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Absorption and circular-dichroism spectra of LaBGeO5−Cr4+ single crystal
Optical Materials 33 (2010) 63–65
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Absorption and circular-dichroism spectra of LaBGeO5Cr4+ single crystal V. Burkov, O. Lysenko ⇑ Moscow Institute of Physics and Technology, 141701, 9, Institutskii per., Dolgoprudny, Moscow Region, Russia
a r t i c l e
i n f o
Article history: Received 3 March 2010 Received in revised form 7 July 2010 Accepted 2 August 2010
Keywords: Circular-dichroism Crystal LaBGeO5ACr4+
a b s t r a c t The absorption and circular-dichroism (CD) spectra of LaBGeO5Cr4+ crystal have been measured at 85 and 300 K in the range 10,000–37,500 cm–1. The spectra have been interpreted by the use of crystal field theory for the states of the 3d2 configuration acted on by a potential of predominantly Td symmetry. Fundamental details of the spectra, especially of the circular-dichroism spectra, are accounted for by contributions from the field of low symmetry C1, and spin–orbit interactions in the intensities of the electronic transitions of the Cr4+ ion. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Using Cr4+ ion as an activator in laser crystals [1–3] had stimulated investigation of its spectroscopic characteristics in different crystal matrixes [4–10]. The results of the measurements of CD spectra of crystals from langasite family doped with chromium had shown clearly the presence of Cr4+ ions [11]. In case of optically active LaBGeO5Cr crystal, there is an opportunity to use CD spectroscopy method for investigation of electronic states and oxidation state of the chromium ions in this crystal. The results received from CD spectra give additional information about the electronic transitions types. The growth of LaBGeO5 crystals was described in [12,13], where data on crystal structure and spectroscopic characteristics were also given. In LaBGeO5 structure (sp. gr. P31), Ge4+ and B3+ ions are situated inside the distorted oxygen tetrahedra of two types, which are quite different in sizes. In defining the positions of Cr ions in the crystal, there are some reasons to say that they occupy bigger tetrahedra in the oxidation state of 4+ by substitution of Ge4+ ions due to their sizes and valences are close. All atoms in the structure are situated in general position with the local symmetry C1. Hence, it should be expected the appearance of CD on electronic transitions of the Cr4+ ion split by the crystal field.
2. Results 2.1. Experimental For measurements, z-cuts of crystal activated with chromium (0.1 at.% of Ge) with the thickness 1.5 mm were used. The crystal ⇑ Corresponding author. Tel.: +7 495 4086011. E-mail address: [email protected] (O. Lysenko). 0925-3467/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2010.08.003
has salmon colour. The absorption spectra were recorded on Hitachi-330 spectrophotometer and circular-dichroism spectra were measured on Mark-3S (Jobin Yvon) dichrograph. Spectra were obtained at 300 and 85 K.
2.2. Absorption and CD spectra At 300 K, four bands are seen in absorption spectra with the maximums at 920, 685, 505 and 240 nm (Fig. 1). The first band has very small intensity and is observed as the bend on the long wave edge of the second band at 300 K as well as 85 K. On the second broad band contour with low intensity any clear structure was not observed even at low temperature. The third band is a superposition of three bands; maximums and two bends on its contour are observed at 300 K. The fourth band (in the UV range) is observed as a bend on the edge of absorption. Since LaBGeO5 crystal is transparent in the range 0.19–8.5 lm [13], all observed lines in the spectrum of the chromium doped crystal should be related to