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Optical absorption properties and valence states of uranium in CaF2 crystals grown by TGT
ARTICLE IN PRESS
Journal of Crystal Growth 270 (2004) 150–155
Optical absorption properties and valence states of uranium in CaF2 crystals grown by TGT Liangbi Sua,b, Weiqiao Yanga, Jun Xua,*, Yongjun Donga,b, Guoqing Zhoua a
Shanghai Institute of Optics and Fine Mechanics, R&D Center for Laser and Opto-Electronic Materials, Chinese Academy of Sciences, Qinghe Road No 390, Jiading, Shanghai 201800, China b Graduate School of Chinese Academy of Science, Beijing 100039, China Received 6 May 2004; accepted 6 June 2004 Available online 15 July 2004 Communicated by M. Roth
Abstract Calcium fluoride single crystals doped with uranyl nitrate were grown by an improved temperature gradient technique under different conditions. Absorption spectra, energy levels and unit cell parameters were studied to analyze the possible color centers and valence states of uranium ions in as-grown U:CaF2 crystals. Uranium in U:CaF2 crystals grown in the presence of PbF2 as an oxygen scavenger is trivalent. F-centers and other defects related to oxygen, with respective absorption lines at 604 and 526 nm, and impure valence states of uranium ions exist in U3+:CaF2 when the molar ratio of PbF2 to U is less than 25. In the absence of PbF2, U:CaF2 crystals are multicolor, consisting of red, cerise, yellow and green volumes from inside to outside where the red part in the core is still U3+:CaF2. Mixed valence states of uranium ions exist in the crystal. The valences of uranium ions are inferred to gradually increase from +3 to +6 according to the graded changes of the absorption spectra and unit cell parameters. r 2004 Elsevier B.V. All rights reserved. PACS: 78.30.Hv; 78.40.Ha; 81.10.Fq; 61.10.Nz Keywords: A1. Absorption spectrum; A1. Color center; A1. Unit cell parameter; B1. Uranium; B1. Calcium Fluoride
1. Introduction Uranium-doped calcium fluoride single crystals have been widely studied for their spectroscopic, chemical and physical properties due to several *Corresponding author. Tel.: +8602169918485; +8602169918485. E-mail address: [email protected] (J. Xu).
fax:
interesting features exhibited by the U-ions in different valence states. Particular interest has been evoked by the laser action [1,2] of U3+:CaF2 crystals in the 2.20–2.6 mm spectral range. The saturable absorption of U2+ or U4+ ions in the broad band centered at 1.55 mm has prompted considerable investigation of U:CaF2 crystals as effective passive Q-switches in Er:glass laser systems [3,4]. However, the detailed
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.06.013
ARTICLE IN PRESS L. Su et al. / Journal of Crystal Growth 270 (2004) 150–155
mechanism of the U3+:CaF2 emission at several wavelengths, such as 2.61,2.51,2.44 and 2.24 mm, has not yet been fully understood. One explanation [5] is related to mixed valence states of uranium ions, another [2] presumes different charge compensation. Moreover, there is a great deal of controversy about the valence states of uranium ions in CaF2 crystals. Hargreaves has established the respective optical spectra of U2+, U3+ and U4+ ions in CaF2 crystals based upon systematic studies [6,7]. Earlier, McLaughlin et al. [8] gained inconsistent conclusions by chemical analyses: the U2+ and U4+ affirmed by Hargreaves were considered as U4+ and U6+, respectively. As a result, there exist self-contradictory reports [3,4] according to which the U2+ or U4+ ions give rise to the 1.5 mm saturable absorption used for passive Q-switching. Therefore, a conclusion can be made, based on a comprehensive review previous work, that several different kinds of effects should be included in the explanations: namely, the valence states of U-ions, charge compensation mechanisms, site symmetries of U-ions in the lattice and the atmosphere influence (oxidizability or reducibility) during crystal growing. Uranium-doped CaF2 single crystals have been grown under different conditions by a modified temperature gradient technique (TGT) in this work. Absorption spectra, energy levels and unit cell parameters of the U:CaF2 crystals have been determined. The possible defects and valence states of uranium ions in various U:CaF2 crystals have been analyzed in combination with varying the growth conditions, such as adding different amounts of PbF2 as an oxygen scavenger.
