Aluminum and Gallium Substitution in Yttrium and Lutetium Aluminum

Oct 10, 2016 - Department of Electronics, Ivan Franko National University of Lviv, Gen. Tarnavskyj Street 107, 70017 Lviv, Ukraine. ⊥. Institute for...
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Aluminum and Gallium Substitution in Yttrium and Lutetium Aluminum−Gallium Garnets: Investigation by Single-Crystal NMR and TSL Methods Valentin Laguta,*,†,‡ Yuriy Zorenko,*,§,∥ Vitaliy Gorbenko,§,∥ Ayzhan Iskaliyeva,§ Yuriy Zagorodniy,⊥ Oleg Sidletskiy,# Paweł Bilski,∇ Anna Twardak,∇ and Martin Nikl† †

Institute of Physics Academy of Science of Czech Republic, Cukrovarnická 10, 16200 6 Prague, Czech Republic Institute of Physics, Opole University, Kopernika 11a, Opole 45-052, Poland § Institute of Physics, Kazimierz Wielki University in Bydgoszcz, Powstańców Wielkopolskich Street, 2, 85-090 Bydgoszcz, Poland ∥ Department of Electronics, Ivan Franko National University of Lviv, Gen. Tarnavskyj Street 107, 70017 Lviv, Ukraine ⊥ Institute for Problems of Material Science, Ukrainian Academy of Sciences, 3 Krjijanovsky Street, 03142 Kyiv, Ukraine # Institute for Scintillation Materials, National Academy of Sciences of Ukraine, Nauki Avenue 60, 61001 Kharkiv, Ukraine ∇ Institute of Nuclear Physic, Polish Academy of Science, Radzikowskiego Street, 156, 31-342 Krakow, Poland ‡

ABSTRACT: The work reports the results on 71Ga and 27Al NMR investigation of the gallium and aluminum ions distribution over tetrahedral and octahedral positions in the Y3Al5−x GaxO12:Ce single crystals and Lu3Al5−xGaxO12:Ce single-crystalline epitaxial films. The gallium content x varies between 0 and 5 in crystals and between 0.3 and 2 in films.. We find that in both the Y- and Lu-based solid solutions the larger gallium ions are preferably located at the tetrahedral position while the smaller aluminum ions prefer the octahedral position of the garnet host. Based on NMR data, the dependence of fractional occupation parameters of the tetrahedral site of Ga and Al ions on the Ga content is determined. In particular, in the Y3Al2Ga3O12:Ce crystal only 28% of Ga ions occupy octahedral sites, whereas 72% occupy tetrahedral ones. NMR investigations suggest that observed nonmonotonic dependences of electron trap depths monitored by thermally stimulated luminescence of Y3Al5−xGaxO12:Ce complex garnets on the Ga content are related to preferential localization of the Ga and Al ions over the tetrahedral and octahedral positions of the garnet lattice, respectively. Our data confirm that the tetrahedral site preference (over the octahedral site) for the Ga occupation is an intrinsic property of the mixed Y3(Lu3)Al5−xGaxO12 garnets. of cations in two different sites of the O10 h −Ia3d garnet lattice.13−15 In particular, a part of Al3+ cations in the octahedral a and tetrahedral d positions are replaced by Ga3+ ions, and Y3+ cations in dodecahedral positions are replaced by large Gd3+ ions which provided similar light yield advance. These Cedoped (Gd,Y)3(Al,Ga)5O12 materials were prepared both in ceramic16 and single-crystal17 forms. Scintillation performance of both the Ce-doped Gd3(Ga,Al)5O12 ceramics and single crystals have been recently directly compared.18,19 Further improvement of these materials especially with respect to the speed of scintillation response has recently been accomplished by the alkali earth ion codoping.20−23 Such materials become comparable regarding time coincidence resolution with commercial LYSO:Ce,Ca24,25 and thus have

1. INTRODUCTION Ce-doped (Lu,Gd)3(Al,Ga)5O12 multicomponent garnets derived from a high-density Lu3Al5O12 host became an important area in the field of high-density, efficient, and fast scintillators (see review, ref 1). Several laboratories around the globe have recently reported about their preparation and characterization.2−7 The (Gd,Y)3(Al,Ga)5O12:Ce single crystals with the optimized Ga:Al ratio are considered nowadays to belong to the most efficient oxide scintillators with very high light yield (LY) up to8 or even above9 60 000 photon/MeV, thus reaching the theoretical limit given by the energy necessary for free electron−hole pair creation in the garnet host.10 Scintillation performance of these garnet compounds was optimized by a combination of the band gap engineering and positioning of Ce3+ levels within the band gap of the garnet host.11,12 Similar modification of the electronic structure of garnet hosts and their Ce3+ activator levels was performed in the classical scintillation material Y3Al5O12 (YAG) by a partial substitution © 2016 American Chemical Society

Received: August 25, 2016 Revised: October 9, 2016 Published: October 10, 2016 24400