electronic transitions of the chromium ions. It should be noted that according to the band positions in the 350–1000 nm range, the spectrum of the crystal is similar to spectra of other crystals doped with Cr4+ in tetrahedral coordination [3–8], therefore one can conclude that in doped LaBGeO5 crystal chromium ions occupy tetrahedral positions in the oxidation state of 4+. High intensity of absorption of LaBGeO5Cr4+ crystal in the UV range is due to charge-transfer transitions of Cr4+ ions [3]. CD spectra confirm these conclusions. If the first and the second bands in absorption spectrum have very low and low intensities correspondingly, the corresponding bands in CD spectrum have enough large intensity. The transitions in the UV range in CD spectrum are also clearly seen. At 85 K the vibronic structure with frequency intervals Dv = 660–690 cm–1 is observed on the short wavelength edge of
64
V. Burkov, O. Lysenko / Optical Materials 33 (2010) 63–65
approximation; the intensity of such transition should be negligible. On the other side, the transition 3A ? 3T1 (t2e) must be more intensive as allowed in the electric-dipole approximation. Since the site symmetry of the Cr4+ ion is C1, any transition in the crystal field could be considered as allowed in electric-dipole, magnetic-dipole and electric-quadrupole approximations. As CD is observed in all transitions, all these transitions are allowed in electric-dipole and magnetic-dipole approximations. Thus, the experimental results of absorption and CD spectra allow definitely make a conclusion about the chromium ions positions inside the oxygen tetrahedra in the crystal lattice of LaBGeO5 in the oxidation state of 4+. Using the result of absorption and CD spectra, one can easily calculate parameters of the crystal field Dq and B from Tabane– Sugano diagram for 3d2 configuration [12]. The band position in the spectra related to 3A2 ? 3T2 transition (allowed by symmetry in magnetic-dipole approximation) gives the value 10Dq = 14585 cm–1. Because the frequency relation of the band maximums in spectra of the first spin allowed transitions is 1.36 at Dq/B = 3.65, then B 400 cm–1. The calculated Dq/B is typical for the strong field approximation. Absorption and CD spectra allow to calculate the dipole strength (Dom), the rotation strength (Rom) and the anisotropy factor (g), which are used generally in analysis of spectroscopic characteristics of optically active compounds [15] Fig. 1. Absorption (dotted line) and CD spectra (solid line) of LaBGeO5Cr4+ at 300 K.
-1
-1
-1
Δε, 10 cm mol l
-1
-1
ε, cm mol l
4
2
-1 670 cm -1 660 cm
-1 690 cm
0 0 -3 -6 -9
445,5 459 -1 660 cm
-1 670 cm
490 -1 690 cm
450
503,5
500 λ, nm
550
Fig. 2. Absorption (dotted line) and CD spectra (solid line) of LaBGeO5Cr4+ in the range 400–600 nm at 85 K.
the third band (Fig. 2). The similar vibronic structure with Dv 700 cm1 was observed in absorption spectrum of Ca2GeO4– Cr4+ crystal, where Cr4+ ion is situated inside the distorted oxygen tetrahedron [4]. 3. Discussion According to Tabane–Sugano [14], the levels positions of the tetrahedral coordinated Cr4+ (3d2) in the energy scale differ dramatically depending on Dq/B parameter. At Dq/B = 2.8 there is the following sequence of excited levels: 1 Eðt22 Þ, 3 T2 ðt2 eÞ, 1 A1 ðt22 Þ, 3 T1 ðt22 Þ, 3 T1 ðt2 eÞ. The transitions from the fundamental state 3A2 to singlet are spin-forbidden. The spin–orbit interaction partly removes this restriction and the transitions could occur in absorption spectra as bands with low intensity. In the frame of Td symmetry, the transition 3A2 ? 3T2 (t2e) is allowed in the magnetic-dipole
3ð2303Þhcn
Z
edk; N4p02 kmax b2 Z 3ð2303Þhc Dedk; Rom ¼ N16p2 kmax b 4n Rom g¼ ; b Dom
Dom ¼
ð1Þ ð2Þ ð3Þ