2. Experimental procedure Uranium-doped CaF2 crystals were grown by an improved TGT as reported previously [9]. The heating elements in the crystal growth furnace were made of highly pure graphite. The tapered 78 mm inner diameter cylindrical graphite crucible was treated at high temperature for more than 50 h, and a [1 1 1]-oriented CaF2 seed crystal was attached at the bottom. In order to prevent melt
151
volatilization, the crucible was sealed by a graphite lid having a 2 mm diameter central hole to permit the escape of volatile gases. The raw materials were uranyl nitrate (UO2(NO3)2 6H2O), CaF2 and PbF2 as a scavenger. Mixtures containing 0.02– 0.1 at% of uranium nitrate and 0–4.0 wt% of PbF2 of were completely mixed, pressed into blocks, and loaded into the crucible. The growth was initiated after the whole furnace was vacuumed to 103 Pa. The characteristics of as-grown U:CaF2 single crystals with a diameter of 75 mm were described in an earlier paper [9]. U:CaF2 crystals were cut into slices from the bottom, center, top and periphery of the boule, respectively, after being orientated by X-ray diffraction. The sliced plates were polished mechanically for recording optical absorption spectra by the Model V-570 UV/VIS/NIR spectrophotometer at room temperature. The crystal structure and the unit cell parameters of various U:CaF2 crystals were analyzed using the Model XDC-1000 Guinier-H.agg Camera.
3. Results and discussion 3.1. Optical spectra of U:CaF2 grown in the presence of PbF2 Three U:CaF2 crystals, with the uranium dopant concentrations of 0.02, 0.05 and 0.10 at% in the melt, respectively, have been grown with a constant PbF2 content of 2.0 wt%. The mole ratios of PbF2 to U are 32, 12.8 and 6.4, respectively. The main valence state of uranium ions in U:CaF2 crystals grown under such conditions have been confirmed elsewhere [9] as U3+ resulting from reduction of U6+ in the raw material. A mild deoxidization atmosphere exists in the TGT furnace due to the presence of graphite resistance heating elements and the graphite crucible. During the process of heating the raw materials under vacuum or inert atmosphere, the following reactions [10] occur: 200o C
1000o C
UO2 ðNO3 Þ2 6H2 O ! UO3 -U3 O8 ! UO2 ; >1000o C
UO2 ðsÞ þ PbF2 ðlÞ ! UF4 ðlÞ þ PbOðlÞ:
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Fig. 1. Absorption spectra of U3+:CaF2 crystals with uranium doping concentrations, 0.02, 0.05 and 0.10 at%, respectively.
PbO with a boiling point of 1470 C evaporate primarily at the melting temperature. The fact that uranium ions in as-grown U:CaF2 crystals are trivalent (U3+) indicates that the complete valence change from U4+ to U3+ occurs either while the U:CaF2 melt is at a high temperature in the crucible or in the process of melt solidification. Optical absorption spectra in the wavelength regions of interest are displayed in Fig. 1. The left graphs show the spectra in the visible range, where color centers absorption bands are usually located. The right part shows the spectra from 1300 to 1800 nm, or the range comprising the 1.55 mm saturable absorption band. With the uranium concentration increasing, exceptional variations of several absorption bands in the spectra occur in addition to the general enhancement of all bands. In the left curve ‘‘a’’ (0.02 at%), the absorption line at 550 nm is the strongest between 500 and 600 nm, being the characteristic absorption band of U3+ ions. With the uranium doping increasing to 0.05 and 0.10 at%, a very strong specific enhancement of the 526 nm band is produced. At the same time, a new band at 604 nm appears. In the 1.5 mm area, only one weak line at 1526 nm exists in the low concentration U3+:CaF2 crystal. With the uranium concentration increasing, the line at 1554 nm arises and grows more rapidly than the 1526 nm line, with the intensities of both lines becoming equal when the concentration of uranium reaches 0.10 at%.
