Disorder response of 3d3 ions zero-phonon lines in the luminescence spectra of Yttrium-Aluminum-Gallium garnet solid solution ceramics

Author’s Accepted Manuscript Disorder response of 3d3 ions zero-phonon lines in the luminescence spectra of Yttrium-AluminumGallium garnet solid solution ceramics S.P. Feofilov, A.B. Kulinkin, P.A. Rodnyi, V.M. Khanin, A. Meijerink www.elsevier.com/locate/jlumin PII: DOI: Reference: S0022-2313(18)30349-1 https://doi.org/10.1016/j.jlumin.2018.04.017 LUMIN15532 To appear in: Journal of Luminescence Received date: 21 February 2018 Revised date: 6 April 2018 Accepted date: 9 April 2018 Cite this article as: S.P. Feofilov, A.B. Kulinkin, P.A. Rodnyi, V.M. Khanin and A. Meijerink, Disorder response of 3d3 ions zero-phonon lines in the luminescence spectra of Yttrium-Aluminum-Gallium garnet solid solution ceramics, Journal of Luminescence, https://doi.org/10.1016/j.jlumin.2018.04.017 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. 1 Disorder response of 3d3 ions zero-phonon lines in the luminescence spectra of YttriumAluminum-Gallium garnet solid solution ceramics S.P. Feofilova,*, A.B. Kulinkina, P.A. Rodnyib, V.M. Khaninc, A. Meijerinkc a Ioffe Institute, St. Petersburg, 194021, Russia b Peter the Great St.Petersburg Polytechnic University, St. Petersburg, 195251, Russia c Utrecht University, Utrecht, 3508 TC, The Netherlands Keywords: solid solution garnet ceramics, impurity ions, zero-phonon lines, fluorescence spectroscopy *Corresponding author: email: sergey.feofilov@mail.ioffe.ru, phone: +7(812)2978188 Abstract The zero-phonon R-line luminescence spectra of 3d3 ions (Cr3+ and Mn4+) in a series of Y3Al5-5yGa5yO12 (0≤y≤1) solid solution garnet ceramics were studied. Instead of increased inhomogeneous line broadening which is usually observed in solid solutions, discrete structure in the Cr3+ emission spectra is observed at lower values of y, whereas at high values of y the strong inhomogeneous broadening of the R-lines occurs and no discrete disorder response is observed. The results are explained based on the limited number of possible high-symmetry luminescent center geometries and the non-random preferential occupancy of tetrahedral lattice sites with gallium ions. 1. Introduction The solid solution (mixed crystal) insulating crystals and ceramics doped with rare-earth and transition metal ions attract significant attention in optical spectroscopy studies. Solid solutions are especially interesting from the point of view of potential applications as scintillators, phosphors, laser materials and spectral hole-burning media because they allow variation of the physical properties of the material by variation of the composition. The structural disorder occurs in solid solutions due to the random distribution of ions in the crystal lattice sites. The inhomogeneous, i.e. related to the variations within the ensemble, broadening of electronic 2 transitions (zero-phonon lines, ZPL) of impurity ions in solid solutions is, usually, significantly larger than that in the pure ordered crystals. This broadening was reported in literature [1,2] and is ascribed to the shift of the electronic energy levels in the random crystal field, connected to irregular structure of impurity ions surroundings. The inhomogeneous linewidths for impurity ions in solid solutions are usually similar by value to that observed in glasses of similar composition. In our previous studies it was observed [3,4] that (in contrast to most studied materials) the modification of zero-phonon R-lines (2E–4A2) in the luminescence spectra of Cr3+ impurity ions in the concentration series of Lu3xY3-3xAl5O12 (0≤x≤1) and Tb3zY3-3zAl5O12 (0≤z≤1) garnet crystals occurs in a discrete rather than a continuous fashion and is not accompanied by strong inhomogeneous broadening. The effect was ascribed to high C3i symmetry of octahedral Cr3+Al sites that allows only a limited number of non-equivalent Cr3+ centers in mixed environment of dodecahedral D2 sites randomly occupied with ions of different size when the disorder is introduced into the Y/Lu/Tb sublattice. The observed dependences of energies and radiative