Here, e is the decimal molar extinction coefficient, De is the decimal molar dichroism coefficient, n is the refractive index, b = (n2 + 2)/3 is the Lorentz factor. The calculated values of Dom, Rom and g are shown in Table 1. The anisotropy factor of intensive UV band is dramatically less the value of crystal-field transition. It is commonly observed in case of transitions allowed in electric-dipole approximation. Since the anisotropy factor of the transition 3A2 ? 3T2 has considerable value and the band intensity in absorption spectra is low, hence this transition is related to magnetic-dipole transition by symmetry. Its appearance in the absorption spectrum is connected with borrowing of intensity from the charge-transfer transitions. The band at 505 nm is related to 3A2 ? 3T1 transition, which is allowed in electric-dipole approximation. Its appearance in the absorption spectra is also connected with intensity borrowing from parity allowed transitions, i.e. from transitions with charge transfer. The band at 920 nm in absorption spectrum, related to 3 A2 ? 1E transition has quite low intensity as spin-forbidden. The appearance of this transition in CD spectra with intensity compared with intensity of other transitions is connected with the spin–orbit interaction, which provides borrowing of electric dipole from spin allowed transitions. The appearance of the magnetic dipole on this transition also provides the mechanism of borrowing it from close 3A2 ? 3T2 transition due to the spin–orbit interaction. Table 1 Values of the dipole strengths, rotation strengths, and anisotropy factors of the d–d transitions of the Cr4+ ion in LaBGeO5 crystal. Transition
3
A2 ? 1 E
3
A 2 ? 3 T2
3
A2 ? 3T1
k, nm D, 1040 CGSE R, 1040 CGSE g = De /e
920 4.4 0.34 0.3
685 44.69 1.1 0.10
505 446.4 1.8 0.016
V. Burkov, O. Lysenko / Optical Materials 33 (2010) 63–65
The structure occurrence on the most intensive band contour with maximum at 505 nm is connected with splitting of excited 3T1 state and the appearance of vibronic bands with Dv 680 cm1.
4. Conclusion Absorption and CD spectra of optically active LaBGeO5Cr4+ crystal are investigated. The values of dipole strength, rotation strength and anisotropy factor of d–d transitions are calculated. Analysis of the results has shown that Cr4+ ion occupy tetrahedral position by substituting of Ge4+ ion.
Acknowledgements Mr. Boris V. Mill from Moscow State University is gratefully acknowledged for providing us with the crystal and discussions during the work.
65
References [1] V. Petricevic, S.K. Gayen, R.R. Alfano, K. Yamagishi, H. Anzai, Y. Yamaguchi, Appl. Phys. Lett. 52 (1988) 1040. [2] V. Petricevic, S.K. Gayen, R.R. Alfano, Appl. Phys. Lett. 53 (1988) 2590. [3] H.R. Verdun, L.M. Thomas, D.M. Andrauskas, T. McCollum, A. Pinto, Appl. Phys. Lett. 53 (1988) 2593. [4] M.F. Hazenkamp, N.U. Gudel, M. Atanasov, U. Kesper, D. Rainen, Phys. Rev. B 53 (1996) 2367. [5] J. Weiyi, L. Huimin, S. Jaffe, W.M. Yen, Phys. Rev. B 43 (1991) 5234. [6] N.N. Bakhurova, A.L. Denisov, E.V. Zharikov, Proc. Advance Solid-State Lasers 20 (1994) 210. [7] A. Sugimoto, Y. Nobe, T. Yamazaki, Phys. Chem. Minerals 24 (1997) 326. [8] R.G. Pappalardo, W.J. Miniscalo, T.E. Peters, J. Luminescence 5 (1993) 87. [9] L.D. Merkle, T.H. Allik, B.H.T. Chai, Opt. Mater. 1 (1992) 91. [10] K.A. Soubbotin, V.A. Smirnov, S.B. Kovalev, Opt. Mater. 13 (2000) 405. [11] V.I. Burkov, A.F. Konstantinova, B.V. Mill, T.F. Vereneichik, Yu.N. Pyrkov, V.P. Orekhova, E.V. Fedotov, Crystallogr. Rep. 54 (2009) 652. [12] A.A. Kaminski, B.V. Mill, A.V. Belokoneva, A.V. Butashin, Proc. USSR Acad. of Sci. Inorg. Mater. 26 (1990) 1105. [13] A.A. Kaminski, A.V. Butashin, B.V. Mill, V.D. Shygorin, Proc. USSR Acad. Sci. Phys. Ser. 56 (1992) 121. [14] Y. Tanabe, S. Sugano, J. Phys. Soc. JPN 9 (1954) 753. [15] J.A. Schellman, Chem. Rev. 5 (1975) 323.