Based on the results obtained, it can be inferred that the absorption lines at 526, 604 and 1554 nm are not attributed to U3+ ions, since their intensities do no vary synchronously with other lines associated with optical transitions between the energy levels of U3+ ions. Although 2 wt% PbF2 have been used as a scavenger, oxygen can hardly be eliminated completely. Moreover, the raw uranyl nitrate contains oxygen. With the concentration of the uranyl nitrate dopant increasing, more oxygen diffuses into the crystal lattice. Therefore, the 604 nm band has been attributed to F centers [11] originating from F vacancies, which can be introduced by the presence of oxygen ions in the lattice. Since the 526 nm band appears synchronously with the band at 604 nm, it can be related to oxygen. Further work is necessary to determine the structure of the associated defect center. The assignment of the 1554 nm band to U2+ or U4+ ions has been disputed in a number of earlier works. Hereby, we assign this band to the U4+ ions because addition of uranyl nitrate containing uranium ions in a high oxidation state (U6+) is more likely to produce uranium ions with a higher valence than +3, since sufficiently strong reduction conditions do not exist in our experiment. Another experiment can also substantiate the above conclusions. A U:CaF2 crystal doped with uranium at 0.05 at% has been grown with a PbF2 content of 4.0 wt% in the melt. The mole ratio of PbF2 to U is 25. Optical spectra of this crystal and
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Fig. 2. Absorption spectra of 0.05 at% doped U3+:CaF2 crystals grown with different PbF2 contents, 2.0 and 4.0 wt% respectively.
that of doped with 0.05 at% uranium and 2.0 wt% PbF2 are shown in Fig. 2 for comparison. The former (‘‘b’’ in Fig. 2) is similar to that of 0.02 at.% doped U:CaF2 crystal grown with 2.0 wt% PbF2 in Fig. 1(a). This result indicates that a larger amount of PbF2 can counteract, to a certain extent, the promotion of oxygen into the lattice introduced by an increasing content of uranyl nitrate. When the molar ratio of PbF2 to U is more than 25, as-grown U:CaF2 crystals are expected to contain no color centers related to oxygen and no mixed valence states of uranium ions. 3.2. U:CaF2 crystals grown in the absence of PbF2 As-grown uranium-doped CaF2 crystal boules obtained by the TGT in the absence of PbF2 consist of regions with several kinds of colors, which are mainly red, cerise, yellow and green from inside to outside. A plane-parallel polished plate longitudinally cut through the center of an as-grown crystal boule is shown in Fig. 3, where the differently colored parts are marked. Such crystal allows us to analyze the association between the color, optical spectra, charge compensation and valence states of uranium ions in CaF2 crystals. The major red part of the crystal exhibits the characteristic spectra of U3+:CaF2. This fact indicates that uranium ions in CaF2 crystal are apt to form U3+-ions in the process of melt solidification under the employed moderate conditions, since the corresponding ionic
Fig. 3. Plane-parallel polished plates longitudinally cut through the center of as-grown U:CaF2 crystal boule by TGT in the absence of PbF2, consisting of four parts with different colors (a: green, b: yellow, c: cerise, d: red).
radius of U3+ among all uranium ions is closest to that of Ca2+ (e.g. rca2þ ¼ 114 pm; rU3þ ¼ 116 pm; rU4þ ¼ 104 pm in the six-coordination sites [12]), and the +1 charge difference is easily compensated by native lattice defects. The visible and infrared absorption spectra of the green, yellow and cerise segments are shown in Fig. 4. Since profuse work has been carried out on the uranium-doped calcium fluoride crystals, the analysis of the experimental results is straightforward. The spectra of the green part (curve a in Fig. 4) correspond to U4+:CaF2 in Refs. [6,7], but are attributed to the U6+ ions elsewhere [8].
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Fig. 4. Absorption spectra of green (a), yellow (b), and cerise (c) parts of as-grown U:CaF2 crystals grown in the absence of PbF2.
Fig. 5. Simple schemes for energy levels of as-grown U:CaF2 crystals grown in the absence of PbF2; a: green, b: yellow; c: cerise.