DOI: 10.1021/acs.jpcc.6b08593 J. Phys. Chem. C 2016, 120, 24400−24408

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The Journal of Physical Chemistry C

crystal. However, usually they are so broad that they can hardly be detected. Only the central transition is well-visible in the spectra as it is affected by the quadrupole coupling as the second-order perturbation to the stronger Zeeman interaction, which defines the Larmor frequency.38 This is the case of 27Al and especially 71Ga isotope which has the quadrupole splitting in the range up to 6.5 MHz in the Y3Ga5O12 (YGG) crystal.39 However, even the central transition lines of 27Al and 71Ga isotopes from the tetrahedral and octahedral positions in YAG and YGG crystals are well-separated from each other,39,40 allowing an easy determination of the Al and Ga site occupations from the single-crystal NMR spectra by properly measuring the area of corresponding spectral lines. It should be noted that the ion (atom) occupation of different lattice sites can also be distinguished in NMR spectrum from the isotropic chemical shift of spectral line which depends directly on the coordination number of probed nucleus, for instance, the number of anions in the first coordination sphere of a cation.37 The point is that the chemical shift is usually smaller or comparable with the NMR spectrum width in a solid, but the spectrum narrowing by utilizing fast magic-angle spinning (MAS) of powder sample can be used. In the case of YAG and mixed Y3Al5−xGaxO12 (x = 0, 1, 2, 3, and 4.5) powders prepared by a sol−gel synthesis, two Al crystallographic sites are easily resolved in 27Al MAS spectra.30,41 However, this technique is not applicable for nuclei with strong quadrupole interaction as the effective narrowing of the spectral line is determined by the spinning rate, which is usually limited to 30−40 kHz. For spectra broader than the spinning rate (as is the case of 69Ga and 71Ga isotopes), the MAS technique is much less effective. This method is also not applicable for films or samples with limited volume. For such cases, single-crystal NMR can provide sufficient signal intensity and spectral resolution. Note that extremely broad NMR spectra of mass limited samples can be acquired by using the WURST-CPMG pulse sequence.42 In the present paper, the Ga and Al ions site occupations have been investigated in Ce-doped Y3Al5−xGaxO12 mixed garnet crystals (x = 0, 2, 3, 5) and Lu3Al5−xGaxO12 singlecrystalline films (x = 0.3 and 2) epitaxially grown on (110) oriented YAG substrate by 27Al and 71Ga single-crystal NMR technique. It is currently well-established that synthesis conditions strongly influence the YAG/LuAG crystal perfection. For instance, creation of so-called anti-site defects when Al substitutes for Y or Lu and vice versa is strongly suppressed in garnet films whose growth temperature (∼1000 °C) is much lower than that used in bulk crystal growth at temperatures around 2000 °C.43 Therefore, it is important to clarify the cation occupation in Y(Lu)3Al5−xGaxO12:Ce single-crystalline films as well. NMR data are further compared with optical properties characterized by thermally stimulated luminescence (TSL) measurements in the same Y3Al5−xGaxO12:Ce crystals under excitation by α- and β-particles. TSL in garnet crystals is based on liberation of electrons from the traps formed by garnet host defects (mostly oxygen vacancies and antisite cations) and their recombination with the holes trapped by Ce3+ ions. Therefore, TSL study of Y3Al5−xGaxO12:Ce with varying Ga content x can indicate the change of the defect level localization with respect to the bottom of the conduction band strongly dependent on whether Ga occupies tetrahedral or octahedral sites. Taking into account that the band gap of Y3Al5−xGaxO12:Ce decreases by about 1.5 eV upon the change of parameter x from 0 to 5,15 the shallow trap levels of defects

an application potential also in TOF-PET medical imaging. The Y3Al5−xGaxO12 system, unlike its Gd analogue, provides a continuous series of solid solutions in the crystal form in the entire range of x.14 Therefore, it is an appropriate object for detailed study of relationship between the composition and luminescent properties of mixed garnet compounds. To better understand results of composition engineering strategy aiming at creation of efficient scintillators, we recently investigated the absorption, luminescent, and scintillation properties of the Ce3+-doped Y3Al5−xGaxO12 garnets with the Ga content x varying between 0 and 5, prepared in the form of single crystals by the Czochralski method.26 From changes of positions of the absorption and emission bands, related to the 4f−5d transitions of Ce3+ ions in these solid solutions, we concluded that the optical properties of the Y3Al5−xGaxO12:Ce garnets change nonmonotonically with increase of the Ga concentration x between 0 and 5. Specifically, at the highest values of Ga content, x > 3, we observed quite different behavior of the Y3Al5−xGaxO12:Ce crystals with respect to properties of these crystals having x between 0 and 3. Such a change in the optical properties of Y3Al5−xGaxO12:Ce crystals in the mentioned concentration ranges is explained by respective differences in occupation of the tetrahedral and octahedral positions of the garnet lattice by Ga ions. At the same time, Ga preference to substitute for Al in either its octahedral or tetrahedral position in Ce-doped Y3Al5−xGaxO12 crystals is still not completely clarified. Meanwhile, this is a very important issue in the strategy of cation substitution in the multicomponent garnet compounds in scintillation materials,1,2,8,17 persistent phosphors,27 and luminescent convertors of LED radiation28 and definitely requires clarification. Owing to their similar ionic radii,29 aluminum and gallium atoms have been found in both tetrahedral and octahedral coordinations in oxide compounds.30,31 The ease of aluminum−gallium substitution has been a subject of several studies where the preference of Ga3+ with respect to Al3+ for tetrahedral site occupation has been observed30,32,33,13 and theoretically calculated in garnets.15 This preference cannot be explained by a size effect since the gallium ionic radius is larger than that of aluminum. However, there is also an opinion that Ga can preferably occupy octahedral positions larger in volume.34−36 Since the site occupation essentially influences the band structure and consequently optical properties of crystals, this aspect needs further detailed investigation. Nuclear magnetic resonance (NMR), providing a unique approach to quantify the site occupancy in a specific material,37 is an optimal tool to accomplish such a task. In the Y3Al5−xGaxO12 solid solution both Al and Ga cations are suitable for NMR measurements. The most easily measurable is 27 Al isotope, which has a nuclear spin I = 5/2, Larmor frequency νL = 104.26 MHz at the field of 9.4 T, and natural abundance of 100%. Ga has two isotopes 69Ga and 71Ga with spin = 3/2, natural abundances of 60 and 40%, and the Larmor frequencies of 96 and 122 MHz, respectively. All these nuclei possess a quadrupole moment eQ, which interacts with the electric field gradient (EFG) generated by surrounding ions. The quadrupole interaction produces 2I − 1 satellite lines, whose splittings depend on nuclear spin and quadrupole interaction strength characterized by the quadrupole coupling constant CQ = e2qQ/h,38 where eq is the largest main component of EFG tensor V. The satellites are symmetrically arranged around the central 1/2 ↔ −1/2 transition. The quadrupole satellite transitions can be resolved in a single 24401

