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Electrolytic coloration and spectral properties of natural fluorite crystals


Spectrochimica Acta Part A 82 (2011) 327–331

Contents lists available at ScienceDirect

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa

Electrolytic coloration and spectral properties of natural ?uorite crystals containing oxygen impurities
Hongen Gu ? , Dongliang Ma, Weiwei Chen, Rui Zhu, Yutong Li, Yang Li
Department of Physics, Tianjin University, Tianjin 300072, PR China

a r t i c l e

i n f o

a b s t r a c t
Natural ?uorite crystals containing oxygen impurities are colored electrolytically by using a pointed cathode and a ?at anode at various temperatures and voltages. F and F2 color centers are produced in colored ?uorite crystals. O2? –Va + , O2? –Va + aggregate, Yb2+ , Ce3+ and Sm2+ absorption bands are observed in absorption spectra of uncolored ?uorite crystals. O2? –Va + , O2? –Va + aggregate, Yb2+ , Ce3+ , Sm2+ , F, M (F2 ) absorption bands and group of four absorption bands are observed simultaneously in absorption spectra of colored ?uorite crystals. Current?time curve for electrolytic coloration of natural ?uorite crystal and its relationship with electrolytic coloration process are given. Production and conversion of color centers are explained. ? 2011 Elsevier B.V. All rights reserved.

Article history: Received 24 March 2011 Received in revised form 11 July 2011 Accepted 15 July 2011 Keywords: Natural ?uorite crystal Electrolytic coloration Color center

1. Introduction Pure calcium ?uoride crystal is an excellent optical window material. Some of calcium ?uoride crystals with appropriate color centers or impurities have very good optical and spectral features. They have been paid a deal of attention to their applications in laser [1], hologram [2–5] and spectral holeburning [6,7] etc. Fluorite is a natural calcium ?uoride mineral with general chemical formula CaF2 . Natural pure ?uorite is colorless and transparent. In general, natural ?uorite exhibits various colors due to different color centers or impurities. It is well known that natural ?uorite can also carry some important information on geological structure and radioactivity around ?uorite deposit in the earth. Therefore, the natural ?uorite can be used for dating or dosimetry [8,9]. Various color centers can be produced in ?uorite crystal by additive coloration, high-energy ray irradiation or high-energy particle bombardment [10]. However, the production of the color centers is very sensitive to impurities contained in the ?uorite crystal, in particular to oxygen impurities. Early researches have found that only and (that is the later F and M (F2 )) absorption bands are observed in absorption spectrum of additively colored natural ?uorspar containing oxygen impurities [11]. A group of four absorption bands is observed in absorption spectrum of X-rayed natural or synthetic calcium ?uoride crystals [12,13]. The electrolysis is an effective coloration method for producing color centers in some crystals. The quick-speed, visual observation, real-time monitor and control, environmental protection and

optional production of the color centers are the substantial advantages of the electrolysis. Moreover, the structure of the apparatus in the electrolysis is much simpler than that in the other coloration methods such as the high-energy ray irradiation and high-energy particle bombardment. In previous research, much attention has been paid to electrolytic coloration of alkali halide crystal, but little to that of alkaline earth halide crystal. In our recent work, lithium-doped strontium ?uoride crystals could be colored electrolytically at appropriate temperatures and voltages by using our homemade electrolysis apparatus [14]. In past electrolysis research, it was believed impossible to color directly electrolytically crystals containing anionic impurities because the impurities or their dissociated products, such as oxygen-related impurities, can prevent the formation of the secondary alkali cathode. The formation of the secondary alkali cathode is a very necessary condition to start electrolytic coloration through electron injection by using a pointed cathode and a ?at anode. Some electrolysis experiments used sodium ?uoride [15], sodium chloride, potassium chloride [16] and calcium ?uoride [17] crystals mainly bene?t from the substantial elimination of the hydroxyl impurities, the hydroxylfree or the very high purity of the original material. Therefore, no electrolytic coloration for alkaline earth halide crystal containing anionic impurities has been performed heretofore. In the present work, natural ?uorite crystals containing oxygen impurities are colored electrolytically by using the same electrolysis apparatus with a pointed cathode and a graphite anode matrix. F and F2 color centers are produced in colored ?uorite crystals. 2. Research details