Optical absorption similar to the yellow part spectra (curve b in Fig. 4) have been attributed to uranium ions of a higher than +3 oxidation state, namely +4.370.3 [8]. Indeed, without PbF2 as a scavenger, the crystal growing and cooling proceeds under a mildly oxidizing atmosphere. It is noteworthy that the two bands, at 526 and 604 nm, assigned above to the oxygen-related color centers also appear in spectra a and b of Fig. 4. It is quite significant that some lines are concurrent in the absorption spectra of the green, yellow, and cerise parts of as-grown U:CaF2 crystals grown in the absence of PbF2. This feature is reflected in Fig. 5, which lists the experimental energy levels of uranium ions in the three crystal parts with different coloration. Most of the energy levels in the yellow part can be found in the green
or cerise parts. Concurrently, the yellow part levels lie between the levels corresponding to the green and cerise parts. Therefore, we presume that uranium ions in the yellow crystal, with a valence of +4.3 (as determined by chemical analysis [8]), in fact exhibit a mixed valence state intermediate to the valence states of uranium ions in the green and cerise parts of the crystal. Chemical analysis alone cannot distinguish between single and mixed valence states. The crystal structures of the four parts of the U:CaF2 crystal have been characterized by X-ray diffraction as well. The results show that all crystals keep the cubic structure of calcium fluoride. The unit cell parameters are listed in Table 1 together with the unit cells’ volumes, which increase gradually from the green to red.
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Table 1 Unit cell parameters of different parts in U:CaF2 crystals grown in absence of PbF2 Samples
a b c d
Color
Green Yellow Cerise Red
a (nm)
0.5463713 0.5463931 0.5464325 0.5464503
V (nm3)
0.16310 0.16312 0.16316 0.16317
Uranium ions of higher valence states have smaller radii, which results in the slightly smaller unit cell parameters and volumes. This supports the claim that the valences of uranium ions in the U:CaF2 crystal gradually rise from the red to the green parts. Since the valence state of uranium ions in the yellow crystal is +4.3, uranium ions in the green should be attributed to U6+ and not to U4+. Although we have accumulated a considerable amount of experimental results, including the absorption spectra the 5f2 configuration, the association of the cerise crystal part with U4+ ions still needs further investigation.
4. Conclusion Calcium fluoride single crystals doped with UO2(NO3)2 6H2O were grown by an improved temperature gradient technique with and without the addition of PbF2 as oxygen scavenger. In the presence of PbF2, the uranium ions in as-grown crystals are trivalent. When the molar ratio of PbF2 to U is more than 25, as-grown U:CaF2 crystals do not contain color centers related to oxygen or trace amounts of tetravalent uranium ions. In the absence of PbF2, U:CaF2 crystals are multicolor, consisting of red, cerise, yellow and green volumes from inside the boule to the periphery. Combining the results of absorption spectra with the energy level analysis and unit cell parameters, uranium ions in the yellow crystal part (previously attributed to a +4.3 valence by chemical analysis) are found to exhibit a mixed valence state, and the valence of uranium ions is presumed to increase gradually from red to green. The uranium ions in the green crystal part are
Valence state of uranium ions Refs. [6,7]
Ref. [8]
This work
+4
+6 +4.3
+6 Mixed Undefined +3
+3
found to be hexavalent, while the corresponding optical absorption spectra are attributed to U4+:CaF2 in Refs. [6,7] and to U6+:CaF2 in Ref. [8]. Further study of the effect of color centers, not reported elsewhere, on the optical performance of U:CaF2 crystals may discover new properties and expand the spectrum of applications of this material.
Acknowledgements The authors are indebted to the Optics Science and Technology Foundation of Shanghai City for partial financial support under Grant No. 022261053.