lifetimes of 2E states of locally identical (i.e. having identical configuration of nearest Y/Lu/Tb cations) Cr3+ centers inside different LuYAG/TbYAG hosts on Lu content x and Tb content z were explained by lattice compression and dilation occurring according to Vegard’s law. The purpose of the study presented here is to trace the modification of the zero-phonon Rline spectrum of garnet solid solutions in a more complicated situation of the disorder introduced into the sublattices of tetrahedral and octahedral Al/Ga sites. We report the results of the luminescence spectroscopy studies of 3d3 ions (Cr3+ and Mn4+) zero-phonon lines in Y3Al55yGa5yO12 solid solution garnet ceramics. 2. Experiment The experiments were performed with a series of solid solution Y3Al5-5yGa5yO12 garnet ceramics (0≤y≤1). The garnet ceramics samples were prepared at Philips Research Eindhoven by sintering a mixture of base oxides of 4N-purity in air atmosphere in the form of pills of 14 mm diameter and 1-2 mm thickness. Based on the X-ray diffraction patterns it was concluded that all samples consist of a single garnet phase. 3 Cr3+ luminescence spectra were recorded taking advantage of the trace amounts of Cr present in the samples that were not intentionally doped with chromium. These samples were doped with 0.2 atomic % of cerium (they were originally synthesized for scintillator studies). Mn4+ luminescence spectra were obtained with the samples doped with 0.1 mol. % Mn without charge compensation; in these samples the majority of Mn dopant ions were in the 3+ valence state. For optical measurements the samples were placed on the cold finger (T=10 K) of a liquid helium refrigerator or immersed in liquid nitrogen (T=77 K). The Cr3+ and Mn4+ luminescence was excited with diode-pumped solid state (DPSS) lasers operating at 543 or 473 nm respectively. The luminescence was detected with a double-grating monochromator (0.01 nm resolution) and a photomultiplier operating in a photon-counting mode. 3. Experimental results The luminescence R-line (2E–4A2) spectra of Cr3+ ions for the series of Y3Al5-5yGa5yO12 solid solution garnet ceramics excited at exc=543 nm via the 4T2 absorption band are shown in Fig. 1 (T=10 and 77 K). At low temperatures the exc=543 nm light does not excite Ce3+ ions even taking into account the effect of Ce3+ excitation at energies below the zero-phonon line [5], so only the Cr3+ ions luminescence is observed. Fig.1 The spectra of the ceramics samples with y=0 and y=1 correspond to the spectra of YAG:Cr [6,7] and YGG:Cr [6,8,9] respectively. The narrow R-line (2E–4A2) emission is observed around 690 nm and is split in a doublet (R1 and R2 line) due to a splitting of the 2E excited state. The spectra observed are similar to those in single-component garnet crystals. The additional weak lines observed close to the R-lines in YAG:Cr and YGG:Cr are due to the phenomenon of inversion between Y3+ and Al3+/Ga3+ cations in dodecahedral and octahedral sites of the garnet lattice [10]. This inversion produces local distortions of the crystalline lattice that could perturb the ideal structure of the neighbouring cationic sites. The behavior of the Cr3+ spectra in Y3Al5-5yGa5yO12 samples with the change of y at its lower values is similar to that reported in [3] for Lu3xY3-3xAl5O12: a discrete structure is observed when the smaller aluminum ions are one by one replaced with larger gallium ones. At T=10 K the luminescence lines correspond mostly to the lower-energy (R1) components of the doublet 2E– 4 4 A2 transitions, though some contribution of partially occupied higher-energy (R2) transition is still present, at T=77 K the contribution of R2 transitions becomes more significant due to increased thermal (Boltzmann) population. The similarity between the Cr3+ R-line spectra of Y3Al5-5yGa5yO12 at lower values of y (y=0.2) and the discrete spectrum of Lu3xY3-3xAl5O12 (x=0.3) is shown in Fig.2; note the different (top and bottom) wavelength scales for the two spectra. A good correspondence of the two spectra is achieved when the wavelength scales are not only shifted, but the scale for Lu3xY33xAl5O12 is expanded