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Optical spectroscopy of disordered Ca3Ga2Ge4O14 crystal doped with manganese
Optical Materials 79 (2018) 317–321
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Optical spectroscopy of disordered Ca3Ga2Ge4O14 crystal doped with manganese
T
Vladimir Burkova, Liudmila Alyabyevaa,∗, Boris Millb, Viacheslav Kotovc a
Moscow Institute of Physics and Technology (State University), 141701, Institutskii per. 9, Dolgoprudny, Moscow Region, Russia Moscow State University, 119991, Leninskie Gory 1, Moscow, Russia c Kotel'nikov Institute of Radio-engineering and Electronics (IRE) of Russian Academy of Sciences, 125009, Mokhovaya 11-7, Moscow, Russia b
A R T I C LE I N FO
A B S T R A C T
Keywords: Optical spectroscopy Circular dichroism Absorption Langasite Transitional metal Electron transition
Circular dichroism, absorption and luminescence spectra of single crystalline manganese doped calcium gallogermanate Ca3Ga2Ge4O14:Mn were investigated in 300–850 nm wavelength region in wide temperature range 8–300 K. Careful analysis of experimental results revealed presence of electron transitions typical for sixfold coordinated trivalent manganese ions with d4 electron configuration. Thus, manganese ions doping the crystal matrix of CCG incorporate into lattice in 1a octahedral site-positions substituting Ga3+ ions. The results obtained were compared with investigation of isostructural to CGG manganese doped langasite crystals, La3Ga5SiO14:Mn where dopant is in octahedral Mn4+ state.
1. Introduction
with 3 d transitional metal ions in Refs. [5–8]. Specifically, study of various langasites doped with chromium ions revealed dopant to be in both 3 + and 4 + valence states. Moreover, chemical composition of compound has crucial influence on Cr3+/Cr4+ ratio [8]. Elements of 3 d group, transitional metals, could have variable valence originates from presence of incomplete 3 d electron shell. Due to this fact, 3 d metals in substitutional solid solutions have different oxidation state, which makes it hard to predict the valence of the dopant in the lattice in advance. Authors of [9] investigated optical characteristics of Ca3Ga2Ge4O14:Mn (CGG:Mn) and manganese ions were suggested to be divalent, however, detailed analysis of the spectroscopic results was not performed. Meanwhile, manganese ions in isostructural to CGG langasite crystal La3Ga5SiO14 were shown to occupy octahedral site-positions in Mn4+ valence state [7]. In view of above, the valence and site-position of 3 d ions doping calcium gallogermanate compounds are not obvious. In this study, we present detailed investigation of absorption and, for the first time, circular dichroism and luminescence of CGG:Mn crystals in 300–850 nm spectral region in a wide temperature range 8–300 K. Involving several spectroscopic methods, the valence state and site-position of the manganese dopant were unveiled. The influence of Jahn-Teller effect and lowering of the local symmetry on electron transitions were considered and the magnitude of the Jahn-Teller splitting was estimated.
Calcium gallogermanate crystal, Ca3Ga2Ge4O14 (CGG), is known to be the first member of a rich crystal family of isostructural compounds including both ordered and disordered structures. Crystals of the gallogermanate family are famous for their piezo-electric properties and are widely used in devices of modern acustooptics. Furthermore, the compounds doped with transitional metal ions are prospective for use in laser technologies and optoelectronics. The gallogermanate crystallizes in a trigonal singony and has the space group P321 with Z = 1 [1–3]. Structure of the compound could be described as a stack of alternate layers of two types normal to the threefold axis C3. One type consists of distorted Thomson cubes 3e with symmetry C2 occupied by the largest cation Ca2+ and oxygen octahedra 1a shared by Ga3+ and Ge4+ ions in ratio 1:4 (symmetry D3). The other type consists of two tetrahedral site positions. Ions of Ga3+ and Ge4+ are statistically distributed in large 3f tetrahedra with symmetry C2, while small tetrahedra 2d with symmetry C3 are oxygen coordination of Ge4+ ions only. Threefold axes of 2 d tetrahedra are collinear with optical axis C3 of the crystal and vertices of each neighbor tetrahedra are antiparallel. Coexistence of different ions with different valences and ionic radii within the same site-position results in disorder of CGG crystal structure. Thus, electrical neutrality of structural positions and the unit cell is achieved due to appearance of structural defects, i.e. oxygen vacancies [4]. Circular dichroism and absorption spectra of nominally pure langasite-type crystals have been investigated in Ref. [4], and those doped
∗
Corresponding author. E-mail address: [email protected] (L. Alyabyeva).
https://doi.org/10.1016/j.optmat.2018.03.057 Received 16 November 2017; Received in revised form 12 January 2018; Accepted 27 March 2018 0925-3467/ © 2018 Elsevier B.V. All rights reserved.
