Rare-earth antisites in lutetium aluminum garnets: Influence on lattice parameter and Ce3+ multicenter structure

Optical Materials xxx (2014) xxx–xxx

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Rare-earth antisites in lutetium aluminum garnets: Influence on lattice parameter and Ce3+ multicenter structure H. Przybylin´ska a, A. Wittlin a,b, Chong-Geng Ma c, M.G. Brik d, A. Kamin´ska a, P. Sybilski a, Yu. Zorenko e, M. Nikl f, V. Gorbenko e,g, A. Fedorov h, M. Kucˇera i, A. Suchocki a,e,⇑ a

Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warsaw, Poland ´ ski University in Warsaw, ul. Dewajtis 5, 01-815 Warsaw, Poland Cardinal Stefan Wyszyn College of Mathematics and Physics, Chongqing University of Posts and Telecommunications, Chongqing 400065, PR China d Institute of Physics, University of Tartu, Riia 142, Tartu 51014, Estonia e Institute of Physics, Kazimierz Wielki University, Weyssenhoffa 11, 85-072 Bydgoszcz, Poland f Institute of Physics AS CR, Cukrovarnicka 10, 16253 Prague, Czech Republic g Department of Electronics Ivan Franko National University of Lviv, 107 General Tarnavskyj Str., 79017 Lviv, Ukraine h Institute for Scintillation Materials of NAS of Ukraine (ISMA), Lenin avenue, 60, Kharkov 61158, Ukraine i Charles University, Faculty of Mathematics and Physics, Ke Karlovu 5, 12116 Prague 2, Czech Republic b c

a r t i c l e

i n f o

Article history: Received 28 December 2013 Received in revised form 6 April 2014 Accepted 15 April 2014 Available online xxxx Keywords: Garnets Scintillators Laser materials Phosphors

a b s t r a c t Low temperature, infrared transmission spectra of lutetium aluminum garnet (LuAG) bulk crystals and epitaxial layers doped with Ce are presented. In the region of intra-configurational 4f–4f transitions the spectra of the bulk LuAG crystal exhibit the signatures of several different Ce3+ related centers. Apart from the dominant center, associated with Ce substituting lutetium, at least six other centers are found, some of them attributed to so-called antisite locations of rare-earth ions in the garnet host, i.e., ions in the Al positions. X-ray diffraction data prove lattice expansion of bulk LuAG crystals due presence of rareearth antisites. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Lutetium aluminum garnets, with the chemical formula Lu3Al5O12 (LuAG), doped with Ce both in the form of single crystals and epitaxially grown layers belong to the class of most successfully applied scintillators [1,2], due to the high density of the matrix. They are also subject of numerous studies as candidates for solid state laser materials [3,4], and phosphors.[5–10] Ce3+ ions in LuAG form very efficient luminescence centers, originating from parity allowed inter-configurational 4f05d1 ? 4f15d0 transitions, with the relatively simple 4f15d0 ground state configuration. Trivalent rare-earths ions either as constituents of the host or as dopants are usually located in dodecahedral sites in the garnet structure. However, in Czochralski grown bulk crystals the so-called antisites are also found, i.e., rare-earth ions replacing aluminum in sites with octahedral symmetry [11]. The idea of antisite formation in garnets first was proposed on a basis of differences in ⇑ Corresponding author at: Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warsaw, Poland. Tel.: +48 228436861. E-mail address: [email protected] (A. Suchocki).

lattice parameters of Czochralski grown crystals and powders prepared by solid state reaction [12–14]. Later on, the problem of antisite defect formation in garnets was discussed thoroughly in Refs. [15–17], and [18], along with proposed models of such defects. In addition, other types of rare-earth and transition metal centers occur often in garnets [19,20]. In contrast, epitaxially grown garnet layers contain much less antisites of rare-earth ions [21,20]. This is due to the significantly lower (in the 950–1050 °C range) growth temperature of the epitaxial layers in comparison with their bulk counterpart (around 2000 °C). This is of importance, since antisite defects influence immensely the optical properties of scintillator materials, having sometimes detrimental effects on their characteristics, as, for example, slowing-down the scintillation response and decreasing the light yield. One of the methods used to improve the scintillation properties of LuAG crystals is either to avoid the formation of Ce multicenters by the use of low temperature grown epitaxial layers or by quenching their luminescence with use of band-gap engineering. The knowledge about the energy structure of the Ce multicenters and their nature in LuAG crystals and epitaxial layers may greatly help to improve the properties of LuAG as scintillator material and lead to its further development.

http://dx.doi.org/10.1016/j.optmat.2014.04.015 0925-3467/Ó 2014 Elsevier B.V. All rights reserved.