by a factor of 1.43. This is a manifestation of larger relative difference between the ionic radii of tetrahedral Ga and Al (0.47 Å and 0.39 Å) and between dodecahedral Y and Lu (1.019 Å and 0.977 Å) whereas the distances between the Cr3+ ion on a octahedral site and the tetrahedral Ga/Al and dodecahedral Y/Lu ions are the same. As a result, replacement of a nearby tetrahedral Al by Ga causes a larger shift in R-line energy than the replacement of nearby dodecahedral Y by Lu (12.3 cm-1 vs. 8.6 cm-1). Fig.2 At high values of y the behavior of the Cr3+ spectra in Y3Al5-5yGa5yO12 samples with the change of y is different from the discrete spectra observed at (y=0.2) and from the discrete spectra of the Lu3xY3-3xAl5O12 series [3]. The strong inhomogeneous broadening of the R-lines occurs and no discrete disorder response is observed. Mn4+ ions have the same 3d3 electronic configuration as Cr3+, so it is interesting to compare the R-line (2E–4A2) spectra of Mn4+ and Cr3+ ions in the series of Y3Al5-5yGa5yO12 solid solution garnet ceramics. Though in the Mn-doped ceramics samples the luminescence spectra were dominated by the broadband Mn3+ luminescence [11], under the exc=473 nm excitation it was still possible to detect Mn4+ R-line spectra which are shown in Fig.3. The spectrum of the ceramics sample with y=0 corresponds to the spectrum of YAG:Mn4+ [12,13] with much larger (60 cm-1) R1-R2 splitting than that for Cr3+ (19 cm-1), so the sets of R1 and of R2 lines do not overlap in the spectra. The behavior of the Mn4+ spectra with changing y is somewhat similar to that of Cr3+ ones: some structure is visible at y=0.3 whereas at y=0.7 the strong inhomogeneous broadening of the R-lines obscures any discrete structure. The additional complication of the spectra is introduced by the significant contribution of Mn4+ multisites [13] that are probably due to local charge compensation of Mn4+. Nevertheless the conclusion about different behavior of the spectra with changing y at low and high y may be done. 5 Fig.3 4. Discussion In [3] the discrete modification of R-line spectra of Cr3+ in LuYAG crystals with the change of Lu/Y ratio was explained by the high symmetry of Cr3+ center in the crystal lattice. In the cubic Oh garnet lattice Cr3+ ions replace the Al3+ ions in the octahedral sites of a C3i symmetry and are surrounded by six oxygen ions [3]. In the next cation coordination sphere the six Lu/Y ions nearest to the Cr3+Al site also form an octahedron with C3i symmetry with a Cr3+Al ion in its center [6,7]. It follows from the C3i symmetry (including trigonal axis and inversion) that all six Lu/Y positions are equivalent in relation to Cr3+Al. For the octahedrally-coordinated Cr3+Al site in Y3Al5-5yGa5yO12 solid solution the nearest randomly occupied Al/Ga sites are tetrahedral and form an octahedron with C3i symmetry around this site – see Fig.4. Thus in full analogy with the consideration for LuYAG [3] it can be concluded that in Y3Al5-5yGa5yO12 there is a limited number of different surroundings of a Cr3+Al center by tetrahedrally-coordinated Al/Ga ions. We will label the Cr3+ centers surrounded with m gallium and n aluminum ions as Cr3+(mGanAl) centers (m+n=6). In pure YAG all six nearest tetrahedrally-coordinated cations are aluminum ions (Cr3+(6Al) center); the replacement of Al3+ ions one by one with Ga3+ in solid solutions produces a few types of Cr3+ centers: one Cr3+(1Ga5Al), three non-equivalent Cr3+(2Ga4Al) centers, three Cr3+(3Ga3Al), three Cr3+(4Ga2Al), one Cr3+(5Ga1Al), and one Cr3+(6Ga). The total number of possible non-equivalent Cr3+ centers is 13. Each additional larger Ga ions introduced into the Cr3+ surrounding shifts the R line towards longer wavelengths. The statistics of occurrence of different Cr3+(mGanAl) centers determines the shape of the R-line spectrum at different Ga content y. The octahedrally-coordinated Al/Ga ions are about 1.5 times further from the Cr3+Al center than the tetrahedrally-coordinated ones, so the influence of the disorder in the octahedrons sublattice on the Cr3+ R-line spectra is expected to be significantly less. Fig.4 The above considerations explain the discrete behavior of the Cr3+ R-line spectra of Y3Al55yGa5yO12 at lower values of y as well as the similarity of the spectrum to that of LuYAG crystal clearly visible in Fig.2. Indeed, the replacement of smaller Al ions with larger Ga ions in the six tetrahedral cation sites around Cr3+ yields the qualitatively same effect as the replacement of smaller Lu ions with larger Y ions in the six surrounding dodecahedral cation sites. 