Optical Materials 79 (2018) 317–321
V. Burkov et al.
2. Material and methods
temperature range. In the room temperature luminescence spectra only one intensive band at λ = 610 nm is observed (Fig. 2). At T∼120 K in the region of 780 nm the second band appears, and its intensity increases with further decrease of the temperature. Eventually, at T = 8 K intensities of the two bands are comparable. Such change of intensivity of emission is due to a fact that with temperature decrease a contribution of nonradiative transitions from the luminescent level also decreases. No other resonances were observed in absorption, CD and luminescence spectra of studied CGG:Mn crystals.
Single crystal of CGG:Mn have been grown by a Czochralski method in 2%О2–98%N2 atmosphere [10]. The crystal has pink color and concentration of Mn ions is 0.1 at.% (molar concentration С = 5.7⋅10−3 mol/l). Samples of 0.5–2.5 mm thickness for optical study were cut from the boule in orientation perpendicular to the optical axis C 3. Measurements were performed in the region 300–850 nm employing several different setups. Absorption spectra were obtained using Hitachi-330 spectrophotometer, and a standard Fourier-transform Bruker Vertex-80v spectrometer, circular dichroism spectra were measured on MarkIII (Jobin-Yvon) dichrograph. Luminescence was excited using continuous-wave YAG:Nd laser LCM-S-111 (λ = 532 nm, Р = 55 mW) along with two light-diodes (λ = 400 nm and λ = 450 nm, Р = 0.5 W). A monochromator MDR-23 with digital registration of the signal was used to register the luminescence signal. The experiments on CGG:Mn were performed at temperatures from 8 to 300 K. A closedcicle cryostat CCS-150 (Janis Research Company) was used for cooling the samples.
4. Discussion The analysis of the experimental data obtained is aimed at definition of the valence state and site-position of the manganese. It should be noted, that in the region of λ < 550 nm in absorption and CD spectra, the electronic transitions of structure defects (oxygen vacations) of pure gallogermanate are located [4]. The intensities of these lines are of the same order as intensities of electronic transitions of impurity manganese ions at low concentrations (about 0.1 at.%) as it seen in our spectra. Thus, we should specify, that the line in CD at 390 nm is a manifestation of electronic transition of structural defect of undoped disordered Ca3Ga2Ge4O14 [4]. According to previous studies of manganese-doped crystals, the band observed in CGG:Mn absorption at ∼460 nm could be assigned to either 5Еg→ 5T2g (Mn3+ [11–13]) or to 4А2g→ 4Т2g (Mn4+ [14–16]) electronic transitions in octahedral coordination of the ion. Absorption spectra of manganese ions in other valence states (Mn2+, Mn5+, Mn6+, Mn7+) and other coordinations [17–21] are absolutely different from the results obtained for CGG:Mn in this work. Nevertheless, the results of [7] clearly depicts that our experimental data are not likely to be similar with optical spectra of sixfold-coordinated Mn4+ in langasite crystal. In CD spectra of LGS:Mn4+ a negative at 360 nm and weak negative CD in the region of 600–800 nm are observed [7]. However, CD spectra of CGG:Mn did not reveal any circular dichroism of negative sign, which is a manifestation of absolutely different configuration of electronic states. In light of above, we connect the band at 460 nm in CGG:Mn spectra with presence of trivalent manganese in octahedral coordination 1a substituting trivalent Ga, this band is a manifestation of the only one spin-allowed transition, 5Еg→ 5T2g (Mn3+) of the d4 configuration. In addition, magnitudes of ionic radii of Mn3+ and Ga3+ in octahedral coordination are close together [22].