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delay of 1 s, much longer than the spin−lattice relaxation time which is about 0.2 s. Note that paramagnetic Ce3+ ions substantially shortened the relaxation time of 27Al nuclei allowing usage of relatively short relaxation delays. NMR spectra were recorded for several separated crystal orientations with respect to the external magnetic field in order to distinguish the resonance lines related to the tetrahedral and octahedral positions of Ga and Al ions. A homemade goniometer was utilized for this purpose. 71 Ga NMR spectra are referenced to external standard GaF3. As a reference for 27Al, Al(NO3)3 was used. The TSL glow curves of the Y3Al5−xGaxO12:Ce crystals after excitation by α- and β-particles from Am241 and Sr90+Y90 sources, respectively, were additionally measured to characterize the dependence of optical properties of the Ce3+ doped Y3Al5−xGaxO12 on the Ga content x; for technical details, see refs 14 and 26. The TSL measurements were performed in the 300−800 K temperature range using a commercial Risoe DA20 TL/OSL reader (Denmark). The registration of TSL signal was performed using the green filter transmitting in the spectral range of Ce3+ luminescence.

can even be buried in the conduction band and corresponding TSL peaks fully eliminated.

2. SAMPLES AND EXPERIMENTAL TECHNIQUES 27 Al, 71Ga, and 69Ga NMR and TSL studies were performed on Y3Al5−xGaxO12:Ce single crystals with x = 0, 2, 3, and 5 grown from the melt by the Czochralski method at temperatures from 1970 to 1830 °C depending on the Ga content (see ref 14 for details). The concentration of Ce was about 0.1 atom %.14 The samples were parallelepipeds with the size of about 2.5 × 2.5 × 0.5 mm3 cut in the (110) plane. 71 Ga and 69Ga NMR investigations were also performed on Lu3Al5−xGaxO12:Ce single-crystalline films (SCF) with the Ga content close to x = 0.3 and 2.0 and the thickness of 110 and 7 μm, respectively. The Lu3Al5−xGaxO12:Ce SCF with the nominal content of Ga x = 1.0 and 3.5 in the melt were epitaxially grown on (110) oriented Y3Al5O12 substrates by the liquid-phase epitaxy (LPE) method from PbO-B2O3 flux at temperatures from 980 to 1025 °C. The thickness of films was determined by the weighing method. It is important to note that the segregation coefficient of Ga ions in the LuAG host is significantly lower in the case of SCF crystallization with respect to the growth by the Czochralski method, where the segregation coefficient of Ga ions is close to 1.14 Specifically, from the result of SCF content measurements performed using the EDX microanalyzer with IXRF 500 and LN2 Eumex detectors at JEOL JSM-820 electronic microscope, we found, that the segregation coefficient of Ga ions in LuAG SCF was linearly increasing from 0.3 at x = 1.0, to 0.57 at x = 3.5. Therefore, the real content of LuAl5−xGaxO12:Ce SCF samples, grown at nominal values x = 1.0 and 3.5 and selected later for NMR investigations, was Lu 3 Al 4 . 7 Ga 0 . 3 O 1 2 :Ce and Lu3Al3Ga2O12:Ce. The NMR measurements were carried out at room temperature using a commercial Bruker Avance 400 MHz solid-state NMR spectrometer with 9.4 T wide-bore magnet. The 71Ga NMR spectra were obtained with the conventional 90x−τ−90y−τ spin echo pulse sequence, with 90° pulse lengths of 1.2 μs. With this pulse sequence, due to short pulses, the spin system could be irradiated well within a broad frequency region, which is extended for the central transition up to 300 kHz. Spectral lines are also broad (up to 50 kHz) in the Y3Al5−xGaxO12:Ce crystals and Lu3Al5−x GaxO12:Ce films. However, this broadening is inhomogeneous with the intrinsic line width smaller than 1 kHz. The time delay between pulses τ = 20 μs was taken much shorter in comparison with the transverse relaxation time T2 to correctly reproduce all 71Ga spectral components. About 60 000 scans were accumulated for each spectrum of the crystal, with a relaxation delay of 1 s. It was at least four times longer compared to the spin−lattice relaxation time estimated by the saturation recovery method. For Lu3Al5−x GaxO12:Ce films, each spectrum was accumulated for 2−3 days. To check whether the solid−echo pulse sequence correctly reproduces 71Ga spectral intensities, test measurements were performed in Y3Ga5O12 crystal. They showed that the intensities of 71Ga NMR lines from tetrahedral and octahedral Ga positions correspond well to those expected in the garnet lattice (for details, see section 3). Since the 27Al NMR spectrum of the central transition is quite narrow (∼15 kHz) as well as spectral lines (4−5 kHz), it was recorded with a single-pulse sequence (90° single pulse of 0.3 μs), which minimizes possible artifacts in the spectrum. For each spectrum, we accumulated 1024 scans with a relaxation