6 The different behavior of the spectra at higher values of y (Fig.1) requires different explanation. Indeed, in Lu3xY3-3xAl5O12 crystals both the replacement of a fraction of (smaller) lutetium ions with (larger) yttrium ones in LuAG and vice versa in YAG result it the clear discrete series of R-lines (Fig.2 in [3]). Similar behavior should be expected for Y3Al5-5yGa5yO12 while replacing a fraction of (smaller) aluminum ions with (larger) gallium ones in YAG and vice versa in YGG assuming random distribution of Al and Ga ions in the tetrahedral sites sublattice. Contrary to such expectation, replacement of a fraction of gallium ions in YGG with Al ions results in inhomogeneous broadening of the R-lines without any discrete disorder response – Fig.1. Also, the overall extent of the broadening (spectral width) is smaller, which can be observed by comparing the spectra for y =0.2 and y = 0.8 in Fig. 1. Such R-lines behavior suggests non-random distribution of Al and Ga ions in the tetrahedral sites sublattice. The observations can be explained when it is assumed that at high values of y Al ions do not occupy tetrahedral sites statistically, but preferentially replace Ga ions in octahedral sites. This preferential replacement of gallium ions in YGG with Al ions in octahedral sites, retains the purely “tetrahedral Ga” surroundings of the Cr3+Al luminescent probe ions up to significant values of Al content (1-y). As a result, the R- lines of Cr3+(6Ga) centers are observed in a wide range of the values of y. The observed inhomogeneous broadening of the Cr3+(6Ga) R- lines of may be ascribed to the disorder in the octahedral Al/Ga sublattice: the octahedral Al/Ga ions are located about 1.5 times further form a given octahedral Cr3+(6Ga) center than the tetrahedral ones and provide more possible configurations. As a result of weaker influence of ions substitution in octahedral positions and of more numerous different R-line positions due to disorder in these positions the structureless broadening occurs and the overall spectral broadening is less. The preferential occupation of tetrahedral sites by Ga and of octahedral sites by Al also clearly follows from the spectra presented in Fig.2. The spectrum of Y3Al5-5yGa5yO12 at low value of larger-size Ga ions content y=0.2 is similar to the spectrum of Lu3xY3-3xAl5O12 with larger-size Y ions content (1-x)=0.7. As soon as the distribution of Lu/Y ions in LuYAG is supposed to be random, the preferential (non-random) occupation of tetrahedral sites in Y3Al55yGa5yO12 with Ga ions is necessary to explain the discrepancy in larger ions content which accounts for the spectra compared in Fig.2. 7 The surprising conclusion about the preferential occupation of the smaller tetrahedral sites by larger Ga ions and of larger octahedral sites by smaller Al ions is in full agreement with the results of x-ray diffraction [14,15] and NMR [16] studies, where the same conclusion was made. The effect is explained [15] by the greater covalency of the Ga-O bonds than that of the Al-O bonds. It should be noted that the preferential occupancy of tetrahedral sites with Ga ions may be different for the tetrahedra surrounding the octahedral Al3+ site and the octahedral Cr3+Al center. The stronger preference in occupation of tetrahedral sites with Ga ions around the octahedral Cr3+Al center may further contribute to the “asymmetry” of the R-line spectra response to the variations of Ga content at high and low values of y. In Fig.1 it may be seen that the shift of the inhomogeneously-broadened R-lines of Cr3+(6Ga) centers with the decrease of y at high Ga content occurs in the shorter-wavelength direction. In contrast, the shift of the R-lines of locally-identical Cr3+ centers observed in [3,4] when the larger ions were replaced by smaller ones occurred in longer-wavelength direction (opposite to the shift of the whole R-line set barycenter) and was explained by the whole lattice compression according to Vegard’s law [3,4,17]. This effect may be ascribed to the peculiarities of lattice distortions at intermediate distances in conditions when significant deviations from Vegard’s law occur [15]. The results obtained for the Mn4+ ions (Fig.3) are qualitatively similar to that for Cr3+ ones and thus further confirm the suggested interpretation. 