3. Results Room temperature axial absorption spectra reveal a wide band with a maximum at ∼460 nm (Fig. 1a). On a long wavelength shoulder of the band, the two weak bends are detected at 480 and 520 nm. In addition, a very weak band is peaked at about 860 nm (k∼0.1 cm−1). A sharp increase of absorption takes place in 350–380 nm. A broad intensive band with a peak at ∼520 nm and two prominent shoulders at 480 and 460 nm are observed in room temperature circular dichroism (CD) spectra (line 2 in Fig. 1a). Additionally, a weak bend of positive sign is detected at about 390 nm. Thus, the peak and the bends on the shoulder of absorption resonance coincide with the bends and the peak of CD spectra. In the region of λ < 350 nm positive CD signal sharply increases. The weak band in 660–900 nm is detected in CD spectra. With cooling intensive band in the absorption spectra in 450–550 nm range resolves into three maxima (Fig. 1b), and on the long wavelength shoulder at 520–600 nm a set of weak bends unveils. Meanwhile the peak in CD spectra does not move with temperature change and is located in the region of the bend in absorption both at 300 K and at 8 K. This depicts that distribution of intensity over the complex band in CD and absorption are different throughout the whole measured
Fig. 1. Absorption (1, 3) and circular dichroism (2,4) spectra of Ca3Ga2Ge4O14 crystal doped with Mn ions at 300 K (a) and 8 K (b). 318
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As was shown in Refs. [11,12], the spin-allowed transition 5Еg→5Т2g of the octahedral high-spin Mn3+ (electron configuration d4) is located at 20000-22000 cm−1, which is in a reasonable agreement with our experimental data. Using Tanabe-Sugano diagram for octahedral d4 electrons (Fig. 3 [23]) we estimated the parameters of the crystal field of our manganese doped gallogermanate crystal, 10Dq/B ≈ 24–27, Dq ≈ 2160 cm−1, B ≈ 800 cm−1. Consequently, all three bands in absorption and CD spectra at 520, 490 and 460 nm are manifestation of the same 5Еg→5Т2g transition splitted in the crystal field. Moreover, a weak band in the red region (∼860 nm) in absorption and CD spectra is usually attributed to the transition between JahnTeller components of the splitted 5Еg ground state of Mn3+. Intensity of this line along with its position in the spectra of different compounds could vary in a wide range. Authors of [11] also discuss given band more likely to be a manifestation of 5Еg→ 3T2g transition. In addition, an attention in Ref. [11] is paid to the influence of the crystal field on electronic states of Mn3+ while in other studies, i.e. [12], the interpretation does not go beyond the scopes of local symmetry Oh of the octahedral manganese. Detailed study of the CGG crystal structure is given in Refs. [1,2]. Local symmetry of 1a octahedral positions is stated to be lowered to D3, and, therefore, 5Т2g excited state, as well as the other T-states splits in
Fig. 2. Luminescence spectra of Mn doped Ca3Ga2Ge4O14 crystal in wide temperature range.
Fig. 3. Tanabe-Sugano diagram for ions of d4 electron configuration in octahedral coordination [23]. 319
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Fig. 4. Energy terms transformation with lowering the symmetry Оh —D3
the crystal field into 5Е and 5А2 states, while octahedral 5Еg ground state transforms into 5Е state in the framework of the D3 symmetry (Fig. 4). In such a case, in the absorption spectra at low temperatures only two bands instead of three would be present in the region of 5 Еg→5Т2g transition. When local symmetry of a crystallographic position is lowered from cubic Oh to D3, not only octahedral threefold degenerated T-terms split, but also, due to the mechanism of borrowing an electric dipole from odd states T1u, the transitions to all excited states become active in CD spectra. Given Mn3+ to be a Jahn-Teller ion, E-states interact with etype vibration, and as a result, the symmetry decreases further to D2 (C2) [24] and E-states split into B2 and B3. states. The symmetry D2 (С2) was selected among other symmetries to which D3 symmetry could lowers as it predicted by a group theory since CD effect on a Mn3+ ions is allowed only if the local site-symmetry of the Mn3+ would be D2 (С2). The presence of three maxima in the bandshape of intensive spin-allowed absorption band could be explained by insignificant Jahn-Teller splitting of the excited E-term (Figs. 1 and 3). The maximum of CD band is peaked at 520 nm, where in absorption spectra, the long-wavelength shoulder of 5Еg→5Т2g transition is located. In the other words, according to the Tanabe-Sugano diagram, this CD maximum is located in the region of the spin-forbidden transitions to T-states which energies are in the vicinity of the 5Т2g term. The maximum of the first intensive luminescence band at 610 nm is significantly shifted to the long-wave region relative to the corresponding maximum of the absorption band, which indicates that the luminescence occurs from a level located in the energy scale below the 5 Т2g state. The peak-positions of both maxima in luminescence do not change with the temperature. Authors of [12] associate corresponding two bands in the low temperature emission spectra of Mn3+ in garnet crystals with transitions from the 1Т2 level to the Jahn-Teller splitted terms of the 5E ground state, where the splitting of the ground state is about 2200–2600 cm−1. Our estimations give the value of the JahnTeller splitting of the Mn3+ ground state in CGG crystal lattice to be about 3700 cm−1.
—
D2 of d4 electron configuration ions.