References [1] P.P. Sorokin, M.J. Stevenson, Phys. Rev. Lett. 5 (1960) 557. [2] G.D. Boyd, R.J. Collins, S.P.S. Porto, A. Yariv, Phys. Rev. Lett. 8 (1962) 269. [3] R.D. Stultz, M.B. Camargo, S.T. Montgomery, M. Birnbaum, K. Spariosu, Appl. Phys. Lett. 64 (1994) 948. [4] R.D. Stultz, M.B. Camargo, M. Birnbaum, J. Appl. Phys. 78 (1995) 2959. [5] R.S. Title, P.P. Sorokinm, M.J. Stevenson, G.D. Pettit, J.E. Scardefield, J.R. Lankard, Phys. Rev. 128 (1962) 62. [6] W.A. Hargreaves, Phys. Rev. 156 (1967) 331. [7] W.A. Hargreaves, Phys. Rev. B 44 (1991) 5293. [8] R. Mclaughlin, U. Abed, J.G. Conway, N. Edelstein, E.H. Huffman, J. Chem. Phys. 53 (1970) 2031. [9] L.B. Su, J. Xu, Y.J. Dong, W.Q. Yang, G.Q. Zhou, G.J. Zhao, J. Crystal Growth 261 (2004) 496. [10] E.H.P. Cordfunke, The Chemistry of Uranium, Including its Application in Nuclear Technology, Elsevier Publishing Company, New York, 1969. [11] W.J. Scouler, A. Smakula, Phys. Rev. 120 (1960) 1154. [12] R.D. Shannon, C.T. Prewitt, Acta Cryst. B 25 (1969) 925.
Journal of Crystal Growth 270 (2004) 150–155
Optical absorption properties and valence states of uranium in CaF2 crystals grown by TGT Liangbi Sua,b, Weiqiao Yanga, Jun Xua,*, Yongjun Donga,b, Guoqing Zhoua a
Shanghai Institute of Optics and Fine Mechanics, R&D Center for Laser and Opto-Electronic Materials, Chinese Academy of Sciences, Qinghe Road No 390, Jiading, Shanghai 201800, China b Graduate School of Chinese Academy of Science, Beijing 100039, China Received 6 May 2004; accepted 6 June 2004 Available online 15 July 2004 Communicated by M. Roth
Abstract Calcium fluoride single crystals doped with uranyl nitrate were grown by an improved temperature gradient technique under different conditions. Absorption spectra, energy levels and unit cell parameters were studied to analyze the possible color centers and valence states of uranium ions in as-grown U:CaF2 crystals. Uranium in U:CaF2 crystals grown in the presence of PbF2 as an oxygen scavenger is trivalent. F-centers and other defects related to oxygen, with respective absorption lines at 604 and 526 nm, and impure valence states of uranium ions exist in U3+:CaF2 when the molar ratio of PbF2 to U is less than 25. In the absence of PbF2, U:CaF2 crystals are multicolor, consisting of red, cerise, yellow and green volumes from inside to outside where the red part in the core is still U3+:CaF2. Mixed valence states of uranium ions exist in the crystal. The valences of uranium ions are inferred to gradually increase from +3 to +6 according to the graded changes of the absorption spectra and unit cell parameters. r 2004 Elsevier B.V. All rights reserved. PACS: 78.30.Hv; 78.40.Ha; 81.10.Fq; 61.10.Nz Keywords: A1. Absorption spectrum; A1. Color center; A1. Unit cell parameter; B1. Uranium; B1. Calcium Fluoride
1. Introduction Uranium-doped calcium fluoride single crystals have been widely studied for their spectroscopic, chemical and physical properties due to several *Corresponding author. Tel.: +8602169918485; +8602169918485. E-mail address: [email protected] (J. Xu).
fax:
interesting features exhibited by the U-ions in different valence states. Particular interest has been evoked by the laser action [1,2] of U3+:CaF2 crystals in the 2.20–2.6 mm spectral range. The saturable absorption of U2+ or U4+ ions in the broad band centered at 1.55 mm has prompted considerable investigation of U:CaF2 crystals as effective passive Q-switches in Er:glass laser systems [3,4]. However, the detailed
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.06.013
ARTICLE IN PRESS L. Su et al. / Journal of Crystal Growth 270 (2004) 150–155
mechanism of the U3+:CaF2 emission at several wavelengths, such as 2.61,2.51,2.44 and 2.24 mm, has not yet been fully understood. One explanation [5] is related to mixed valence states of uranium ions, another [2] presumes different charge compensation. Moreover, there is a great deal of controversy about the valence states of uranium ions in CaF2 crystals. Hargreaves has established the respective optical spectra of U2+, U3+ and U4+ ions in CaF2 crystals based upon systematic studies [6,7]. Earlier, McLaughlin et al. [8] gained inconsistent conclusions by chemical analyses: the U2+ and U4+ affirmed by Hargreaves were considered as U4+ and U6+, respectively. As a result, there exist self-contradictory reports [3,4] according to which the U2+ or U4+ ions give rise to the 1.5 mm saturable absorption used for passive Q-switching. Therefore, a conclusion can be made, based on a comprehensive review previous work, that several different kinds of effects should be included in the explanations: namely, the valence states of U-ions, charge compensation mechanisms, site symmetries of U-ions in the lattice and the atmosphere influence (oxidizability or reducibility) during crystal growing. Uranium-doped CaF2 single crystals have been grown under different conditions by a modified temperature gradient technique (TGT) in this work. Absorption spectra, energy levels and unit cell parameters of the U:CaF2 crystals have been determined. The possible defects and valence states of uranium ions in various U:CaF2 crystals have been analyzed in combination with varying the growth conditions, such as adding different amounts of PbF2 as an oxygen scavenger.