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´ ska et al. / Optical Materials xxx (2014) xxx–xxx H. Przybylin

Although LuAG:Ce is very important from the point of view of applications, up to now there is little information on the Ce multicenter structure in this material. It is known that certain antisite-type defects are formed there [22]. The confirmation of the existence of Ce3+ antisite centers was proven in Ref. [20] with use of EPR spectroscopy. In the latter work, apart from Ce3+ Lu ions in regular Lu positions, the Ce3+ Al antisite centers and two other types of Ce3+ Lu centers strongly interacting with LuAl antisite defects were found as well [20]. This work is devoted to low temperature far-infrared transmission studies of LuAG:Ce single crystals and epitaxial layers with the aim to study the origin of Ce multicenters The results are supported by X-ray diffraction (XRD), which confirm the RE-antisite presence in this material. Recently, we demonstrated such spectroscopic evidences for gadolinium gallium garnet bulk crystals [23], and also yttrium aluminum garnet bulk crystals and epitaxial layers [24]. 2. Samples and experimental procedures The LuAG:Ce bulk crystal used in our study, obtained from CRYTUR Company, was grown by the Czochralski method in a molybdenum crucible with use of 5 N raw materials. The Ce content in the crystal was about 0.09 at.%. LuAG:Ce single crystalline films (SCF) were grown by liquid phase epitaxy (LPE) in a platinum crucible from the melt solution, containing the PbO–B2O3 flux and Lu2O3, CeO2 and Al2O3 crystal-forming oxides of 5 N purity, on both sides of (1 1 0) oriented LuAG substrates [25,26]. For transmission studies samples with total thickness of around 127 lm were selected. The average Ce content in the layers was equal to about 0.2 at.%. The infrared absorption spectra were measured with a VERTEX 80v (Bruker) and a BOMEM DA3 Fourier-Transform Infrared (FTIR) spectrometers, with resolution of 1 cm1. For low temperature measurements the samples were placed into a cold finger of Oxford Instruments CF-102 continuous-flow cryostat equipped with KBr windows. The XRD investigations were applied for estimation of the structural disorder in LuAG single crystals, grown by the Czochralski method at a temperature of 2010 °C, and LuAG single crystalline layers, grown by the LPE method in the 1030–1050 °C temperature range from melt-solution based on the PbO–B2O3 flux onto LuAG substrates with (1 1 1) orientation. Due to low growth rate (0.15– 0.25 lm/min) and relatively high temperature range of LuAG SCF growth, we kept the concentration of Pb2+ related dopant in these SCF below 30 ppm. XRD measurements were performed using the DRON-4 double crystal spectrometer with Si (4 0 0) monochromator (Cu Ka radiation). 3. Experimental results and discussion The energy structure of Ce3+ ions with 4f1 configuration consists of a 2F5/2 ground state and a 2F7/2 excited state, which arise from the 2F term due to spin-orbit interaction. The crystal field (CF) splits these two states further into three (levels #1–3) and four (levels #4–7) energy levels, respectively. The absorption spectrum of the LuAG:Ce(0.09 at.%) bulk sample taken at 5 K in the region of the 4f–4f transitions is presented in Fig. 1. A spurious background was subtracted from the spectra. The four groups of lines in Fig. 1 are attributed to optical transitions between the lowest-lying level (#1) of the ground 2F5/2 state and four levels of the 2F7/2 excited state of Ce3+. Each group consists of several lines, which means that several Ce3+ – related center contribute to the spectrum. The lines around 2360 cm1, associated with transitions between levels #1 and #6 are partially overlapped, therefore the lines associated with various Ce3+ centers are

Fig. 1. Absorption spectrum of the bulk LuAG:Ce (0.09 at.%) sample taken at 5 K in the region of Ce3+ 4f–4f transitions. Transitions designations are marked on the graph.