5. Conclusions Spectroscopic studies of CGG:Mn revealed complete correspondence of the lines observed in absorption, circular dichroism and luminescence spectra with transitions of sixfold-coordinated high-spin Mn3+ (d4). The parameters of the crystal field were defined using TanabeSugano diagram and were estimated to be 10Dq/B ≈ 24–27, Dq ≈ 2160 cm−1, B ≈ 800 cm−1. Transitions to the Jahn-Teller splitted ground state are active in the luminescence spectra, and the value of the Jahn-Teller effect was measured to be about 3700 cm−1. The obtained results indicate that the manganese impurity ions are trivalent and substitute Ga3+ ions in the crystal lattice of CGG in 1a octahedral sitepositions. Acknowledgements This work was performed in Kotel'nikov Institute of Radio-engineering and Electronics (IRE) of Russian Academy of Sciences and in Moscow Institute of Physics and Technology (State University), the study of circular dichroism and luminescence is supported by Russian Science Foundation, Project 14-22-00279, the study of absorption is supported by the Ministry of Education and Science of the Russian Federation, Project 5top100. References [1] E.L. Belokoneva, M.A. Simonov, B.V. Mill, A.V. Butashin, N.V. Belov, Crystal structure of cagallogermanate, Ca3Ge[(Ga2Ge)Ge2O14] and its analog, Ba3Fe [(FeGe2)Ge2O14], Dokl. Akad. Nauk SSSR 255 (5) (1980) 1099–1104. [2] A.A. Kaminskii, E.L. Belokoneva, B.V. Mill, Y.V. Pisarevskii, S.E. Sarkisov, I.M. Silvestrova, A.V. Butashin, G.G. Khodzhabagyan, Pure and Nd3+-doped Ca3Ga2Ge4O14 and Sr3Ga2Ge4O14 single crystals, their structure, optical, spectral luminescence, electromechanical properties, and stimulated emission, Phys. Status Solidi 86 (1) (1984) 345–362. [3] A.A. Kaminskii, B.V. Mill, C.E. Sarkisov, Physics and Spectroscopy of Laser Crystals, Nauka, Moscow, 1986. [4] V.I. Burkov, E.P. Perederei, E.V. Fedotov, B.V. Mill, YuV. Pisarevskii, Circular dichroism spectra of langasite family crystals in the range of electronic transitions of structure defects, Crystallogr. Rep. 53 (5) (2008) 843–846. [5] V.I. Burkov, L.N. Alyabyeva, YuV. Denisov, B.V. Mill, Optical spectroscopy of a La3Ga5SiO14:Co2+ crystal, Inorg. Mater. 50 (11) (2014) 1119–1124.
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[15] A. Suchocki, J.D. Allen, R.C. Powell, G.M. Loiacono, Spectroscopy and four-wave mixing in Li4Ge5O12: Mn 4+ crystals, Phys. Rev. B 36 (1987) 6729. [16] A. Brenier, A. Suchocki, S. Pedrini, G. Boulon, C. Madej, Spectroscopy of Mn 4+doped Ca-substituted gadolinium gallium garnet, Phys. Rev. B 46 (1992) 3219. [17] A.B.P. Lever, Inorganic Electronic Spectroscopy, second ed., Elsevier Science Publishers B.V., Amsterdam, 1984. [18] K. Ando, Optical study of Mn2+ intraionic transitions in zinc-blende MnTe, Phys. Rev. B 47 (1993) 9350. [19] J.A. Capobianco, C. Cormier, R. Moncorg, H. Manaa, M. Betinelli, Gain measurements of Mn5+(3d2) doped Sr5(PO4)3Cl and Ca2PO4Cl, Appl. Phys. Lett. 60 (1992) 163. [20] R. Borromei, L. Oleari, P. Day, Electronic spectrum of the manganate(V) ion in different host lattices, J. Chem. Soc., Faraday Trans. 2 (77) (1981) 1563. [21] L.D. Merkle, A. Pinto, H.R. Verdun, B. McIntosh. Laser action from Mn5+ in Ba3(VO4)2, Appl. Phys. Lett.., 61 Laser action from Mn5+ in Ba3(VO4)2 2386. [22] J.A. Schellman, Circular dichroism and optical rotation, Chem. Rev. 75 (1975) 323. [23] Tanabe sugano diagrams via spreadsheets by Prof. Robert J. Lancashire, created Feb. 1992, visited Jan 2018. URL:http://wwwchem.uwimona.edu.jm/courses/ Tanabe-Sugano/. [24] L.D. Landau, E.M. Lifshitz, Course of theoretical physics, Quantum Mechanics, vol. 3, Pergamon Press, London, 1958.