2. Experimental procedure Uranium-doped CaF2 crystals were grown by an improved TGT as reported previously [9]. The heating elements in the crystal growth furnace were made of highly pure graphite. The tapered 78 mm inner diameter cylindrical graphite crucible was treated at high temperature for more than 50 h, and a [1 1 1]-oriented CaF2 seed crystal was attached at the bottom. In order to prevent melt
151
volatilization, the crucible was sealed by a graphite lid having a 2 mm diameter central hole to permit the escape of volatile gases. The raw materials were uranyl nitrate (UO2(NO3)2 6H2O), CaF2 and PbF2 as a scavenger. Mixtures containing 0.02– 0.1 at% of uranium nitrate and 0–4.0 wt% of PbF2 of were completely mixed, pressed into blocks, and loaded into the crucible. The growth was initiated after the whole furnace was vacuumed to 103 Pa. The characteristics of as-grown U:CaF2 single crystals with a diameter of 75 mm were described in an earlier paper [9]. U:CaF2 crystals were cut into slices from the bottom, center, top and periphery of the boule, respectively, after being orientated by X-ray diffraction. The sliced plates were polished mechanically for recording optical absorption spectra by the Model V-570 UV/VIS/NIR spectrophotometer at room temperature. The crystal structure and the unit cell parameters of various U:CaF2 crystals were analyzed using the Model XDC-1000 Guinier-H.agg Camera.
3. Results and discussion 3.1. Optical spectra of U:CaF2 grown in the presence of PbF2 Three U:CaF2 crystals, with the uranium dopant concentrations of 0.02, 0.05 and 0.10 at% in the melt, respectively, have been grown with a constant PbF2 content of 2.0 wt%. The mole ratios of PbF2 to U are 32, 12.8 and 6.4, respectively. The main valence state of uranium ions in U:CaF2 crystals grown under such conditions have been confirmed elsewhere [9] as U3+ resulting from reduction of U6+ in the raw material. A mild deoxidization atmosphere exists in the TGT furnace due to the presence of graphite resistance heating elements and the graphite crucible. During the process of heating the raw materials under vacuum or inert atmosphere, the following reactions [10] occur: 200o C
1000o C
UO2 ðNO3 Þ2 6H2 O ! UO3 -U3 O8 ! UO2 ; >1000o C
UO2 ðsÞ þ PbF2 ðlÞ ! UF4 ðlÞ þ PbOðlÞ:
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Fig. 1. Absorption spectra of U3+:CaF2 crystals with uranium doping concentrations, 0.02, 0.05 and 0.10 at%, respectively.
PbO with a boiling point of 1470 C evaporate primarily at the melting temperature. The fact that uranium ions in as-grown U:CaF2 crystals are trivalent (U3+) indicates that the complete valence change from U4+ to U3+ occurs either while the U:CaF2 melt is at a high temperature in the crucible or in the process of melt solidification. Optical absorption spectra in the wavelength regions of interest are displayed in Fig. 1. The left graphs show the spectra in the visible range, where color centers absorption bands are usually located. The right part shows the spectra from 1300 to 1800 nm, or the range comprising the 1.55 mm saturable absorption band. With the uranium concentration increasing, exceptional variations of several absorption bands in the spectra occur in addition to the general enhancement of all bands. In the left curve ‘‘a’’ (0.02 at%), the absorption line at 550 nm is the strongest between 500 and 600 nm, being the characteristic absorption band of U3+ ions. With the uranium doping increasing to 0.05 and 0.10 at%, a very strong specific enhancement of the 526 nm band is produced. At the same time, a new band at 604 nm appears. In the 1.5 mm area, only one weak line at 1526 nm exists in the low concentration U3+:CaF2 crystal. With the uranium concentration increasing, the line at 1554 nm arises and grows more rapidly than the 1526 nm line, with the intensities of both lines becoming equal when the concentration of uranium reaches 0.10 at%.