? Corresponding author. Tel.: +86 022 27891344; fax: +86 022 27890681. E-mail address: jthgu@163.com (H. Gu). 1386-1425/$ – see front matter ? 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2011.07.056

Natural ?uorite crystals containing oxygen impurities are used and obtained commercially. The actual concentrations of the diva-

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Fig. 1. Structure scheme of electrolysis apparatus.

lent oxygen impurity ions are determined from the parameter of the characteristic absorption bands of the oxygen impurity ions. The ?uorite crystals are transparent and light green. Samples with size of several millimeters are cleaved from a large ?uorite crystal bulk. The samples are then colored electrolytically in a homemade apparatus at various temperatures (200–600 ? C) and DC voltages (300–1500 V) for several hours. The structure scheme of the apparatus used in the electrolytic coloration is shown in Fig. 1. A pointed tungsten cathode and a ?at stainless steel anode are used. Some coarse graphite powders damped with alcohol are used between the sample and anode in order to ensure good contact. The coarse graphite grains structure a graphite anode matrix. The sample is held in slowly ?owed dry and pure nitrogen during the electrolytic coloration to protect the electrodes against oxidation. The sample is put on a copper bulk for quenching to room temperature (RT) after the electrolytic coloration. Absorption spectra of the samples are measured with a spectrophotometer model UV-240 at RT. 3. Main results Fig. 2 shows the typical absorption spectrum of a natural ?uorite crystal containing oxygen impurities before electrolytic coloration. The thickness of the ?uorite crystal is 3.5 mm. In the absorption spectrum, the strong absorption band near 205 nm corresponds to O2? –Va + aggregate color centers [18]. The 259, 274 and 362 nm absorption bands correspond to Yb2+ impurity ions [19]. The 304 nm absorption band corresponds to Ce3+ impurity ions [20]. The 428 and 609 nm absorption bands correspond to Sm2+ impurity ions [21]. The two absorption bands in the blue and red light regions lead the uncolored ?uorite crystals to exhibit

Fig. 3. The absorption spectrum of the same ?uorite crystal as used in Fig. 2 after electrolytic coloration at temperature 294 ? C and voltage 900 V for 240 min. Inset shows a local enlarged curve.

Fig. 2. The absorption spectrum of a natural ?uorite crystal containing oxygen impurities before electrolytic coloration. The thickness of the ?uorite crystal is 3.5 mm. Dashed curves show resolved absorption peaks. Inset shows local enlarged curves.