3. NMR INVESTIGATION OF GA3+ AND AL3+ DISTRIBUTION IN THE Y3AL5−XGAXO12:CE AND LU3AL5−XGAXO12:CE HOSTS As pointed out above, 27Al and 71Ga NMR lines related to tetragonal and octahedral sites of Al and Ga cations in the garnet structure can be resolved even in the central transition spectrum due to the large difference in EFG tensors at these crystallographic sites (e.g., see refs 39−41.). The resonance line positions of the 1/2 ↔ −1/2 central transition in a crystal depend on crystal orientations with respect to the external magnetic field and are given by the equation which contains contributions from the quadrupole and chemical shift mechanisms:44 ν1/2 = −

νQ2 ⎛ 3 ⎞⎟ ⎜I (I + 1) − f (θ , φ) 16νL ⎝ 4⎠ η

+ νL[1 + δiso + δax(3 cos2 θ − 1) + δaniso sin 2 θ cos 2φ] (1)

where νQ =

3e 2qQ h2I(2I − 1)

is the quadrupole frequency, eq = Vzz,

and η = (Vxx − Vyy)/Vzz is the largest eigenvalue of the EFG tensor and its asymmetry parameter, respectively. The function fη(θ, φ) describes the dependence of the frequency shift on the mutual orientations of external magnetic field and EFG tensor; x, y, and z form the principal EFG tensor reference frame. The second term in eq 1 describes the chemical shift of the Larmor frequency, where δiso, δax, and δaniso are isotropic, axial, and anisotropic components of the chemical shift (CS) tensor (e.g., see ref 44). For convenience, Table 1 presents main NMR parameters of 27 Al and 71Ga in YAG and YGG determined previously. One can see, in particular, that the EFG tensor for gallium and aluminum in the tetrahedral (GaIV, AlIV) and octahedral (GaVI, AlVI) sites has an axial (cylindrical) symmetry. There are three magnetically inequivalent tetrahedral sites and four magnetically inequivalent octahedral sites with the principal EFG axis directed along the and cubic directions, respectively.39,40 The ratio of the tetrahedral to octahedral position values is equal to 3/2 in stoichiometric compounds. Altogether, aluminum and gallium ions have seven magnetically 24402

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The Journal of Physical Chemistry C Table 1. 27Al and 71Ga Quadrupole Coupling Parameters (CQ, νQ) and Isotropic Chemical Shifts δiso for Three AlVI,GaVI and Four AlIV,GaIV sites in YAG and YGGa site AlVI AlIV GaVI GaIV a

CQ (MHz)

νQ (MHz)

± ± ± ±

0.0955 0.915 2.05 6.55

0.637 6.103 4.10 13.1

0.001 0.001 0.06 0.2

δiso (ppm)

ref

± ± ± ±

40 40 39 39

2.1 81.5 5.6 219

0.2 0.4 1.2 19

random distribution of Al over tetrahedral and octahedral sites is expected. To check whether the population of GaIV and GaVI sites also depends on Ga concentration, we measured the 71Ga NMR spectra first in the Y3Ga5O12:Ce single crystal. A typical 71Ga spectrum is shown in Figure 2.

The EFG tensor asymmetry parameter η is zero for both nuclei.

inequivalent positions in the garnet lattice. It should lead to seven different lines of the central transitions in the 27Al and 71 Ga NMR spectra which, however, can partly coincide along symmetrical axes or planes. Figure 1 shows the 27Al NMR spectrum measured in the mixed crystals of Y3Al5−xGaxO12:Ce at x = 2 and 3. These spectra represent only the 1/2 ↔ −1/2 central transition, whose frequency is defined by the chemical shift mechanism and second order quadrupole interaction according to eq 1. Because the AlVI site is characterized by small quadrupole frequency (in the Y3Al5−xGaxO12 solid solution CQ increases from 0.637 to ∼1.4 MHz),41 corresponding NMR lines remain almost unchanged under the crystal rotation. In contrast, the NMR lines responsible for the AlIV sites are markedly (8−12 kHz) shifted from the Larmor frequency by the isotropic chemical shift and quadrupole coupling, which are much larger for the AlIV site comparing to the AlVI site (Table 1). Therefore, the strong line around the Larmor frequency in Figure 1 corresponds to four overlapping resonances from AlVI. Respectively, remaining lines in the spectra are assigned to the AlIV sites. It can be easily seen that the relative intensity of AlIV NMR lines as compared to that of AlVI lines is about two times stronger in Y3Al3Ga2O12:Ce than that in Y3Al2Ga3O12:Ce. This suggests that the population of AlIV and AlVI sites in Y3Al5−xGaxO12:Ce single crystals depends on Ga concentration. Under the reasonable assumption that the area under NMR line is proportional to the concentration of Al ions in the corresponding lattice site, the occupation number (fractional parameter) of Al in the tetrahedral coordination can be easily calculated as fAl = AlIV/(AlVI + AlIV), where AlIV (AlVI) represent the total NMR intensities of Al in tetrahedral (octahedral) sites. This gives fAl = 0.50 and 0.38 for Y3Al3Ga2O12:Ce and Y3Al2Ga3O12:Ce, respectively. These numbers are considerably smaller than the value fAl = 0.6 measured in Y3Al5O1240 where a

Figure 2. 71Ga NMR spectrum measured in Y3Ga5O12:Ce crystal (black points) at the magnetic field orientation B ⊥ (110) and its Gaussian decomposition into separated lines from GaVI (two blue solid lines around the zero frequency shift) and GaIV (three green solid lines) sites. Fitted spectrum is shown by the red solid line.