not resolved for these transitions. The lines close to 3000 cm1 are associated with the hydroxyl group vibrations. Evident multicenter structure of Ce3+ dopant in LuAG can be the more easily observed in the region of the strongest absorption associated with the transitions from the levels #1 to levels #7 of various Ce3+-related centers, presented in Fig. 2. In addition to the major line with a peak at 3841 cm1, which we associate with the Ce3+ ions in regular Lu dodecahedral positions, at least six other lines are visible on both sides of this line. The three strongest lines occur at 3826, 3858, and 3889 cm1, the remaining ones are located at the wings of the dominating 3841 cm1 line. The total intensity ratio of these three lines to the ones grouped around 3841 cm1 is about 20%, which is a relatively large value. Full width at half of the maximum of the particular lines in the spectrum is between 4 and 6 cm1. At an elevated temperature of 105 K, there are additional lines in the transmission spectrum of the LuAG:Ce Czochralski grown crystal. These lines are associated with the population of the second levels of the 2F5/2 state and transitions from these levels to the different levels of the various Ce3+ centers. Fig. 3 shows these additional lines in the region of transitions terminating at level #7. Two partially overlapped lines can be distinguished around 3620 cm1. Assuming that the stronger of these lines, peaked at 3618 cm1, is associated with the major Ce3+ center, and the weaker one with a peak at 3631 cm1, associated with the second most abundant Ce3+ center, we obtain the separation between the two lowest levels of the ground 2F5/2 state as equal to 223 cm1 for the major Ce3+ center and 254 cm1 for the second most abundant Ce3+ center in LuAG. The low temperature absorption spectrum of the LuAG epitaxial layer is presented in Fig. 4. Three major lines associated with Ce3+ absorption are observed, related to optical transitions between level #1 and levels #4, #5, and #7. Comparison of this spectrum with that measured in the LuAG bulk crystal in the region of #1 ? #7 transitions (see Fig. 2) shows that though the additional lines around the major peak are still easily discernible their intensities, with reference to the major line at 3841 cm1, are much lower in the epitaxial layer than in the bulk crystal. This confirms that epitaxial grown layers contain much less Ce3+ multicenters, in particular the Ce antisites (Ce3+ Al ). Dominating lines associated with Ce3+ on Lu sites (Ce3+ Lu ) are visible, except of the low intensity line associated with transition between levels #1 and #6. The FWHM of the lines associated with transitions between levels #1 and #7 are similar in the epitaxial layer and in the bulk crystal. A large number of Ce3+ multisites is observed in examined crystals, with quite abundant concentrations as compared to the concentration of the major Ce3+ – related center. That observation remains valid, even taking into account that the absorption

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´ ska et al. / Optical Materials xxx (2014) xxx–xxx H. Przybylin

Fig. 2. Expanded view of the absorption spectra of the LuAG:Ce (0.09 at.%) bulk crystal (full line) and epitaxial layer (dashed line) at 5 K in the region of 1 ? 7 intrashell 4f–4f transitions of Ce3+ ions.

Fig. 3. Absorption spectrum of Ce-doped LuAG bulk crystal at 105 K in the region of intra-configurational 4f ? 4f transitions to level #7.

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This difference is so large, that a relatively large concentration of RE antisites can affect the lattice parameter of the garnet crystal containing such defects. In order to check this possibility, we performed precision XRD measurements of undoped LuAG single crystals, grown by Czochralski method, which are used as substrates for liquid phase epitaxy, and single crystalline layers grown by LPE. The results are presented in Fig. 5, which shows the XRD pattern (Cu Ka radiation) of the LuAG substrate (trace 1) and the epitaxial layer (trace 2). Two peaks Ka1 and Ka2 are related to the reflections from (1 0 0) plane of the LuAG lattice, with the total half-width of 0.05° for the substrate and 0.06° for the main reflection peak from the LuAG epitaxial layer. The XRD pattern of the (8 0 0) plane of LuAG epitaxial layer grown by LPE onto LuAG substrate shows that the film has exactly the same (1 0 0) crystalline orientation as the LuAG substrate but a notably smaller lattice constant. The misfit between the lattice constants of LuAG epitaxial 0 0 layer (11.9002 Å A and substrate (11.9062 Å A) is equal to 0.05%. We relate this difference in the lattice parameters to the different structural disorder of LuAG epitaxial grown layers and bulk single crystals. Namely, due to the formation of LuAl antisite defects at the high (2010 °C) growth temperature of LuAG bulk crystals, their real chemical formula can be described as Lu3LuxAl2xAl3O12, where x is the concentration of Lu cations in the octahedral sites of Al ions [12,27,14,28,29,15,16]. In contrast, due to the absence of LuAl antisite defects at the low (about 1000 °C) crystallization temperature of LuAG epitaxial layers [25,30,31], the LuAG SCF can be described by the stoichiometric formula Lu03Al5O12. The significantly large ionic radius of Lu3+ ions (0.861 Å A) replacing Al3+ 0 ions (of atomic radius equal to 0.535 Å A) [32] can, hence, lead to an expansion of the lattice in the presence of LuAl antisite defects. Recently, with use of spectroscopic methods, it was demonstrated that the concentration of LuAl antisite defects in the Lu3LuxAl2xAl3O12 single crystals can reach even the value x  0.12.[31,32] In this work we estimate the concentration of the LuAl antisite defects, based on the difference in the lattice constants of the LuAG epitaxial layer and the bulk crystal. We assume, that the lattice parameter of Lu3X2Al3O12 is linearly proportional to the ionic radius of the X ion (Vegard’s law), i.e. a = a0 + k.R, where R is the ionic radius of the X ion. Such a dependence is valid for the system of Gd 3X2Ga3O12 garnets (X = Ga, Sc, In), with k  1.56 [33]. Assuming the same k coefficient for the Lu3X2Al3O12 system, the lattice parameter of the Lu3X2Al3O12 (abbreviated as LuXAG) will be equal to: aLuXAG = aLuAG + kDR (Ns/N), where DR is the difference of ionic radii of the dopant and the host cation, and NS/N is the concentration ratio of the dopant and the host ion (aluminum ion in our case). Substituting the proper values to the last equation, we obtain the value of Ns/N = 0.0118. Thus, the concentration of the