the light green color. The 828 nm absorption band does not correspond to any known color centers or absorption bands. The high-energy side of the absorption spectrum can be resolved into three Gaussian-type absorption peaks at 185, 195 and 217 nm. The corresponding bandwidths are 0.80, 0.80 and 0.50 eV, respectively. The corresponding maximum absorption coef?cients are 2.44, 2.30 and 2.44 cm?1 , respectively. The absorption spectrum is plotted against photon energy in the resolution procedure. The absorption spectrum and resolved absorption peaks are replotted against light wavelength after the resolution. The 185 and 195 nm absorption peaks correspond to the O2? –Va + and O2? –Va + aggregate color centers, respectively [19]. The 217 nm absorption peak corresponds to the Sm2+ impurity ions [20]. The concentration of the divalent oxygen impurity ions in the ?uorite crystal is estimated at 4.70 × 1017 cm?3 according to the spectral parameters of the 185 and 195 nm absorption peaks and the Smakula’s formula. The calculated results show that the ?uorite crystals contain indeed more oxygen impurity ion than other ones. Fig. 3 depicts the absorption spectrum of the same ?uorite crystal as used in Fig. 2 after electrolytic coloration at temperature 294 ? C and voltage 900 V for 240 min. In the absorption spectrum, the 195 and 205 absorption bands correspond to the O2? –Va + aggregate color centers. The 216, 258, 273 and 363 nm absorption bands correspond to the Yb2+ impurity ions. The 304 nm absorption band corresponds to the Ce3+ impurity ions. The 383 and 550 nm absorption bands correspond to F and M (F2 ) absorption bands, respectively [10]. The 417 nm absorption band corresponds to the Sm2+ impurity ions. The 865 nm absorption band does not correspond to any known color centers or absorption bands. Fig. 4 shows the typical absorption spectrum of another natural ?uorite crystal containing oxygen impurities before electrolytic coloration. The thickness of the ?uorite crystal is 1.8 mm. In the absorption spectrum, the 194 nm absorption band corresponds to the O2? –Va + aggregate color centers. The 215 nm absorption band corresponds to the Sm2+ impurity ions. The 260, 273 and 363 nm absorption bands correspond to the Yb2+ impurity ions. The 304 nm absorption band corresponds to the Ce3+ impurity ions. The 428 and 618 nm absorption bands correspond to the Sm2+ impurity ions. By comparing the absorption spectra in Figs. 2 and 4, one can see that the distribution of the impurity ions in the large ?uorite crystal bulk is inhomogeneous. The 828 nm absorption band does not correspond to any known color centers or absorption bands. Fig. 5 presents the typical absorption spectrum of the same ?uorite crystal as used in Fig. 4 after electrolytic coloration at temperature 552 ? C and voltage 1200 V for 75 min. The local absorption

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Fig. 4. The absorption spectrum of a natural ?uorite crystal containing oxygen impurities before electrolytic coloration. The thickness of the ?uorite crystal is 1.8 mm.

Fig. 6. Curve of current?time for electrolytic coloration of a natural ?uorite crystal containing oxygen impurities by using a pointed cathode and a ?at anode at 552 ? C and 1200 V.

4. Discussion spectrum near 573 nm is resolved into two Gaussian-type absorption peaks at 550 and 580 nm. Both corresponding bandwidths are 0.40 eV. Similarly, the absorption spectrum is plotted against photon energy in the resolution procedure. The absorption spectrum and resolved absorption peaks are replotted against light wavelength after the resolution. In the absorption spectrum, the 200 nm absorption band corresponds to the O2? –Va + aggregate color centers. The 215, 259 and 273 nm absorption bands correspond to the Yb2+ impurity ions. The 304 nm absorption band corresponds to the Ce3+ impurity ions. The 573 nm absorption band consists of the 550 and 580 nm absorption peaks. The 386 and 550 nm absorption peaks correspond to the F and M absorption bands, respectively. The 225, 335, 400 absorption bands and 580 nm absorption peak correspond to the group of four absorption bands [12]. The 225, 335, 400 and 580 nm absorption bands are connected to electrons trapped by Ca2+ interstitials, holes trapped in Ca2+ vacancies, neutral ?uorine atoms in interstitials and general lattice distortion, respectively [22]. The typical current?time curve for electrolytic coloration of a natural ?uorite crystal containing oxygen impurities by using a pointed cathode and a ?at anode at temperature 552 ? C and voltage 1200 V is presented in Fig. 6. The current?time curve is very similar to that of the alkali halide crystal containing anionic impurities [23], and displays three different zone-regimes, as indicated. As shown above, the used natural ?uorite crystals contain indeed more O2? –Va + and O2? –Va + aggregate color centers before the electrolytic coloration (see Figs. 2 and 4). In the formation process of the natural ?uorite crystal, oxygen-related impurities are directly included in the ?uorite crystal. O2? impurity ions may form in the ?uorite crystal. Some O2? impurity ions are combined with neighboring anion vacancies (Va + ) to form O2? –Va + color centers for maintaining local electric neutralities. Some O2? –Va + color centers may aggregate to form O2? –Va + aggregate color centers. Therefore, the absorption bands of the O2? –Va + and O2? –Va + aggregate color centers appear in the absorption spectra of the uncolored ?uorite crystals (Figs. 2 and 4). Some O2? –Va + and O2? –Va + aggregate color centers may dissociate during the electrolytic coloration, which results in the decrease of the corresponding absorption bands (see Fig. 3). Similarly, the Yb2+ , Ce3+ and Sm2+ impurity ions are directly included in the ?uorite crystal in the formation process of the ?uorite crystal. The corresponding absorption bands of the impurity ions appear in the absorption spectra of the uncolored and colored ?uorite crystals. The absence of the absorption bands of the Sm2+ impurity ions (see Fig. 5) is very likely due to their disappearance inside the other intense absorption bands. As described above, the natural ?uorite crystals containing oxygen impurities can be colored electrolytically at various temperatures and voltages. F and F2 color centers are produced in the colored ?uorite crystals. The electrolytic coloration of the ?uorite crystals should mainly bene?t from appropriate electrolysis temperatures and voltages as well as anode structure of used electrolysis apparatus. Moreover, the various color centers can be produced optionally or simultaneously in the natural ?uorite crystals by using the present electrolysis method. This electrolysis may become a very ef?cient method for preparing dating, dosimetry and optoelectronic materials using synthetic or natural crystals in the future. In a general electrolytic coloration of pure alkali halide crystal by using a pointed cathode and a ?at anode, the formation of the secondary alkali cathode is a very necessary condition to start the electrolytic coloration through electron injection [24]. After the secondary alkali cathode is formed, electrons can be emitted from the secondary alkali cathode and injected into the alkali halide crystal. F color centers are produced directly in the alkali halide crystal. Therefore, we think that it is the secondary metal cathode to emit the electrons. The corresponding metal ion must be a constituent part of the used crystal. In our recent electrolytic coloration of alkaline earth ?uoride crystal [14], the secondary alkaline earth cathode