One can see that the 71Ga NMR spectrum is much broader compared to that from 27Al NMR. This is due to the about ten times larger quadrupole frequencies of 71Ga nuclei compared to those of 27Al (Table 1I). In accordance with eq 1 (for details, see also ref 39), the rotation of the Y3Ga5O12 crystal in the external magnetic field leads to a shift of the 71Ga NMR lines belonging to three magnetically inequivalent GaIV sites in the range of more than 200 kHz. However, four partially overlapping lines related to GaVI are located at the center of the spectrum and experience only slight (∼15 kHz) variation depending on the crystal orientation in an external field. Accordingly, the two lines located in the center of the 71Ga NMR spectrum shown in Figure 2 correspond to four overlapping lines originating from the GaVI, whereas other three lines located far from the Larmor frequency correspond to GaIV. The fractional parameter f Ga calculated from the integral NMR intensities of Ga in the tetrahedral and octahedral sites is equal to 0.60 with a deviation of 5% in agreement with the crystal structure.

Figure 1. 27Al NMR spectra (black lines) of Y3Al3Ga2O12:Ce (a) and Y3Al2Ga3O12:Ce (b) single crystals at the magnetic field orientation B ⊥ (110) crystal plane with corresponding Lorentzian fits (red solid lines). Separated spectral lines from AlIV and AlVI sites are shown by green and blue solid lines, respectively. 24403

DOI: 10.1021/acs.jpcc.6b08593 J. Phys. Chem. C 2016, 120, 24400−24408

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The Journal of Physical Chemistry C Typical 71Ga NMR spectra of Y3Al3Ga2O12:Ce single-crystal are shown in Figure 3 for a few crystal orientations. One can

crystal, spectral lines are about two times broader than those in the Y3Al3Ga2O12:Ce crystal (EFGs increase with increase of Ga concentration). Therefore, the decomposition of the spectrum into the GaIV and GaVI components has some uncertainty due to partial overlapping of the components around zero frequencies. Based on the above analysis, the fractional parameters of gallium in the tetrahedral coordination, f Ga = GaIV/(GaVI + GaIV) have been calculated for all the studied compounds and together with Al data are presented in Table 2 and plotted in Table 2. Al and Ga Distribution Parameters in Y3[Al1−pGap]2[Al(3−x+2p)/3Ga(x−2p)/3]3O12:Ce Mixed Crystals x

p

fAl

0 2 3 5

0 0.28 0.45 1

0.6 0.50 0.38

f Ga 0.72 0.70 0.6

Figure 3. 71Ga NMR spectra of Y3Al3Ga2O12:Ce single crystal for different alignments of the (110) crystal face with respect to external magnetic field. The spectrum at θ = 0° (B ⊥ (110)) is decomposed into Lorentzian lines from GaIV (green solid lines) and GaVI (blue solid line) sites. Fitted spectrum is shown by the red solid line.

see that the 71Ga NMR lines are markedly broader than those in Y3Ga5O12:Ce crystal (full width at half-maximum (fwhm) is up to 50 kHz). The spectrum broadening is obviously related to quadrupole parameters distribution as the Ga (or Al) lattice sites are randomly distorted by Al (or Ga) ions. The spectral lines are thus inhomogeneously broadened. The line width depends on the crystal orientation as well. Nevertheless, the 71 Ga NMR lines from GaIV and GaVI sites can be well-resolved even in such broad spectra by choosing appropriate crystal orientation, for instance, at θ = 0°. Accordingly, the narrow line located in the center of the 71Ga NMR spectrum (the lowest spectrum in Figure 3) corresponds to four overlapping lines originating from the GaVI, whereas other two broad lines correspond to GaIV. Similar 71Ga spectra were measured for Y3Al2Ga3O12:Ce crystal. One of the spectra decomposed into separate lines related to the GaIV and GaVI sites is shown in Figure 4. In this

Figure 5. Fractional occupation parameters of tetrahedral sites of Ga and Al ions in the Y3Al5−xGaxO12:Ce single crystal garnets (solid symbols) and Ga fractional occupation parameter in Lu3Al5−xGaxO12:Ce single-crystalline films (empty symbols) as a function of the total Ga content. Dashed line (f Ga = fAl = 0.6) corresponds to a random distribution of Al and Ga over tetrahedral and octahedral sites.

Figure 5. They markedly differ from the value fAl = f Ga = 0.6 expected for a random distribution of Al and Ga over tetrahedral sites (Figure 5, dashed line). For instance, in Y3Al3Ga2O12:Ce compound, the fractional parameter fAl is 0.5, wherein f Ga = 0.72. Nevertheless, these two values are in good correspondence within an experimental error of 4−5%. This can be checked by using the structural formula of the Y 3 Al 5−x Ga x O 12 system, which can be expressed as Y3[Al1−pGap]2[Al(3−x+2p)/3Ga(x−2p)/3]3O12, where p is the occupancy parameter of Ga3+ ions in the octahedral sites.32,33 Based on the structural formula, one can obtain that in the case of fAl = 0.5 it should be f Ga = 0.75. Similar calculations carried out for Y3Al2Ga3O12:Ce lead to the values f Ga = 0.7 and fAl = 0.38, which are again reasonably corresponding to each other. As it follows from the structural formula for x = 3 and fAl = 0.38, the expected f Ga value is equal to 0.72. This means that at x = 3 only 28% of the Ga ions occupy the octahedral sites whereas 72% occupy the tetrahedral sites. At the concentration of Ga ions x = 2, only 38% of the Al ions occupy the tetrahedral sites while 62% occupy the octahedral sites.