Fig. 4. Absorption spectrum of the LuAG:Ce SCF sample taken at 5 K in the region of Ce3+ 4f–4f transitions. Large noise-like signal between 2800 and 3600 cm1 is associated with the very low reference signal in this spectral region, lines around 3000 cm1 are due to hydroxyl group vibrational levels.

intensity cannot be directly taken as a probe of concentration of the particular center, since their relative oscillator strengths are unknown. The lower growth temperature of the epitaxial layers reduces the number of antisite defects, but the other defects, forming some of the additional Ce3+ multicenters, still remain there. The plausible models of possible Ce-related defect centers were already proposed in a few publications [14,15,17,18]. However it is difficult to distinguish basing on the spectroscopic results, which particular lines can be assigned to the proposed model center. The Ce antisites (i.e. Ce in position of aluminum ions) are formed in the Czochralski grown crystals at a much higher concentration, in spite of the very different ionic radii of the Ce3+ and Al3+ ions. Also the atomic radius of Lu3+ is much larger than that of Al3+.

Fig. 5. XRD pattern of single crystal substrate (1) and LuAG epitaxial layer (2). The difference in the lattice constants between the epitaxial layer and single crystal 0 substrate, being equal to -0.006 A Å (misfit of 0.05%), is proportional to the concentration of LuAl antisite defects.

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´ ska et al. / Optical Materials xxx (2014) xxx–xxx H. Przybylin

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LuAl antisite defects in the bulk crystal can be estimated as 1.18% of the total content of cations in the octahedral sites of garnet lattice. Taking into account that one molecule of LuAG contains two Al3+ ions in octahedral sites, we can thus determine the fraction [ = 2*0.0118 = 0.0236 in the formula Lu3(LuxAl2x)Al3O12. This value is about 5 times lower than the fraction of LuAl antisites, x  0.12, estimated with use of spectroscopic methods [15–17]. However, the intensity of luminescence or absorption spectra without precise knowledge of the oscillator strengths of the appropriate optical transitions are not proper methods for estimation of the concentration of defects. Assuming, that the epitaxially grown layer is free from antisite defects, and further, that there is further no significant incorporation of the flux components used for LPE growth (such as Pb2+ with a large ionic radius) its lattice constant can be taken as standard for the stoichiometric Lu3Al5O12. We obtain then a0 simple, approximate relation for the lattice parameter a (in Å A) of Lu3(LuxAl2x) Al3O12 as a = 11.9002 + 0.2542x. It has been confirmed by absorption measurements in the UV spectral region that indeed only a very small amount of Pb ions is incorporated into the lattice of the epitaxial layer and should therefore not lead to lattice expansion. The presence of such unintentional dopants at a significant concentration would result in underestimation of the antisite defect concentration in the bulk crystals. It is partly possible to identify the nature of each Ce3+ luminescence center from the presented spectroscopic measurements. The dominant center, with the highest absorption intensity is obviously associated with Ce3+ ions in dodecahedral sites, substituting lutetium ions. We associate some of the other centers, observed both in the bulk crystals and epitaxial layers, but at a much lower concentration in the latter, with Ce3+ ions in octahedral (antisite) positions and with Ce3+ ions in dodecahedral sites in the close vicinity of lutetium antisites [20]. Thus the lines with the peaks at 3826, 3858, and 3889 cm1 for the transitions terminating at level #7 are related to the various antisite defects in LuAG. The remaining Ce3+ related centers are most probably related to the presence of some other defects close to Ce3+ in dodecahedral sites, which exist both in the single crystals as well as in the epitaxial layers, as for example, OH groups, which were detected by us in the IR spectra. 4. The crystal field theoretical analysis The calculation of the energies of the Ce3+ 4f levels and crystal field parameters (CFP) for the dodecahedral site with D2 symmetry in LuAG were performed in the framework of the exchange charge model (ECM) [34]. Details of the calculation procedure can be found in Ref. [23]. Using the standard notation of Wybourne [35] and Table 1.7 of Ref. [36] the parameterized effective Hamiltonians for the 4f1 configuration of Ce3+ ions in LuAG can be written as:

