0→ F2 7

587

92MD

39

604

43

ED

21

51

ED

22

705

D0→ F4 7

822

D0→ F6

31.8

ED

Eu:Lu3Al5O12 (Ω2 = 0.545·10 5

D0→7F1 7

D0→ F2 7

2

cm , Ω4 = 1.076·10–20 cm2)

586

91MD

43

605

45

ED

23

54

ED

28

5

D0→ F4

705

5

D0→7F6

–

4.23

13.5 –20

5

236

–

194

5.10

–

TABLE 4. CIE 1931 Color Coordinates of Luminescence of Eu:GdAlO3, Eu:Y3Al5O12, and Eu:Lu3Al5O12 Samples (λex = 400 nm) Sample Eu:GdAlO3

Eu:Y3Al5O12

Eu:Lu3Al5O12

x

y

λ d, nm

init.

0.637

0.363

600.6

o

800 С

0.631

0.369

599.3

o

1300 С

0.632

0.368

599.6

init.

0.628

0.371

598.7

o

800 С

0.615

0.384

596.0

o

1300 С

0.611

0.388

595.2

init.

0.606

0.393

594.2

o

0.594

0.406

591.8

0.588

0.411

590.8

800 С o

1300 С

Relatively few previous studies have focused on SCS of materials based on Eu:RE3Al5O12 and Eu:REAlO3 [14, 16, 26, 31]. Structural and morphological features of the materials were mainly studied previously whereas the spectral and kinetic characteristics of Eu3+ luminescence were not studied in detail. Future work on Eu:GdAlO3, Eu:Y3Al5O12, and Eu:Lu3Al5O12 prepared by SCS will be directed toward determining the optimum Eu3+ concentrations for low concentrational quenching of the luminescence. Co-activation of these materials by the Yb3+/Eu3+ pair to produce up- and down-conversion processes also seems promising [32, 33]. Conclusions. SCS of nanocrystals (average Scherrer diameter 20–40 nm) of Eu:GdAlO3, Eu:Y3Al5O12, and Eu:Lu3Al5O12 containing high concentrations (10–20 at%) of Eu3+ followed by calcination was demonstrated to be possible. The structural properties of these materials and the Eu3+ luminescence characteristics were studied. The obtained porous 873

materials could be used to fabricate red ceramic luminophores. Excitation at 400 nm by a commercial GaN-diode laser produced intense long-lived (0.5–1.0 ms) red Eu3+ luminescence. The color coordinates in the CIE 1931 system were x = 0.632 and y = 0.368 for Eu:GdAlO3; 0.611 and 0.388 for Eu:Y3Al5O12; and 0.588 and 0.411 for Eu:Lu3Al5O12. J–O parameters Ω2, Ω4, and Ω6; luminescence branching coefficients; and the radiative lifetime of the Eu3+ 5D0 level were determined. The proposed synthetic method for nanocrystalline materials could be used to prepare Re3Al5O12 and REAlO3 nanocrystals with various RE ions including those producing up- and down-conversion materials.

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