Figure 4. 71Ga NMR spectrum measured in Y3Al2Ga3O12:Ce crystal (black points) at the magnetic field orientation B ⊥ (110) and its Gaussian decomposition into separate lines from GaIV (green solid lines) and GaVI (blue solid line) sites. Fitted spectrum is shown by the red solid line. 24404

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Figure 6. 71Ga NMR spectrum measured in (a) Lu3Al4.7Ga0.3O12:Ce and (b) Lu3Al3.Ga2O12:Ce films (black points) and its Gaussian decomposition into separate lines from GaIV (green solid lines) and GaVI (blue solid lines) sites. Fitted spectra are shown by the red solid lines.

Figure 7. TSL glow curves of Y3Al5−xGaxO12:Ce crystals with different Ga content x after β-particles (a) and α-particles (b) irradiations; dependence of the TSL peak position (c) and energy of excitation (d) of high- and low-energy peaks on Ga content x.

noise ratio. The decomposition of the spectrum into separate GaIV and GaVI components for the Lu3Al4.7Ga0.3O12:Ce film (Figure 6a) leads to the fractional parameter f Ga = 0.76−0.78. The signal is too noisy for the second film (Figure 6b) with the thickness of only 7 μm but a larger Ga concentration x = 2. The spectrum in this film is also broader than that with x = 0.3 due to an increase of quadrupole coupling with increasing Ga concentration.41 This factor also reduces the intensity of NMR spectrum. We estimated the fractional parameter in the Lu3Al3Ga2O12:Ce film as f Ga = 0.69−0.73. One can thus conclude that the Ga occupation of tetrahedral and octahedral sites in Lu3Al5−xGaxO12:Ce films has the same tendency as that in Y3Al5−xGaxO12:Ce crystals, namely, at low concentrations Ga prefers to occupy the smaller tetrahedral sites. Obviously, this is

We also tested the possibility to measure NMR spectra in the Lu3Al5−xGaxO12:Ce single-crystalline films. Because the films were grown on the Y3Al5O12 substrate containing aluminum, only occupation numbers of Ga ions can be determined. However, the NMR technique is apparently the only method that allows us to estimate the cation occupation parameters in thin-film mixed garnets. Moreover, with this measurement we can check whether the Lu substitution for Y influences the Al/ Ga occupation of the octahedral and tetrahedral sites. Figure 6 shows the 71Ga NMR spectra measured in two films with the Ga concentration close to x = 0.3 and 2. One can see that despite the small volume of films (the thickness was 110 and 7 μm for x = 0.3 and 2, respectively) the 71Ga NMR spectrum for the thicker film has a quite satisfactory signal-to24405

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Table 3. TSL Peak Positions and Corresponding Trap Depths in Y3Al5−xGaxO12:Ce Crystals with Different Ga Content x crystal

high-energy peak T (K); trap depth (eV) under β-particles excitation

low-energy peak T (K); trap depth (eV) under β-particles excitation

high-energy peak T (K); trap depth (eV) under α-particles excitation

low-energy peak T (K); trap depth (eV) under α-particles excitation

Y3Al5O12:Ce Y3Al3Ga2O12:Ce Y3Al2Ga3O12:Ce Y3Ga5O12:Ce

437; 590 (1.43 eV) 417; 528 (0.62 eV) 429 (0.54 eV) 357 (0.50 eV)

361; 426 (1.09 eV); 390 (0.76 eV); 360 (0.48 eV);

443; 586 (1.34 eV) 427; 530 (0.76 eV) 381; 421 (0.59 eV) 355 (0.5 eV)

372; 423 (1.04 eV); 389 (0.6 eV) 359 (0.64 eV)

model of Al/Ga distribution predict lower energy of the system as compared with the model assuming random distribution of cations.15 However, the DFT calculation itself cannot explain the Ga occupation preference. It needs correct input parameters which can be provided, in particular, by NMR.

an intrinsic property of the garnet structure which should be taken into account in interpretation of all optical results where the concentration of Ga stands as a parameter. It is worth noting that the Ga fractional occupation parameters of tetrahedral sites only slightly differ from those obtained in ref 33 by the X-ray technique. We can thus conclude that our NMR data strongly support the conclusion that despite Ga3+ being larger than Al3+ it preferentially occupies the tetrahedral (four-coordinated) site rather than the octahedral (six-coordinated) site in complex Y3Al5−xGaxO12 garnets. As can be seen from Figure 6 due to slightly different slopes of displayed curves related to the Y3Al5−xGaxO12:Ce crystals and Lu3Al5−xGaxO12:Ce films, the local cation ordering can depend on (i) technology of crystal growth (the Czochralski method at temperatures 1970−1830 °C or LPE method at temperatures 950−1050 °C) and (ii) type of cation (Y or Lu) at the dodecahedral site. Detailed clarification of the nature of these differences needs further investigation.