[6] L. Alyabyeva, V. Burkov, B. Mill, Optical spectroscopy of La3Ga5SiO14 disordered crystals doped with Fe3+ ions, Opt. Mater. 43 (2015) 55–58. [7] V.I. Burkov, S.V. Gudenko, L.N. Alyabyeva, Optical and EPR spectroscopy of a La3Ga5SiO14:Mn crystal, J. Exp. Theor. Phys. Lett. 119 (4) (2014) 723–736. [8] V.I. Burkov, A.F. Konstantinova, B.V. Mill, T.F. Veremeichik, YuN. Pyrkov, V.P. Orekhova, E.V. Fedotov, The absorption and circular dichroism spectra of langasite family crystals doped with chromium ions, Crystallogr. Rep. 54 (4) (2009) 613–618. [9] A.E. Nosenko, R.Y. Leshchuk, B.V. Padlyak, Optical and epr spectroscopy of impurity manganese ions in disordered Ca3Ga2Ge4O14 single crystals, Radiat. Eff. Defect Solid 135 (1–4) (1995) 55–60. [10] B.V. Mill, A.V. Butashin, A.M. Ellern, A.A. Maier, Phase formation in the CAOGA2O3-GEO2 system, Inorg. Mater. 17 (9) (1981) 1216–1220. [11] T.S. Davis, J.P. Fackler, M.J. Weeks, Spectra of manganese (III) complexes. Origin of the low-energy band, Inorg. Chem. 7 (10) (1968) 1994–2002. [12] S. Kück, S. Hartung, S. Hurling, K. Petermann, G. Huber, Optical transitions in Mn 3+-doped garnets, Phys. Rev. B 57 (4) (1998) 2203. [13] D.S. McClure, Optical spectra of transition-metal ions in corundum, J. Chem. Phys. 36 (10) (1962) 2757–2779. [14] S. Geschwind, P. Kisliuk, M.P. Klein, J.P. Remeika, D.L. Wood, Sharp-line fluorescence, electron paramagnetic resonance, and thermoluminescence of Mn 4+ in αAl2O3, Phys. Rev. 126 (1962) 1684.
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Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Optical spectroscopy of disordered Ca3Ga2Ge4O14 crystal doped with manganese
T
Vladimir Burkova, Liudmila Alyabyevaa,∗, Boris Millb, Viacheslav Kotovc a
Moscow Institute of Physics and Technology (State University), 141701, Institutskii per. 9, Dolgoprudny, Moscow Region, Russia Moscow State University, 119991, Leninskie Gory 1, Moscow, Russia c Kotel'nikov Institute of Radio-engineering and Electronics (IRE) of Russian Academy of Sciences, 125009, Mokhovaya 11-7, Moscow, Russia b
A R T I C LE I N FO
A B S T R A C T
Keywords: Optical spectroscopy Circular dichroism Absorption Langasite Transitional metal Electron transition
Circular dichroism, absorption and luminescence spectra of single crystalline manganese doped calcium gallogermanate Ca3Ga2Ge4O14:Mn were investigated in 300–850 nm wavelength region in wide temperature range 8–300 K. Careful analysis of experimental results revealed presence of electron transitions typical for sixfold coordinated trivalent manganese ions with d4 electron configuration. Thus, manganese ions doping the crystal matrix of CCG incorporate into lattice in 1a octahedral site-positions substituting Ga3+ ions. The results obtained were compared with investigation of isostructural to CGG manganese doped langasite crystals, La3Ga5SiO14:Mn where dopant is in octahedral Mn4+ state.
1. Introduction
with 3 d transitional metal ions in Refs. [5–8]. Specifically, study of various langasites doped with chromium ions revealed dopant to be in both 3 + and 4 + valence states. Moreover, chemical composition of compound has crucial influence on Cr3+/Cr4+ ratio [8]. Elements of 3 d group, transitional metals, could have variable valence originates from presence of incomplete 3 d electron shell. Due to this fact, 3 d metals in substitutional solid solutions have different oxidation state, which makes it hard to predict the valence of the dopant in the lattice in advance. Authors of [9] investigated optical characteristics of Ca3Ga2Ge4O14:Mn (CGG:Mn) and manganese ions were suggested to be divalent, however, detailed analysis of the spectroscopic results was not performed. Meanwhile, manganese ions in isostructural to CGG langasite crystal La3Ga5SiO14 were shown to occupy octahedral site-positions in Mn4+ valence state [7]. In view of above, the valence and site-position of 3 d ions doping calcium gallogermanate compounds are not obvious. In this study, we present detailed investigation of absorption and, for the first time, circular dichroism and luminescence of CGG:Mn crystals in 300–850 nm spectral region in a wide temperature range 8–300 K. Involving several spectroscopic methods, the valence state and site-position of the manganese dopant were unveiled. The influence of Jahn-Teller effect and lowering of the local symmetry on electron transitions were considered and the magnitude of the Jahn-Teller splitting was estimated.