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Absorption and circular-dichroism spectra of LaBGeO5Cr4+ single crystal V. Burkov, O. Lysenko ⇑ Moscow Institute of Physics and Technology, 141701, 9, Institutskii per., Dolgoprudny, Moscow Region, Russia
a r t i c l e
i n f o
Article history: Received 3 March 2010 Received in revised form 7 July 2010 Accepted 2 August 2010
Keywords: Circular-dichroism Crystal LaBGeO5ACr4+
a b s t r a c t The absorption and circular-dichroism (CD) spectra of LaBGeO5Cr4+ crystal have been measured at 85 and 300 K in the range 10,000–37,500 cm–1. The spectra have been interpreted by the use of crystal field theory for the states of the 3d2 configuration acted on by a potential of predominantly Td symmetry. Fundamental details of the spectra, especially of the circular-dichroism spectra, are accounted for by contributions from the field of low symmetry C1, and spin–orbit interactions in the intensities of the electronic transitions of the Cr4+ ion. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Using Cr4+ ion as an activator in laser crystals [1–3] had stimulated investigation of its spectroscopic characteristics in different crystal matrixes [4–10]. The results of the measurements of CD spectra of crystals from langasite family doped with chromium had shown clearly the presence of Cr4+ ions [11]. In case of optically active LaBGeO5Cr crystal, there is an opportunity to use CD spectroscopy method for investigation of electronic states and oxidation state of the chromium ions in this crystal. The results received from CD spectra give additional information about the electronic transitions types. The growth of LaBGeO5 crystals was described in [12,13], where data on crystal structure and spectroscopic characteristics were also given. In LaBGeO5 structure (sp. gr. P31), Ge4+ and B3+ ions are situated inside the distorted oxygen tetrahedra of two types, which are quite different in sizes. In defining the positions of Cr ions in the crystal, there are some reasons to say that they occupy bigger tetrahedra in the oxidation state of 4+ by substitution of Ge4+ ions due to their sizes and valences are close. All atoms in the structure are situated in general position with the local symmetry C1. Hence, it should be expected the appearance of CD on electronic transitions of the Cr4+ ion split by the crystal field.
2. Results 2.1. Experimental For measurements, z-cuts of crystal activated with chromium (0.1 at.% of Ge) with the thickness 1.5 mm were used. The crystal ⇑ Corresponding author. Tel.: +7 495 4086011. E-mail address: [email protected] (O. Lysenko). 0925-3467/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2010.08.003
has salmon colour. The absorption spectra were recorded on Hitachi-330 spectrophotometer and circular-dichroism spectra were measured on Mark-3S (Jobin Yvon) dichrograph. Spectra were obtained at 300 and 85 K.
2.2. Absorption and CD spectra At 300 K, four bands are seen in absorption spectra with the maximums at 920, 685, 505 and 240 nm (Fig. 1). The first band has very small intensity and is observed as the bend on the long wave edge of the second band at 300 K as well as 85 K. On the second broad band contour with low intensity any clear structure was not observed even at low temperature. The third band is a superposition of three bands; maximums and two bends on its contour are observed at 300 K. The fourth band (in the UV range) is observed as a bend on the edge of absorption. Since LaBGeO5 crystal is transparent in the range 0.19–8.5 lm [13], all observed lines in the spectrum of the chromium doped crystal should be related to electronic transitions of the chromium ions. It should be noted that according to the band positions in