ISSN 1064-2269, Journal of Communications Technology and Electronics, 2017, Vol. 62, No. 2, pp. 175–177. © Pleiades Publishing, Inc., 2017. Original Russian Text © L.N. Alyabyeva, V.I. Burkov, V.A. Kotov, 2017, published in Radiotekhnika i Elektronika, 2017, Vol. 62, No. 2, pp. 175–178.
RADIO PHENOMENA IN SOLIDS AND PLASMA
Luminescence of Disordered Crystals with Langasite Structure Doped with Chromium Ions L. N. Alyabyevaa, V. I. Burkova, and V. A. Kotovb, * a
Moscow Institute of Physics and Technology (State University), Dolgoprudnyi, Moscow oblast, 141700 Russia Institute of Radio Engineering and Electronics, Russian Academy of Sciences, Moscow, 125009 Russia *e-mail:
[email protected]
bKotel’nikov
Received December 7, 2015
Abstract—The luminescence, absorption and circular dichroism spectra of langasite family crystals La3Ga5SiO14 and Sr3Ga2Ge4O14 doped with chromium ions (0.1 at %) are studied in the region of 300–1200 nm. It is demonstrated that these spectra contain bands typical of both a Cr3+ ion in an octahedral environment (position 1a) and a Cr4+ ion in a tetrahedral site (position 2d). The intensities of these bands differ considerably. DOI: 10.1134/S1064226917020012
INTRODUCTION The absorption spectra in polarized light, the luminescence spectra, and the luminescence quenching times of La3Ga5SiO14 (LGS), Ca3Ga2Ge4O14 (CGG), Sr3Ga2Ge4O14 (SGG), La3Ta0.5Ga5.5O14 (LGT), and La3Nb0.5Ga5.5O14 (LGN) crystals doped with chromium ions have been studied in detail in [1–3]. It has been affirmed that luminescence of these crystals is defined by both R-lines (Cr3+) and a broad band (transition 4T2g → 4A2g) of the Сr3+ ion in an octahedral site. However, the results of investigation of absorption spectra (most notably, circular dichroism (CD) spectra) have shown that chromium ions are present in these crystals both in the oxidation state (+3) (Cr3+ in octahedral position 1а) and in the oxidation state (+4) (Cr4+ in tetrahedral position 2d [4]). If this is the case, transitions of these ions should manifest themselves in the luminescence spectra. In this study, we analyzed absorption, circular dichroism, and luminescence spectra of La3Ga5SiO14:Cr and Sr3Ga2Ge4O14:Cr crystals with disordered structure of calcium gallogermanate (CGG). The choice of the studied objects is justified by the appearance of characteristic features typical of the presence of ion Cr4+ in absorption and CD spectra. 1. SAMPLES AND THE MEASUREMENT PROCEDURE The La3Ga5SiO14 (LGS) and Sr3Ga2Ge4O14 (SGG) crystals doped with chromium ions (0.1 at %) were grown by the Czochralski method. The LCM-S111, YAG:Nd (λ = 532 nm, Р = 55 mW) and helium– neon (λ = 632.8 nm, Р = 10 mW) lasers were used to
excite luminescence. The structure of crystals was described in detail in [5]. The luminescence spectra were measured using a setup with an MDR-23 monochromator. The absorption spectra were obtained with the use of a Hitachi330 spectrophotometer. The circular dichroism spectra were recorder with the help of a Jobin-Yvon Mark III dichroism spectrometer. A closed cycle cryostat Janis CCS-150 was used for lowtemperature measurements. 2. RESULTS AND DISCUSSION Figure 1 shows the luminescence spectra of chromium-doped LGS and SGG crystals. R-lines, which are characteristic of a Cr3+ ion in an octahedral environment, are clearly seen in both spectra in the region of 700 nm. The maxima of these lines coincide with bends at the long-wavelength wing of the CD band. A distinct broad band of a considerable intensity is also seen in the luminescence spectrum of the LGS crystal in the region of 900–1200 nm. It can be seen from the results presented in Fig. 2 that R-lines (characteristic of Cr3+ ions in octahedral sites) in the form of bends at the long-wavelength wing of the intense CD band are present in the circular dichroism spectra of both crystals. Since the band in the region of 900–1200 nm in the luminescence spectrum may hardly be attributed to a Cr3+ ion in octahedral oxygen coordination, it was assumed that another luminescence center should be present in the crystal. Analysis of the chemical composition of langasite crystals doped with chromium ions allows us to assume that this band in the near IR region is the manifestation of electron transitions of chromium ions in some other valence state, which is different from the
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ALYABYEVA et al. 2T , 2E 1g g
(а)
→ 4A2g
6.4
Cr3+
1
5.6
1
3A 1
1
→ 3T1(3F) Cr4+
2 3T (F) 2
εc, cm–1
I, arb. units
4.8
→ 3A2
Cr4+
4.0 3.2 2.4 1.6 0.8
2 → 4T1g Cr3+
4A 2g
0 700
800
900 λ, nm
1000
1100
350 400 450 500 550 600 650 700 750 λ, nm (b)
Fig. 1. Band positions in luminescence spectra of (1) LGS:Cr and (2) SGG:Cr crystals.