Calcium gallogermanate crystal, Ca3Ga2Ge4O14 (CGG), is known to be the first member of a rich crystal family of isostructural compounds including both ordered and disordered structures. Crystals of the gallogermanate family are famous for their piezo-electric properties and are widely used in devices of modern acustooptics. Furthermore, the compounds doped with transitional metal ions are prospective for use in laser technologies and optoelectronics. The gallogermanate crystallizes in a trigonal singony and has the space group P321 with Z = 1 [1–3]. Structure of the compound could be described as a stack of alternate layers of two types normal to the threefold axis C3. One type consists of distorted Thomson cubes 3e with symmetry C2 occupied by the largest cation Ca2+ and oxygen octahedra 1a shared by Ga3+ and Ge4+ ions in ratio 1:4 (symmetry D3). The other type consists of two tetrahedral site positions. Ions of Ga3+ and Ge4+ are statistically distributed in large 3f tetrahedra with symmetry C2, while small tetrahedra 2d with symmetry C3 are oxygen coordination of Ge4+ ions only. Threefold axes of 2 d tetrahedra are collinear with optical axis C3 of the crystal and vertices of each neighbor tetrahedra are antiparallel. Coexistence of different ions with different valences and ionic radii within the same site-position results in disorder of CGG crystal structure. Thus, electrical neutrality of structural positions and the unit cell is achieved due to appearance of structural defects, i.e. oxygen vacancies [4]. Circular dichroism and absorption spectra of nominally pure langasite-type crystals have been investigated in Ref. [4], and those doped
∗
Corresponding author. E-mail address: [email protected] (L. Alyabyeva).
https://doi.org/10.1016/j.optmat.2018.03.057 Received 16 November 2017; Received in revised form 12 January 2018; Accepted 27 March 2018 0925-3467/ © 2018 Elsevier B.V. All rights reserved.
Optical Materials 79 (2018) 317–321
V. Burkov et al.
2. Material and methods
temperature range. In the room temperature luminescence spectra only one intensive band at λ = 610 nm is observed (Fig. 2). At T∼120 K in the region of 780 nm the second band appears, and its intensity increases with further decrease of the temperature. Eventually, at T = 8 K intensities of the two bands are comparable. Such change of intensivity of emission is due to a fact that with temperature decrease a contribution of nonradiative transitions from the luminescent level also decreases. No other resonances were observed in absorption, CD and luminescence spectra of studied CGG:Mn crystals.
Single crystal of CGG:Mn have been grown by a Czochralski method in 2%О2–98%N2 atmosphere [10]. The crystal has pink color and concentration of Mn ions is 0.1 at.% (molar concentration С = 5.7⋅10−3 mol/l). Samples of 0.5–2.5 mm thickness for optical study were cut from the boule in orientation perpendicular to the optical axis C 3. Measurements were performed in the region 300–850 nm employing several different setups. Absorption spectra were obtained using Hitachi-330 spectrophotometer, and a standard Fourier-transform Bruker Vertex-80v spectrometer, circular dichroism spectra were measured on MarkIII (Jobin-Yvon) dichrograph. Luminescence was excited using continuous-wave YAG:Nd laser LCM-S-111 (λ = 532 nm, Р = 55 mW) along with two light-diodes (λ = 400 nm and λ = 450 nm, Р = 0.5 W). A monochromator MDR-23 with digital registration of the signal was used to register the luminescence signal. The experiments on CGG:Mn were performed at temperatures from 8 to 300 K. A closedcicle cryostat CCS-150 (Janis Research Company) was used for cooling the samples.