5. TSL MEASUREMENTS The results of the thermoluminescence measurements of β- and α-irradiated Y3Al5−xGaxO12:Ce crystals above RT are shown in Figure 7a,b, respectively. The TSL peak positions of Y3Al5−xGaxO12:Ce crystal samples with different Ga content x and respective trap depths are collected in Table 3. The TSL mechanism in these crystals is connected with the electron liberation from deep traps, which most probably include oxygen vacancies,46,47 and their subsequent recombination with the holes localized at the Ce3+ ions. The main highenergy TSL peak of Y3Al5O12:Ce crystal is observed at about 586−590 K (Figure 7a,b, curves 1; Table 3). With increasing Ga content in crystals up to x = 2, the position of the highenergy TSL peak is shifted to lower temperatures 528−530 K for Y3Al3Ga2O12:Ce (Figure 7a,b, curves 2). Following increase of the Ga content at x > 2.0, further shifts are observed of this TSL peak to 421−429 K in Y3Al2Ga3O12:Ce crystal (Figure 7a,b, curves 3; Table 3) and to 357−355 K for Y3Ga5O12:Ce crystal (Figure 7a,b, curves 4; Table 3). Thus, the dependence of the main TSL peak position on Ga content in two intervals x = 0−2 and x = 2−5 can be presented by two lines with different slopes. Moreover, this kind of dependence on the Ga content also appears in other optical properties of Y3Al5−xGaxO12:Ce crystals, namely, in the position of the Ce3+ absorption and emission bands as well as energies of creation of an exciton bound around Ce3+ ions (see ref 26 for detail). Taking into account the results presented in Figure 7, we can conclude that the nonlinear dependence of optical properties of solid solution of Y3Al5−xGaxO12:Ce garnet on the Ga content (parameter x) correlates with the preferential gallium and aluminum distribution in the tetrahedral and octahedral positions of the garnet host, respectively. The Ga admixture in Y3Al5−xGaxO12:Ce crystals also leads to a significant decrease of the intensity of the main TSL peaks in the 425−650 K temperature range. This result is in good correlation with significant increase of overall scintillation efficiency in Y3Al2Ga3O12:Ce crystal,26 most probably due to elimination of trap-related centers in the scintillation mechanism of the samples with the mentioned content. At the same time, in the crystals with higher gallium content x > 3, namely, in the Y3Ga5O12:Ce crystal, the intensity of TSL above RT is practically negligible. Apart from the high-energy peak, the low-energy peak at about 423−426 K is also observed in the TSL glow curves of Y3Al5O12:Ce crystals. This peaks is shifted to lower temperatures 389−390 K for Y3Al3Ga2O12:Ce (Figure 7a,b, curves 2) and 359−360 K for Y3Al2Ga3O12:Ce crystals (Figure 7a,b,

4. INTERPRETATION OF THE GA/AL DISTRIBUTION At present, there is no clear explanation why Ga3+ preferentially occupies the tetrahedral site in the mixed Y3(Lu3)Al5−xGaxO12 garnets. Meanwhile, the precise determination of Ga/Al distribution in the Y and especially Lu garnets is an important problem as the cation site occupation essentially influences the band structure and positioning of the 5d energy levels of the Ce3+ activator as well as energy levels of other impurities and lattice defects within the forbidden gap. Explanation of this phenomenon is also an actual task for theoretical consideration in order to perform the band gap and band-edge engineering of multicomponent garnet scintillators.11,15 The preference of Ga3+ ions to occupy tetrahedral sites is greater than that of Fe3+ ions in the analogous system Y3Fe5−xAlxO1245 despite almost equal ionic radii of Ga3+ and Fe3+.29 This means that conventional ionic radius arguments do not work for Ga3+. Many authors32,33,13,15 thus reasonably pointed out that one of the main reasons for this peculiar cation distribution is a stronger covalency of the Ga−O bond compared with the Al−O bond. The Ga ion thus prefers tetrahedrally coordinated chemical bonds. This is supported by the study of stability of Fe-, Al-, and Ga-containing garnets at high pressures and high temperatures. In particular, decomposition to the YXO3 compound (X = Fe or Al) with perovskite-like (octahedral-coordinated) structure has been found to occur, while in the case of Y−Ga garnet no such transformation was found.32 In addition to the effect of covalency, more recently, Nakatsuka et al. proposed13 that the peculiar cation distribution in the Y−Al−Ga system is a consequence of not only the strong covalency of the Ga−O bond but also the need to decrease the cation−cation repulsive force which decreases across the shared polyhedral edges with increasing Ga content as a result of structural geometric restrictions. Note that DFT calculations with appropriate 24406