2
3+ state. It was concluded in [4] that the CD and absorption spectra of crystals listed above are consistent with the presence of not only Cr3+ ions, but also ions of chromium in the oxidation state 4+ in a tetrahedral environment in these crystals. Therefore, it seems natural to associate the long-wavelength luminescence band with Cr4+ ions. The +4 oxidation state of a chromium ion is quite unstable. However, crystalline compounds Cr(0tBu)4 and Cr(OCHBu t2 ) 4
[6, 7] with Сr ions in the oxidation state of +4 are known to exist. The spectroscopic properties of a Cr4+ ion in forsterite (Mg2SiO4) and Ca2GeO4 crystals have already been examined [8–10]. The chromium Сr4+ ions occupy tetrahedral sites in garnet crystals [11–19] and heavily distorted tetrahedral sites in Mg2SiO4 (forsterite) and Ca2GeO4 crystals [9, 10]. The CD and absorption spectra of a chromium-doped LaBGeO5 crystal were investigated in [20]. It has been demonstrated that chromium ions in the oxidation state (4+) occupy tetrahedral in the lattice sites whose local symmetry is lowered to С1. At the same time, the majority of studies [11–18] were application-oriented; specifically, it was stated in [11, 12] that garnet crystals doped with a small amount (0.05–0.1 at %) of chromium ions in the oxidation state (+4) (Y3Al5O12:Cr4+) may be used as passive Q-switches for Y3Al5O12:Nd3+ lasers. When garnet crystals doped with Cr4+ were grown, Mg2+ or Ca2+ ions (0.01–0.9 at %) were introduced into the mixture for charge compensation. Passive Q-switches based on Y3Al5O12:Cr4+ crystals are now available commercially [21]. It has also been demonstrated in the last two decades that a Cr4+ ion in a tetrahedral site is a prom-
Δεc, cm–1
0.04 0.03
3A 1
→ 3T1(3F) Cr4+
0.02 0.01
→ 4T2g Cr3+
4A 2g
1
0 350 400 450 500 550 600 650 700 750 λ, nm Fig. 2. The (a) absorption and (b) circular dichroism spectra of the (1) LGS:Cr and (2) SGG:Cr crystals.
ising activator for tunable solid-state lasers in the nearIR region (1.1–2.0 μm) [22]. The band in the luminescence spectrum and the laser emission band are tied under the Тd symmetry to the transition from excited state 3Т2 to ground state 3А , which is allowed in the magnetic dipole approxi2 mation. The broad luminescence band of crystals doped with Cr4+ is attributed to the interaction of electron states with vibrations of the lattice. According to the literature, lasing has already been achieved in Y3Al5O12:Cr4+ [23, 24], Y2SiO5:Cr4+ [25], Y3ScxAl5 – xO12:Cr4+ [26], and Mg2SiO4:Cr4+ [27–31] crystals.
The absorption and luminescence spectra in [1–3] are almost the same as the spectra obtained in this study, but authors declined any possibility that chromium ions in any oxidation state other than +3 are present in langasite crystals. However, the results presented in [4, 8, 9], where the presence of Cr4+ in crystals was proven reliably, point to the possibility of this phe-
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LUMINESCENCE OF DISORDERED CRYSTALS WITH LANGASITE STRUCTURE
nomenon. The broadband luminescence of a Cr3+ ion in chromium-doped langasite crystals in the region of 0.95–1.3 μm was interpreted using a model that assumes mixing of close crystalline 4Т2g and 2Еg states of a Cr3+ ion owing to the spin-orbit interaction through vibronic tunneling. This model was proposed in [32, 33] in order to explain the broadband luminescence and laser generation in certain garnet crystals (YGG, YSGG, GGG, GSGG, and LLGG) doped with Cr3+ ions [34–36]. However, this luminescence model, which involves the 4T2g → 4A2g transition of a Сr3+ ion in an octahedral environment, is inapplicable to chromium-doped crystals with the calcium gallogermanate structure. CONCLUSIONS It has been demonstrated that chromium ions in the oxidation states 3+ and 4+ may be found in chromium-doped disordered langasite crystals of different compositions. The Cr4+/Cr3+ ratio varies widely in the series of the studied crystal samples. ACKNOWLEDGMENTS We wish to thank B.V. Mill for providing us with crystal samples and stimulating discussion of the results. The examination of luminescence spectra of langasite family crystals La3Ga5SiO14 and Sr3Ga2Ge4O14 was supported by the Russian Science Foundation (grant no. 14-22-00279). The optical characteristics of these crystals were studied at the Kotel’nikov Institute of Radio Engineering and Electronics. These studies were supported by the Russian Foundation for Basic Research (project nos. 16-07-00734 and 16-0700735). REFERENCES 1. A. A. Kaminskii, A. M. Butashin, A. A. Demidovich, et al., Phys. Status Solidi A 112, 197 (1989). 2. P. I. Macfarlane, T. P. J. Han, B. Henderson, and A. A. Kaminskii, Opt. Mater, 3 (15), 15 (1994). 3. M. Casalboni, A. Luci, and Grassabo, et al., Phys. Rev. B 48, 3781 (1994). 4. V. I. Burkov, A. F. Konstantinova, B. V. Mill, et al., Crystallogr. Rep. 54, 613 (2009). 5. A. A. Kaminskii, Laser Crystals (Nauka, Moscow, 1975, Springer-Verlag, New York, 1981). 6. E. C. Alyea, J. S. Basil, D. C. Bradley, and M. H. Chisholm, J. Chem. Soc. A, 772 (1971). 7. M. Bochmann, G. Wilkinson, Brent G. Young, et al., J. Chem. Soc. Dalton Trans., No. 10, 1863 (1980). 8. D. Reinen, U. Kesler, M. Atanasov, and J. Roos, Inorg. Chem. 34 (1), 184 (1995).
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