Table 1 The 4f CF energy levels of Ce3+ ions doped in LuAG (data given in cm1). Level No.

2

I.R.

Ecalc

1 2 3 4 5 6 7

2

C6 C7 C7 C6 C6 C7 C7

0 221 770 2091 2267 2430 3836

LJ F5/2

2

F7/2

Table 2 The energy parameters of Ce3+ ions doped in LuAG in the framework of ECM (unit: cm1). Refer to the text for their explanations. pc

ec

corr

Total

B20

464

198

696

34

B22

243

73

365

195

B40

336

347

12

B42

1854

851

2705

zB44

997

858

1855

B60

1030

690

1720

B62

379

206

585

B64

422

432

854

B66 G(O1) G(O2)

354

502

a Eavg f

856 1.87 3.99 1.50 1660 602

Notes: The abbreviations ‘‘pc’’, ‘‘ec’’ and ‘‘corr’’ stand for the CFP contributions from point charges, exchange charges and the correction due to other factors, such as dipoles, respectively, and then ‘‘total’’ represents their sum.

  ð2Þ ð2Þ ð2Þ ð4Þ Hð4f 1 Þ ¼ Eav g þ f4f sf  lf þ B20 C 0 þ B22 C 2 þ C 2 þ B40 C 0     ð4Þ ð4Þ ð4Þ ð4Þ ð6Þ þ B42 C 2 þ C 2 þ B44 C 4 þ C 4 þ B60 C 0     ð6Þ ð6Þ ð6Þ ð6Þ þ B62 C 2 þ C 2 þ B64 C 4 þ C 4

ð1Þ

where the notation and meanings of various operators and parameters are defined according to the standard practice [37,38]. The resulting calculated energies of the 4f levels of Ce3+ in dodecahedral sites in LuAG are also listed in Table 1, the appropriate parameters of the theoretical fit to the experimental data are given in Table 2. An excellent agreement between the experimentally observed and theoretically calculated energy levels has been achieved for the dominating Ce3+ Lu center. 5. Conclusions Low temperature infrared absorption of LuAG:Ce3+ bulk crystals grown by the Czochralski method shows evidence of several multicenter incorporation of this dopant in the bulk crystals. In contrast, the Ce3+ multicenter formation in the epitaxial grown LuAG layers is considerably reduced, due to the much lower growth temperature. A comparison of the spectra of the bulk crystals and epitaxial layers allows us to identify some antisite defects occurring in the former. The theoretical crystal field calculations based on exchange-charge model describe very well the experimental data for the major Ce3+ Lu center. The antisite formation in this garnet increases significantly the lattice parameter of the bulk crystals. The concentration of the antisite defects x in the in the formula of Lu3(LuxAl2x)Al3O12 bulk crystals estimated on this basis is equal to about 0.0236, e. g., 1.2% of the total content of Al3+ ions in the octahedral sites of garnet lattice.

Eexp 0 223 2120 2275 2372 3841

Notes: All CF energy states have the same C5 representations in D2 symmetry. The symbol ‘‘I.R.’’ stands for the double-value irreducible representation of the parent group of D2 (i.e., D2d point-group). Ecalc and Eexp correspond to the calculated and observed CF energy level values, respectively.

Acknowledgements The cooperation program between Estonian and Polish Academies of Sciences for the years 2013–2015 is kindly acknowledged. This work was partially supported by the European Union within the European Regional Development Fund through the Innovative Economy grant MIME (POIG.01.01.02-00-108/09), Polish National Science Center (project No 2012/07/B/ST5/02376), and Czech Science Foundation project P204/12/0805. Thanks are due to K. Nejezchleb from CRYTUR for providing us the bulk LuAG:Ce crystal

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for this study. MGB and MN acknowledge Marie Curie Initial Training Network LUMINET, grant agreement no. 316906.

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