4. Discussion The analysis of the experimental data obtained is aimed at definition of the valence state and site-position of the manganese. It should be noted, that in the region of λ < 550 nm in absorption and CD spectra, the electronic transitions of structure defects (oxygen vacations) of pure gallogermanate are located [4]. The intensities of these lines are of the same order as intensities of electronic transitions of impurity manganese ions at low concentrations (about 0.1 at.%) as it seen in our spectra. Thus, we should specify, that the line in CD at 390 nm is a manifestation of electronic transition of structural defect of undoped disordered Ca3Ga2Ge4O14 [4]. According to previous studies of manganese-doped crystals, the band observed in CGG:Mn absorption at ∼460 nm could be assigned to either 5Еg→ 5T2g (Mn3+ [11–13]) or to 4А2g→ 4Т2g (Mn4+ [14–16]) electronic transitions in octahedral coordination of the ion. Absorption spectra of manganese ions in other valence states (Mn2+, Mn5+, Mn6+, Mn7+) and other coordinations [17–21] are absolutely different from the results obtained for CGG:Mn in this work. Nevertheless, the results of [7] clearly depicts that our experimental data are not likely to be similar with optical spectra of sixfold-coordinated Mn4+ in langasite crystal. In CD spectra of LGS:Mn4+ a negative at 360 nm and weak negative CD in the region of 600–800 nm are observed [7]. However, CD spectra of CGG:Mn did not reveal any circular dichroism of negative sign, which is a manifestation of absolutely different configuration of electronic states. In light of above, we connect the band at 460 nm in CGG:Mn spectra with presence of trivalent manganese in octahedral coordination 1a substituting trivalent Ga, this band is a manifestation of the only one spin-allowed transition, 5Еg→ 5T2g (Mn3+) of the d4 configuration. In addition, magnitudes of ionic radii of Mn3+ and Ga3+ in octahedral coordination are close together [22].
3. Results Room temperature axial absorption spectra reveal a wide band with a maximum at ∼460 nm (Fig. 1a). On a long wavelength shoulder of the band, the two weak bends are detected at 480 and 520 nm. In addition, a very weak band is peaked at about 860 nm (k∼0.1 cm−1). A sharp increase of absorption takes place in 350–380 nm. A broad intensive band with a peak at ∼520 nm and two prominent shoulders at 480 and 460 nm are observed in room temperature circular dichroism (CD) spectra (line 2 in Fig. 1a). Additionally, a weak bend of positive sign is detected at about 390 nm. Thus, the peak and the bends on the shoulder of absorption resonance coincide with the bends and the peak of CD spectra. In the region of λ < 350 nm positive CD signal sharply increases. The weak band in 660–900 nm is detected in CD spectra. With cooling intensive band in the absorption spectra in 450–550 nm range resolves into three maxima (Fig. 1b), and on the long wavelength shoulder at 520–600 nm a set of weak bends unveils. Meanwhile the peak in CD spectra does not move with temperature change and is located in the region of the bend in absorption both at 300 K and at 8 K. This depicts that distribution of intensity over the complex band in CD and absorption are different throughout the whole measured
Fig. 1. Absorption (1, 3) and circular dichroism (2,4) spectra of Ca3Ga2Ge4O14 crystal doped with Mn ions at 300 K (a) and 8 K (b). 318
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As was shown in Refs. [11,12], the spin-allowed transition 5Еg→5Т2g of the octahedral high-spin Mn3+ (electron configuration d4) is located at 20000-22000 cm−1, which is in a reasonable agreement with our experimental data. Using Tanabe-Sugano diagram for octahedral d4 electrons (Fig. 3 [23]) we estimated the parameters of the crystal field of our manganese doped gallogermanate crystal, 10Dq/B ≈ 24–27, Dq ≈ 2160 cm−1, B ≈ 800 cm−1. Consequently, all three bands in absorption and CD spectra at 520, 490 and 460 nm are manifestation of the same 5Еg→5Т2g transition splitted in the crystal field. Moreover, a weak band in the red region (∼860 nm) in absorption and CD spectra is usually attributed to the transition between JahnTeller components of the splitted 5Еg ground state of Mn3+. Intensity of this line along with its position in the spectra of different compounds could vary in a wide range. Authors of [11] also discuss given band more likely to be a manifestation of 5Еg→ 3T2g transition. In addition, an attention in Ref. [11] is paid to the influence of the crystal field on electronic states of Mn3+ while in other studies, i.e. [12], the interpretation does not go beyond the scopes of local symmetry Oh of the octahedral manganese. Detailed study of the CGG crystal structure is given in Refs. [1,2]. Local symmetry of 1a octahedral positions is stated to be lowered to D3, and, therefore, 5Т2g excited state, as well as the other T-states splits in
Fig. 2. Luminescence spectra of Mn doped Ca3Ga2Ge4O14 crystal in wide temperature range.