the 350–1000 nm range, the spectrum of the crystal is similar to spectra of other crystals doped with Cr4+ in tetrahedral coordination [3–8], therefore one can conclude that in doped LaBGeO5 crystal chromium ions occupy tetrahedral positions in the oxidation state of 4+. High intensity of absorption of LaBGeO5Cr4+ crystal in the UV range is due to charge-transfer transitions of Cr4+ ions [3]. CD spectra confirm these conclusions. If the first and the second bands in absorption spectrum have very low and low intensities correspondingly, the corresponding bands in CD spectrum have enough large intensity. The transitions in the UV range in CD spectrum are also clearly seen. At 85 K the vibronic structure with frequency intervals Dv = 660–690 cm–1 is observed on the short wavelength edge of
64
V. Burkov, O. Lysenko / Optical Materials 33 (2010) 63–65
approximation; the intensity of such transition should be negligible. On the other side, the transition 3A ? 3T1 (t2e) must be more intensive as allowed in the electric-dipole approximation. Since the site symmetry of the Cr4+ ion is C1, any transition in the crystal field could be considered as allowed in electric-dipole, magnetic-dipole and electric-quadrupole approximations. As CD is observed in all transitions, all these transitions are allowed in electric-dipole and magnetic-dipole approximations. Thus, the experimental results of absorption and CD spectra allow definitely make a conclusion about the chromium ions positions inside the oxygen tetrahedra in the crystal lattice of LaBGeO5 in the oxidation state of 4+. Using the result of absorption and CD spectra, one can easily calculate parameters of the crystal field Dq and B from Tabane– Sugano diagram for 3d2 configuration [12]. The band position in the spectra related to 3A2 ? 3T2 transition (allowed by symmetry in magnetic-dipole approximation) gives the value 10Dq = 14585 cm–1. Because the frequency relation of the band maximums in spectra of the first spin allowed transitions is 1.36 at Dq/B = 3.65, then B 400 cm–1. The calculated Dq/B is typical for the strong field approximation. Absorption and CD spectra allow to calculate the dipole strength (Dom), the rotation strength (Rom) and the anisotropy factor (g), which are used generally in analysis of spectroscopic characteristics of optically active compounds [15] Fig. 1. Absorption (dotted line) and CD spectra (solid line) of LaBGeO5Cr4+ at 300 K.
-1
-1
-1
Δε, 10 cm mol l
-1
-1
ε, cm mol l
4
2
-1 670 cm -1 660 cm
-1 690 cm
0 0 -3 -6 -9
445,5 459 -1 660 cm
-1 670 cm
490 -1 690 cm
450
503,5
500 λ, nm
550
Fig. 2. Absorption (dotted line) and CD spectra (solid line) of LaBGeO5Cr4+ in the range 400–600 nm at 85 K.
the third band (Fig. 2). The similar vibronic structure with Dv 700 cm1 was observed in absorption spectrum of Ca2GeO4– Cr4+ crystal, where Cr4+ ion is situated inside the distorted oxygen tetrahedron [4]. 3. Discussion According to Tabane–Sugano [14], the levels positions of the tetrahedral coordinated Cr4+ (3d2) in the energy scale differ dramatically depending on Dq/B parameter. At Dq/B = 2.8 there is the following sequence of excited levels: 1 Eðt22 Þ, 3 T2 ðt2 eÞ, 1 A1 ðt22 Þ, 3 T1 ðt22 Þ, 3 T1 ðt2 eÞ. The transitions from the fundamental state 3A2 to singlet are spin-forbidden. The spin–orbit interaction partly removes this restriction and the transitions could occur in absorption spectra as bands with low intensity. In the frame of Td symmetry, the transition 3A2 ? 3T2 (t2e) is allowed in the magnetic-dipole
3ð2303Þhcn
Z
edk; N4p02 kmax b2 Z 3ð2303Þhc Dedk; Rom ¼ N16p2 kmax b 4n Rom g¼ ; b Dom
Dom ¼
ð1Þ ð2Þ ð3Þ