Fig. 5. The absorption spectrum of the same ?uorite crystal as used in Fig. 4 after electrolytic coloration at temperature 552 ? C and voltage 1200 V for 75 min. Dashed curves show resolved absorption peaks.

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should be formed. Then, electrons are emitted from the secondary alkaline earth cathode. The emitted electrons are injected into the lithium-doped strontium ?uoride crystal during the electrolytic coloration, which result in direct production of F color centers in the lithium-doped strontium ?uoride crystal. Therein, there is not too much penetrating oxygen, which can be neutralized by doped lithium ions. That is, the penetrating oxygen has not obstructed the formation of the secondary alkaline earth cathode and electron injection. However, such a process cannot occur in the electrolytic coloration of the natural ?uorite crystal containing oxygen impurities. In the absorption spectra of the ?uorite crystals before and after electrolytic coloration (Figs. 2–5), there are intense absorption bands of the O2? –Va + and/or O2? –Va + aggregate color centers. These results show that more O2? –Va + and O2? –Va + aggregate color centers in the ?uorite crystals have not been neutralized by the Yb2+ , Ce3+ and Sm2+ impurity ions. The O2? –Va + and O2? –Va + aggregate color centers can prevent the formation of the secondary alkaline earth cathode, which results in that F color centers cannot be produced directly in the ?uorite crystals. Like the previous electrolytic coloration of the hydroxyl-doped sodium chloride crystal by using a pointed cathode and a ?at anode [23], the present electrolytic coloration of the natural ?uorite crystal containing oxygen impurities is not through electron injection, too. Our previous researches have proved that V-type color centers are ?rstly produced. F color centers are secondly produced through the photo-conversion of the V-type color centers in the alkali halide crystal doped with anions [23]. Therefore, we think that such basic formation mechanism of color centers is suitable for the present electrolytic coloration of the natural ?uorite crystal containing oxygen impurities. Namely, V-type color centers are ?rstly produced from ?uorite crystal region nearby the graphite anode matrix because the production of the V-type color centers does not depend on a secondary alkaline earth cathode. The main production process is that the halogen ions located at the ?uorite crystal region nearby the graphite anode matrix may exchange electrons with the graphite anode matrix and convert to halogen atoms. Some halogen ions may combine with halogen atoms to form halogen molecular ions. The halogen molecular ions stabilized near impurities or imperfections constitute V-type color centers [25]. However, such electron exchanges can hardly occur without the coarse graphite grains in the graphite anode matrix. The grains act as multiple pointed anodes. These anodes can produce enough high electric ?eld strengths in the crystal region nearby the grains. The electric ?eld strengths facilitate the electron exchanges during the electrolytic coloration. The migration of the V-type color centers results in the coloration of the other region of the ?uorite crystal. The V-type color center migrates through the diffusion of the interstitial halogen atom in the V-type color center. The reason is that the electrons in the coloring ?uorite crystal are de?cient, which is favorable to the diffusion of the interstitial halogen atom. The electron exchanges between the halogen ions and anode result in electron de?ciency inside the ?uorite crystals during the electrolytic coloration [26]. It is observed that the coloration of the ?uorite crystal takes place immediately after voltage is applied. F color centers can be produced from the photo-conversion of the Vtype color centers according to excitonic mechanism [27] because it is not completely dark during electrolytic coloration, quenching and spectral measurements. The V-type color centers in the ?uorite crystal are unstable at RT or higher. They may be metastable products. The corresponding absorption bands cannot be observed at RT. In the meantime, a reaction may occur under light illumination: F + F → F2 . Therefore, the M absorption band is observed in the absorption spectra of the colored ?uorite crystals (see Figs. 3 and 4). During electrolytic coloration, current?time data of natural ?uorite crystal containing oxygen impurities are recorded. The property of the current–time curve of the ?uorite crystal is very