Fig. 3. Tanabe-Sugano diagram for ions of d4 electron configuration in octahedral coordination [23]. 319
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Fig. 4. Energy terms transformation with lowering the symmetry Оh —D3
the crystal field into 5Е and 5А2 states, while octahedral 5Еg ground state transforms into 5Е state in the framework of the D3 symmetry (Fig. 4). In such a case, in the absorption spectra at low temperatures only two bands instead of three would be present in the region of 5 Еg→5Т2g transition. When local symmetry of a crystallographic position is lowered from cubic Oh to D3, not only octahedral threefold degenerated T-terms split, but also, due to the mechanism of borrowing an electric dipole from odd states T1u, the transitions to all excited states become active in CD spectra. Given Mn3+ to be a Jahn-Teller ion, E-states interact with etype vibration, and as a result, the symmetry decreases further to D2 (C2) [24] and E-states split into B2 and B3. states. The symmetry D2 (С2) was selected among other symmetries to which D3 symmetry could lowers as it predicted by a group theory since CD effect on a Mn3+ ions is allowed only if the local site-symmetry of the Mn3+ would be D2 (С2). The presence of three maxima in the bandshape of intensive spin-allowed absorption band could be explained by insignificant Jahn-Teller splitting of the excited E-term (Figs. 1 and 3). The maximum of CD band is peaked at 520 nm, where in absorption spectra, the long-wavelength shoulder of 5Еg→5Т2g transition is located. In the other words, according to the Tanabe-Sugano diagram, this CD maximum is located in the region of the spin-forbidden transitions to T-states which energies are in the vicinity of the 5Т2g term. The maximum of the first intensive luminescence band at 610 nm is significantly shifted to the long-wave region relative to the corresponding maximum of the absorption band, which indicates that the luminescence occurs from a level located in the energy scale below the 5 Т2g state. The peak-positions of both maxima in luminescence do not change with the temperature. Authors of [12] associate corresponding two bands in the low temperature emission spectra of Mn3+ in garnet crystals with transitions from the 1Т2 level to the Jahn-Teller splitted terms of the 5E ground state, where the splitting of the ground state is about 2200–2600 cm−1. Our estimations give the value of the JahnTeller splitting of the Mn3+ ground state in CGG crystal lattice to be about 3700 cm−1.
—
D2 of d4 electron configuration ions.
5. Conclusions Spectroscopic studies of CGG:Mn revealed complete correspondence of the lines observed in absorption, circular dichroism and luminescence spectra with transitions of sixfold-coordinated high-spin Mn3+ (d4). The parameters of the crystal field were defined using TanabeSugano diagram and were estimated to be 10Dq/B ≈ 24–27, Dq ≈ 2160 cm−1, B ≈ 800 cm−1. Transitions to the Jahn-Teller splitted ground state are active in the luminescence spectra, and the value of the Jahn-Teller effect was measured to be about 3700 cm−1. The obtained results indicate that the manganese impurity ions are trivalent and substitute Ga3+ ions in the crystal lattice of CGG in 1a octahedral sitepositions. Acknowledgements This work was performed in Kotel'nikov Institute of Radio-engineering and Electronics (IRE) of Russian Academy of Sciences and in Moscow Institute of Physics and Technology (State University), the study of circular dichroism and luminescence is supported by Russian Science Foundation, Project 14-22-00279, the study of absorption is supported by the Ministry of Education and Science of the Russian Federation, Project 5top100. References [1] E.L. Belokoneva, M.A. Simonov, B.V. Mill, A.V. Butashin, N.V. Belov, Crystal structure of cagallogermanate, Ca3Ge[(Ga2Ge)Ge2O14] and its analog, Ba3Fe [(FeGe2)Ge2O14], Dokl. Akad. Nauk SSSR 255 (5) (1980) 1099–1104. [2] A.A. Kaminskii, E.L. Belokoneva, B.V. Mill, Y.V. Pisarevskii, S.E. Sarkisov, I.M. Silvestrova, A.V. Butashin, G.G. Khodzhabagyan, Pure and Nd3+-doped Ca3Ga2Ge4O14 and Sr3Ga2Ge4O14 single crystals, their structure, optical, spectral luminescence, electromechanical properties, and stimulated emission, Phys. Status Solidi 86 (1) (1984) 345–362. [3] A.A. Kaminskii, B.V. Mill, C.E. Sarkisov, Physics and Spectroscopy of Laser Crystals, Nauka, Moscow, 1986. [4] V.I. Burkov, E.P. Perederei, E.V. Fedotov, B.V. Mill, YuV. Pisarevskii, Circular dichroism spectra of langasite family crystals in the range of electronic transitions of structure defects, Crystallogr. Rep. 53 (5) (2008) 843–846. [5] V.I. Burkov, L.N. Alyabyeva, YuV. Denisov, B.V. Mill, Optical spectroscopy of a La3Ga5SiO14:Co2+ crystal, Inorg. Mater. 50 (11) (2014) 1119–1124.
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