Here, e is the decimal molar extinction coefficient, De is the decimal molar dichroism coefficient, n is the refractive index, b = (n2 + 2)/3 is the Lorentz factor. The calculated values of Dom, Rom and g are shown in Table 1. The anisotropy factor of intensive UV band is dramatically less the value of crystal-field transition. It is commonly observed in case of transitions allowed in electric-dipole approximation. Since the anisotropy factor of the transition 3A2 ? 3T2 has considerable value and the band intensity in absorption spectra is low, hence this transition is related to magnetic-dipole transition by symmetry. Its appearance in the absorption spectrum is connected with borrowing of intensity from the charge-transfer transitions. The band at 505 nm is related to 3A2 ? 3T1 transition, which is allowed in electric-dipole approximation. Its appearance in the absorption spectra is also connected with intensity borrowing from parity allowed transitions, i.e. from transitions with charge transfer. The band at 920 nm in absorption spectrum, related to 3 A2 ? 1E transition has quite low intensity as spin-forbidden. The appearance of this transition in CD spectra with intensity compared with intensity of other transitions is connected with the spin–orbit interaction, which provides borrowing of electric dipole from spin allowed transitions. The appearance of the magnetic dipole on this transition also provides the mechanism of borrowing it from close 3A2 ? 3T2 transition due to the spin–orbit interaction. Table 1 Values of the dipole strengths, rotation strengths, and anisotropy factors of the d–d transitions of the Cr4+ ion in LaBGeO5 crystal. Transition
3
A2 ? 1 E
3
A 2 ? 3 T2
3
A2 ? 3T1
k, nm D, 1040 CGSE R, 1040 CGSE g = De /e
920 4.4 0.34 0.3
685 44.69 1.1 0.10
505 446.4 1.8 0.016
V. Burkov, O. Lysenko / Optical Materials 33 (2010) 63–65
The structure occurrence on the most intensive band contour with maximum at 505 nm is connected with splitting of excited 3T1 state and the appearance of vibronic bands with Dv 680 cm1.
4. Conclusion Absorption and CD spectra of optically active LaBGeO5Cr4+ crystal are investigated. The values of dipole strength, rotation strength and anisotropy factor of d–d transitions are calculated. Analysis of the results has shown that Cr4+ ion occupy tetrahedral position by substituting of Ge4+ ion.
Acknowledgements Mr. Boris V. Mill from Moscow State University is gratefully acknowledged for providing us with the crystal and discussions during the work.
65
References [1] V. Petricevic, S.K. Gayen, R.R. Alfano, K. Yamagishi, H. Anzai, Y. Yamaguchi, Appl. Phys. Lett. 52 (1988) 1040. [2] V. Petricevic, S.K. Gayen, R.R. Alfano, Appl. Phys. Lett. 53 (1988) 2590. [3] H.R. Verdun, L.M. Thomas, D.M. Andrauskas, T. McCollum, A. Pinto, Appl. Phys. Lett. 53 (1988) 2593. [4] M.F. Hazenkamp, N.U. Gudel, M. Atanasov, U. Kesper, D. Rainen, Phys. Rev. B 53 (1996) 2367. [5] J. Weiyi, L. Huimin, S. Jaffe, W.M. Yen, Phys. Rev. B 43 (1991) 5234. [6] N.N. Bakhurova, A.L. Denisov, E.V. Zharikov, Proc. Advance Solid-State Lasers 20 (1994) 210. [7] A. Sugimoto, Y. Nobe, T. Yamazaki, Phys. Chem. Minerals 24 (1997) 326. [8] R.G. Pappalardo, W.J. Miniscalo, T.E. Peters, J. Luminescence 5 (1993) 87. [9] L.D. Merkle, T.H. Allik, B.H.T. Chai, Opt. Mater. 1 (1992) 91. [10] K.A. Soubbotin, V.A. Smirnov, S.B. Kovalev, Opt. Mater. 13 (2000) 405. [11] V.I. Burkov, A.F. Konstantinova, B.V. Mill, T.F. Vereneichik, Yu.N. Pyrkov, V.P. Orekhova, E.V. Fedotov, Crystallogr. Rep. 54 (2009) 652. [12] A.A. Kaminski, B.V. Mill, A.V. Belokoneva, A.V. Butashin, Proc. USSR Acad. of Sci. Inorg. Mater. 26 (1990) 1105. [13] A.A. Kaminski, A.V. Butashin, B.V. Mill, V.D. Shygorin, Proc. USSR Acad. Sci. Phys. Ser. 56 (1992) 121. [14] Y. Tanabe, S. Sugano, J. Phys. Soc. JPN 9 (1954) 753. [15] J.A. Schellman, Chem. Rev. 5 (1975) 323.