Based on the results obtained, it can be inferred that the absorption lines at 526, 604 and 1554 nm are not attributed to U3+ ions, since their intensities do no vary synchronously with other lines associated with optical transitions between the energy levels of U3+ ions. Although 2 wt% PbF2 have been used as a scavenger, oxygen can hardly be eliminated completely. Moreover, the raw uranyl nitrate contains oxygen. With the concentration of the uranyl nitrate dopant increasing, more oxygen diffuses into the crystal lattice. Therefore, the 604 nm band has been attributed to F centers [11] originating from F vacancies, which can be introduced by the presence of oxygen ions in the lattice. Since the 526 nm band appears synchronously with the band at 604 nm, it can be related to oxygen. Further work is necessary to determine the structure of the associated defect center. The assignment of the 1554 nm band to U2+ or U4+ ions has been disputed in a number of earlier works. Hereby, we assign this band to the U4+ ions because addition of uranyl nitrate containing uranium ions in a high oxidation state (U6+) is more likely to produce uranium ions with a higher valence than +3, since sufficiently strong reduction conditions do not exist in our experiment. Another experiment can also substantiate the above conclusions. A U:CaF2 crystal doped with uranium at 0.05 at% has been grown with a PbF2 content of 4.0 wt% in the melt. The mole ratio of PbF2 to U is 25. Optical spectra of this crystal and
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Fig. 2. Absorption spectra of 0.05 at% doped U3+:CaF2 crystals grown with different PbF2 contents, 2.0 and 4.0 wt% respectively.
that of doped with 0.05 at% uranium and 2.0 wt% PbF2 are shown in Fig. 2 for comparison. The former (‘‘b’’ in Fig. 2) is similar to that of 0.02 at.% doped U:CaF2 crystal grown with 2.0 wt% PbF2 in Fig. 1(a). This result indicates that a larger amount of PbF2 can counteract, to a certain extent, the promotion of oxygen into the lattice introduced by an increasing content of uranyl nitrate. When the molar ratio of PbF2 to U is more than 25, as-grown U:CaF2 crystals are expected to contain no color centers related to oxygen and no mixed valence states of uranium ions. 3.2. U:CaF2 crystals grown in the absence of PbF2 As-grown uranium-doped CaF2 crystal boules obtained by the TGT in the absence of PbF2 consist of regions with several kinds of colors, which are mainly red, cerise, yellow and green from inside to outside. A plane-parallel polished plate longitudinally cut through the center of an as-grown crystal boule is shown in Fig. 3, where the differently colored parts are marked. Such crystal allows us to analyze the association between the color, optical spectra, charge compensation and valence states of uranium ions in CaF2 crystals. The major red part of the crystal exhibits the characteristic spectra of U3+:CaF2. This fact indicates that uranium ions in CaF2 crystal are apt to form U3+-ions in the process of melt solidification under the employed moderate conditions, since the corresponding ionic
Fig. 3. Plane-parallel polished plates longitudinally cut through the center of as-grown U:CaF2 crystal boule by TGT in the absence of PbF2, consisting of four parts with different colors (a: green, b: yellow, c: cerise, d: red).
radius of U3+ among all uranium ions is closest to that of Ca2+ (e.g. rca2þ ¼ 114 pm; rU3þ ¼ 116 pm; rU4þ ¼ 104 pm in the six-coordination sites [12]), and the +1 charge difference is easily compensated by native lattice defects. The visible and infrared absorption spectra of the green, yellow and cerise segments are shown in Fig. 4. Since profuse work has been carried out on the uranium-doped calcium fluoride crystals, the analysis of the experimental results is straightforward. The spectra of the green part (curve a in Fig. 4) correspond to U4+:CaF2 in Refs. [6,7], but are attributed to the U6+ ions elsewhere [8].