5. Conclusions In Y3Al5-5yGa5yO12 garnet solid solutions disorder is introduced into both sublattices of tetrahedral and octahedral Al/Ga sites. It was observed experimentally that the modification of zero-phonon R-lines spectra of Cr3+ impurity ions in the concentration series of Y3Al5-5yGa5yO12 (0≤y≤1) solid solution ceramics occurs differently at high Al and high Ga content. Discrete Cr3+ R-line spectra are observed at lower values of y whereas at high values of y inhomogeneous broadening of the R-lines occurs and no discrete disorder response is visible. The effect may be explained by non-random distribution of Al and Ga ions in the tetrahedral sites sublattice, namely, the preferential occupation of tetrahedral sites with Ga ions. It may be concluded that luminescence spectroscopy of probe Cr3+ ions in solid solutions enables to detect the variations in the occupancy of sublattices by different ions and to reveal their non-random distribution. The 8 R-line spectra of Mn4+ impurity ions in Y3Al5-5yGa5yO12 show the same difference in behavior with changing y at its low and high values and thus confirm the conclusion about non-random distribution of Al and Ga ions between the tetrahedral and octahedral sites sublattices. 9 References [1] G.P. Morgan, T.J. Glynn, G.F. Imbusch, J.P. Remeika, Luminescence from Al2xGa2(1x)O3:Cr 3+ , J. Chem. Phys. 69 (1978) 4859–4866. [2] B.M. Tissue, N.J. Cockroft, L. Lu, D.C. Nguyen, W.M. Yen, Comparison of the spectra and dynamics of Er3+:Y2-xScxO3 (x= 0, 1, 2), J. Lumin. 48–49 (1991) 477–480. [3] S. Feofilov, A. Kulinkin, K. Ovanesyan, A. Petrosyan, C. Dujardin, Anomalous discrete disorder response of high-symmetry impurity centers spectra in garnet solid solutions, Phys. Chem. 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Yamanaka, Cation distribution and crystal chemistry of Y3Al5-xGaxO12 (0≤x≤5) garnet solid solutions, Acta Crystallogr. B 55 (1999) 266–272. [16] V. Laguta, Y. Zorenko, V. Gorbenko, A. Iskaliyeva, Y. Zagorodniy, O. Sidletskiy, P. Bilski, A. Twardak, M. Nikl, Aluminum and gallium substitution in Yttrium and Lutetium Aluminum−Gallium Garnets: Investigation by Single-Crystal NMR and TSL methods, J. Phys. Chem. C 120 (2016) 24400−24408. [17] P.R. Wamsley, K.L. Bray, The effect of pressure on the luminescence of Cr3+:YAG, J. Lumin. 59 (1994) 11–17. 11 Figure captions Fig. 1. Fluorescence spectra of Cr3+ ions in Y3Al5-yGayO12 ceramics samples. exc=543 nm, thick lines: T=10 K, thin lines: T=77 K. T=10 & 77 K exc=543 nm 3+ Cr Y3Al5-5yGa5yO12 y=1 y=0.8 y=0.6 y=0.4 y=0.2 y=0 684 686 688 690 wavelength (nm) 692 12 Fig. 2. Comparison of the R-line fluorescence spectra of Cr3+ ions in Y3Al5-yGayO12 (y=0.2) ceramics and Lu3xY3-3xAl5O12 (x=0.3) crystal [3]. T=10 K. Top and bottom wavelength axes are different for the two spectra (correspondence indicated by arrows). 683 Cr 684 685 686 687 3+ Y3GaAl4O12 (y=0.2) 688 689 T=10 K exc=543 nm Lu0.9Y2.1Al5O12 (x=0.3) 683 684 685 686 687 688 689 690 691 692 693 wavelength (nm) Fig. 3. Fluorescence spectra of Mn4+ ions in Y3Al5-yGayO12 ceramics samples. exc=473 nm, T= 77 K. 13 4+ T=77 K exc=473 nm Mn Y3Al5-5yGa5yO12 y=1 y=0.7 R2 R1 y=0.3 y=0 648 650 652 654 656 wavelength (nm) 658 660 14 Fig.4. Al/Ga tetrahedra environment of an octahedral Cr3+Al center with C3i symmetry in the garnet lattice. One of three possible Cr3+(2Ga4Al) centers is shown. C3 is the trigonal symmetry axis. For clarity oxygen ions are shown as small spheres and dodecahedral Y sites are omitted. Al3+ Ga3+ Cr3+ Ga3+ Al3+ Fig.4. Al3+ C3 axis Al 3+