similar to that of the alkali halide crystal containing anionic impurities [23]. Therefore, we think that the basic relationship between the current?time curve and electrolytic coloration process of the alkali halide crystal is very suitable for the present electrolytic coloration process of the ?uorite crystal. That is, through the electron exchange, V-type color centers can be produced in the whole process of the electrolytic coloration of the ?uorite crystal. The electron exchanges can induce a part of the current in the whole process of the electrolytic coloration. Similarly, the ionic motion under the action of the applied voltage at high temperatures can induce a part of the current in the whole process of the electrolytic coloration, too. The current component induced by the ionic motion is dominant in the ?rst-zone current. The current component induced by the electron exchange is dominant in the current of the other zones. Some V-type color centers are produced in the time interval of the ?rst current zone. More V-type color centers are produced by the “snowslide” effect [28] in the time intervals of the other current zones. In addition, herein current undulations also result from electrode effect [28], which forms the current zone 3. Moreover, the color center distribution in an electrolytically colored crystal is not very homogeneous, which is caused mainly by the electric ?eld gradient formed by using only a single-point cathode. In the practice application, a homogeneous color center distribution in a crystal is expected by using a multi-point cathode [29]. 5. Conclusion The present results have shown that the natural ?uorite crystals containing oxygen impurities can be colored electrolytically at various temperatures and voltages. Production and conversion mechanisms of color centers in the colored crystals are explained. V-type color centers were directly produced during the whole process of the electrolytic coloration through the electron exchanges between the halogen ions and graphite anode matrix. The graphite anode matrix plays a key role. Some V-type color centers are produced directly in the time interval of the ?rst current zone. More V-type color centers are produced in the time intervals of the other current zones. The migration of the V-type color centers results in the coloration of the other region of the ?uorite crystal. The V-type color centers migrate through the diffusion of the interstitial halogen atoms in the V-type color centers. The F color centers are formed by the photo-conversion of the V-type color centers under light illumination. F-aggregate color centers are formed by aggregations of F color centers under light illumination. The current of the electrolytic coloration consists of the two current components induced by the electron exchange and ionic motion. The current component induced by the ionic motion is dominant in the ?rst-zone current. The current component induced by the electron exchange is dominant in the current of the other zones. The electrolysis may become a very ef?cient method for preparing dating, dosimetry and opto-electronic materials using synthetic or natural crystals in the future due to the optional or simultaneous production of the various color centers. Acknowledgements This work is partly supported by National Natural Science Foundation of China under Grant no. 69178028. References
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