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Yoshikawa, A. Characterization of GAGG:Ce scintillators with various Al-to-Ga ratio. Nucl. Instrum. Methods Phys. Res., Sect. A 2015, 772, 112−117. (6) Auffray, E.; Augulis, R.; Borisevich, A.; Gulbinas, V.; Fedorov, A.; Korjik, M.; Lucchini, M. T.; Mechinsky, V.; Nargelas, S.; Songaila, E.; Tamulaitis, G.; Vaitkevičius, A.; Zazubovich, S. Luminescence rise time in self-activated PbWO4 and Ce-doped Gd3Al2Ga3O12 scintillation crystals. J. Lumin. 2016, 178, 54−60. (7) Wu, Y.; Luo, J.; Nikl, M.; Ren, G. Origin of improved scintillation efficiency in (Lu,Gd)3(Ga,Al)5 O12:Ce multicomponent garnets: an Xray absorption near edge spectroscopy (XANES) study. APL Mater. 2014, 2, 012101. (8) Kamada, K.; Kurosawa, S.; Prusa, P.; Nikl, M.; Kochurikhin, V. V.; Endo, T.; Tsutumi, K.; Sato, H.; Yokota, Y.; Sugiyama, K.; Yoshikawa, A. Cz grown 2-in. size Ce:Gd3(Al,Ga)5O12 single crystal; relationship between Al, Ga site occupancy and scintillation properties. Opt. Mater. 2014, 36, 1942−1945. (9) Wang, C.; Wu, Y.; Ding, D.; Li, H.; Chen, X.; Shi, J.; Ren, G. Optical and scintillation properties of Ce-doped (Gd2Y1)Ga2.7Al2.3O12 single crystal grown by Czochralski method. Nucl. Instrum. Methods Phys. Res., Sect. A 2016, 820, 8−13. (10) Dorenbos, P. Fundamental limitationin in the performance of Ce3+-, Pr3+ - and Eu3+- activated scintillators. IEEE Trans. Nucl. Sci. 2010, 57, 1162−1167. (11) Fasoli, M.; Vedda, A.; Nikl, M.; Jiang, C.; Uberuaga, B. P.; Andersson, D. A.; McClellan, K. J.; Stanek, C. R. Band-gap engineering for removing shallow traps in rare-earth Lu3Al5O12 garnet scintillators using Ga3+ doping. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 84, 081102. (12) Dorenbos, P. Electronic structure and optical properties of the lanthanide activated RE3(Al1−x Gax)5O12 (RE = Gd, Y,Lu) garnet compounds. J. Lumin. 2013, 134, 310−318. (13) Nakatsuka, A.; Yoshiasa, A.; Yamanaka, T. Cation distribution and crystal chemistry of Y3Al5‑GaxO12 (0 ≤ x ≤ 5) garnet solid solutions. Acta Crystallogr., Sect. B: Struct. Sci. 1999, 55, 266−272. (14) Sidletskiy, O.; Kononets, V.; Lebbou, K.; Neicheva, S.; Voloshina, O.; Bondar, V.; Baumer, V.; Belikov, K.; Gektin, A.; Grinyov, B.; Joubert, M.-F. Structure and scintillation yield of Cedoped Al−Ga substituted yttrium garnet. Mater. Res. Bull. 2012, 47, 3249−3252. (15) Stanek, C. R.; Jiang, C.; Yadav, S. K.; McClellan, K. J.; Uberuaga, B. P.; Andersson, D. A.; Nikl, M. The effect of Ga-doping on the defect chemistry of RE3Al5O12 garnets. Phys. Status Solidi B 2013, 250, 244− 248. (16) Cherepy, N. J.; Kuntz, J. D.; Seeley, Z. M.; Fisher, S. E.; Drury, O. B.; Sturm, B. W.; Hurst, T. A.; Sanner, R. D.; Roberts, J. J.; Payne, S. A. Transparent ceramic scintillators for gamma spectroscopy and radiography. Proc. SPIE 2010, 78050I. (17) Kamada, K.; Yanagida, T.; Pejchal, J.; Nikl, M.; Endo, T.; Tsutumi, K.; Fujimoto, Y.; Fukabori, A.; Yoshikawa, A. Scintillatororiented combinatorial search in Ce-doped (Y,Gd)3(Ga,Al)5O12 multicomponent garnet compounds. J. Phys. D: Appl. Phys. 2011, 44, 505104. (18) Yanagida, T.; Kamada, K.; Fujimoto, Y.; Yagi, H.; Yanagitani, T. Comparative study of ceramic and single crystal Ce:GAGG scintillator. Opt. Mater. 2013, 35, 2480−2485. (19) Wu, Y.; Luo, Z.; Jiang, H.; Meng, F.; Koschan, M.; Melcher, C. L. Single crystal and optical ceramic multicomponent garnet scintillators: A comparative study. Nucl. Instrum. Methods Phys. Res., Sect. A 2015, 780, 45−50. (20) Tyagi, M.; Meng, F.; Koschan, M.; Donnald, S. B.; Rothfuss, H.; Melcher, C. L. Effect of codoping on scintillation and optical properties of a Ce-doped Gd3Ga3Al2O12 scintillator. J. Phys. D: Appl. Phys. 2013, 46, 475302. (21) Wu, Y.; Meng, F.; Li, Q.; Koschan, M.; Melcher, C. L. Role of Ce4+ in the scintillation mechanism of codoped Gd3Ga3Al2O12:Ce. Phys. Rev. Appl. 2014, 2, 044009. (22) Kamada, K.; Nikl, M.; Kurosawa, S.; Beitlerova, A.; Nagura, A.; Shoji, Y.; Pejchal, J.; Ohashi, Y.; Yokota, Y.; Yoshikawa, A. Alkali earth

curves 3; Table 3). Most probably, this TSL peak in the Y3Ga5O12:Ce crystal falls below 323 K, the lowest available temperature of TSL measurements in our setup.

6. CONCLUSIONS The 27Al, 71Ga, and 69Ga NMR study was performed in the multicomponent Y 3 Al 5−x Ga x O 12 :Ce garnet crystals and Lu3Al5−xGaxO12:Ce films. The Ga content x varied between 0 and 5 in studied crystals and between 0.3 and 2 in films to help determine the gallium and aluminum ion distribution over different available sites of the garnet hosts. We found that (i) gallium ions preferably occupy the tetrahedral sites of the garnet host as compared to larger octahedral sites whereas (ii) Al ions preferably localized at the octahedral sites despite having a smaller ionic radius with respect to that of Ga. In our opinion, the main reason for this peculiar cation distribution is stronger covalency of the Ga−O bond compared with the Al− O bond as well as the need for decreasing the cation−cation repulsive force as previously suggested.13 As a consequence, the nonmonotonic dependences of optical properties of Y3Al5−x GaxO12:Ce garnets on the Ga/Al content are observed due to the above-mentioned occupation site preferences. Specifically, we showed that the dependence of the main TSL peak position and corresponding depths of traps related to oxygen vacancies in Y3Al5−xGaxO12:Ce crystals on the Ga content x from intervals 0−2 and 2−5 can be presented by two lines with different slopes.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was performed in the framework of Polish NCN 2012/07/B/ST5/02376 and NANOLUX 2014 No 286 projects and Czech Science Foundation No. 16-15569S project. Partial support of the Ministry of Education and Science of Ukraine in the framework of SF-20 Fk and Fk 64/34 is also gratefully acknowledged. Thanks are due to M. Dusek for structural segment figure preparation.

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REFERENCES

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