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Fig. 4. Absorption spectra of green (a), yellow (b), and cerise (c) parts of as-grown U:CaF2 crystals grown in the absence of PbF2.
Fig. 5. Simple schemes for energy levels of as-grown U:CaF2 crystals grown in the absence of PbF2; a: green, b: yellow; c: cerise.
Optical absorption similar to the yellow part spectra (curve b in Fig. 4) have been attributed to uranium ions of a higher than +3 oxidation state, namely +4.370.3 [8]. Indeed, without PbF2 as a scavenger, the crystal growing and cooling proceeds under a mildly oxidizing atmosphere. It is noteworthy that the two bands, at 526 and 604 nm, assigned above to the oxygen-related color centers also appear in spectra a and b of Fig. 4. It is quite significant that some lines are concurrent in the absorption spectra of the green, yellow, and cerise parts of as-grown U:CaF2 crystals grown in the absence of PbF2. This feature is reflected in Fig. 5, which lists the experimental energy levels of uranium ions in the three crystal parts with different coloration. Most of the energy levels in the yellow part can be found in the green
or cerise parts. Concurrently, the yellow part levels lie between the levels corresponding to the green and cerise parts. Therefore, we presume that uranium ions in the yellow crystal, with a valence of +4.3 (as determined by chemical analysis [8]), in fact exhibit a mixed valence state intermediate to the valence states of uranium ions in the green and cerise parts of the crystal. Chemical analysis alone cannot distinguish between single and mixed valence states. The crystal structures of the four parts of the U:CaF2 crystal have been characterized by X-ray diffraction as well. The results show that all crystals keep the cubic structure of calcium fluoride. The unit cell parameters are listed in Table 1 together with the unit cells’ volumes, which increase gradually from the green to red.
ARTICLE IN PRESS L. Su et al. / Journal of Crystal Growth 270 (2004) 150–155
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Table 1 Unit cell parameters of different parts in U:CaF2 crystals grown in absence of PbF2 Samples
a b c d
Color
Green Yellow Cerise Red
a (nm)
0.5463713 0.5463931 0.5464325 0.5464503
V (nm3)
0.16310 0.16312 0.16316 0.16317
Uranium ions of higher valence states have smaller radii, which results in the slightly smaller unit cell parameters and volumes. This supports the claim that the valences of uranium ions in the U:CaF2 crystal gradually rise from the red to the green parts. Since the valence state of uranium ions in the yellow crystal is +4.3, uranium ions in the green should be attributed to U6+ and not to U4+. Although we have accumulated a considerable amount of experimental results, including the absorption spectra the 5f2 configuration, the association of the cerise crystal part with U4+ ions still needs further investigation.
4. Conclusion Calcium fluoride single crystals doped with UO2(NO3)2 6H2O were grown by an improved temperature gradient technique with and without the addition of PbF2 as oxygen scavenger. In the presence of PbF2, the uranium ions in as-grown crystals are trivalent. When the molar ratio of PbF2 to U is more than 25, as-grown U:CaF2 crystals do not contain color centers related to oxygen or trace amounts of tetravalent uranium ions. In the absence of PbF2, U:CaF2 crystals are multicolor, consisting of red, cerise, yellow and green volumes from inside the boule to the periphery. Combining the results of absorption spectra with the energy level analysis and unit cell parameters, uranium ions in the yellow crystal part (previously attributed to a +4.3 valence by chemical analysis) are found to exhibit a mixed valence state, and the valence of uranium ions is presumed to increase gradually from red to green. The uranium ions in the green crystal part are
Valence state of uranium ions Refs. [6,7]
Ref. [8]
This work
+4
+6 +4.3
+6 Mixed Undefined +3
+3
found to be hexavalent, while the corresponding optical absorption spectra are attributed to U4+:CaF2 in Refs. [6,7] and to U6+:CaF2 in Ref. [8]. Further study of the effect of color centers, not reported elsewhere, on the optical performance of U:CaF2 crystals may discover new properties and expand the spectrum of applications of this material.
Acknowledgements The authors are indebted to the Optics Science and Technology Foundation of Shanghai City for partial financial support under Grant No. 022261053.
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