ISSN 00201685, Inorganic Materials, 2014, Vol. 50, No. 12, pp. 1277–1282. © Pleiades Publishing, Ltd., 2014. Original Russian Text © M.N. Brekhovskikh, V.A. Fedorov, 2014, published in Neorganicheskie Materialy, 2014, Vol. 50, No. 12, pp. 1363–1368.
Purification of Fluorides for Optical Materials Synthesis M. N. Brekhovskikh and V. A. Fedorov Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow, 119991 Russia email:
[email protected] Received June 10, 2014
Abstract—This paper considers processes for the ultrapurification of starting materials for the preparation of heavy metal fluoride glasses with the lowest possible impurity content. Based on the results of physicochem ical studies of reactions between fluorination agents and components of fluoride glasses, we have proposed novel techniques for the preparation of fluorides and glasses free of oxygencontaining impurities. DOI: 10.1134/S0020168514120036
The development of laser engineering and fiber optics for communication systems is impossible with out novel transparent materials for transmitting opti cal radiation. Lowloss fluoride fibers are necessary for creating fiberoptic sensors, lowtemperature pyrom eters, laser microsurgery devices, technical diagnostic tools, and gain media of IR lasers. Rareearthdoped fluoride fibers hold the most promise for producing highperformance fiber amplifiers and lasers [1–7]. The production of fluoride glasses and fibers requires fiber optic grade fluorides. The development of ultrapurification processes for the preparation of fluoride glasses with the minimum possible impurity concentration is a challenging prob lem. The main difficulty in developing the technology of fluorides as key components of optical materials is related to the ultrapurification step, which is compli cated by the high reactivity of fluorides for construc tion materials, their hygroscopicity, and their ten dency toward pyrohydrolysis. To minimize the loss in real singlemode optical fibers, one should ensure high purity (at a level of 10–7 to 10–6 wt %) of starting materials and glasses in terms of impurities absorbing in the near and midIR spec tral regions (rareearth and transition metals, hydroxyl groups). It is well known [8] that the main impurities absorbing in these spectral regions (1–5 μm) are hydroxyl groups, groups containing oxide and halide bridges, a number of transition metal cations, and car bon inclusions captured by molten glass during cool ing. For example, transmission spectra of almost all fluoride glasses contain an absorption band near 2.9 μm (stretching mode), corresponding to the presence of hydroxyl ions in the glass. The absorption bands of other oxygencontaining impurity groups and carbon inclusions are located at long wavelengths (λ > 5 μm), so they have a weaker effect on the absorption of radi
ation with a working wavelength of 2.55 μm. Iron, cobalt, nickel, and copper impurities absorb in the range 1.5–2.5 μm. In the case of fluoride glasses containing LaF3 or GdF3 [8], which have no absorption bands in the range 0.4–7 μmm, undesirable related impurities include Sm, Tb, Ce, Pr, Nd, Dy, and Eu, which have absorp tion bands in the range 0.8–6 μm [8]. The ability to remove such impurities is of great importance from the viewpoint of reducing the optical absorption loss, extending the working range, and increasing the transmission of optical fluoride materi als and eliminating microinhomogeneities. According to the preparation process, fluorides can be divided into two groups: (1) fluorides that are pre pared and purified by a hydrometallurgical process (BaF2, CaF2, SrF2, YF3, AlF3) and (2) fluorides that are purified in the final step by a dry process (ZrF4, HfF4) [9]. Fedorov with Churbanov described a multipurpose apparatus that allows one to precipitate fluorides both during heating (Ba, Na, Y, and Sr fluorides) and, if necessary, during cooling of a working mixture (Ca and Al fluorides). In the former process, use is made of a Teflon reactor with an external heater. The latter process employs a reactor with a submerged flowtype Teflon cooler. In early studies of fluoride glasses, because of the high volatility of ZrF4 and HfF4 the most convenient and effective way to purify them was sublimation at temperatures from 600 to 700°C in a fluorination atmosphere [10, 11]. Using a combination of pro cesses indicated in Table 1, one can achieve the most complete removal of undesirable impurities from ZrF4 and, using other purified components, obtain fluo
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Table 1. Methods for the preparation of extrapuregrade zirconium tetrafluoride [10, 11] Process
Key steps
Chemical purification in the gas phase
Preparation of ZrCl4 by chlorinating metallic Zr, followed by fluorina tion with elemental fluorine
Sublimation, distillation
Chemical vapor transport at t > 800°C
Extraction
Extraction from an aqueous ZrOCl2 solution by organic solvents
Treatment in a fluorination atmosphere
Reaction of gaseous nonmetal fluorides with oxygen impurities in melts
rozirconate glasses and optical fibers with an intrinsic optical loss near 1 dB/km at a wavelength λ = 2.5 μm. Currently, ZrF4 and HfF4 are purified by sublima tion in flowing dry nitrogen [9]. One sublimation cycle reduces the iron content of the material by 20 times and the Ni, Cr, and Cu concentrations to 0.01–0.1 ppm. The final stage of the process includes hightem perature hydrofluorination and fluorination in order to remove oxygen and impurities that form volatile compounds with fluorine. To obtain fluoride glasses
Fig. 1. Quartz reactor with a glassy carbon crucible enclosed in it.
and optical fibers, a team headed by M.F. Churbanov synthesized increased pilot batches of extrapuregrade fluorides: ZrF4, HfF4, BaF2, LaF3, AlF3, NaF, MgF2, CaF2, SrF2, YF3, TmF3, PrF3, NdF3, ErF3, TbF3, PbF2, InF3, and others. The content of socalled col oring impurities in the fluorides obtained was under 1–3 ppm, and the oxygen and OH contents were below 0.01–0.05 wt % [9]. Electrochemical removal of Fe ions is used as a pre liminary step in fluoride glass preparation [12]. Since glasses are chemically inert under ordinary conditions and reactant diffusion rates in the glass bulk and through the glass surface are not high even at elevated temperatures, to obtain impurityfree optical materi als the starting reagents or glass batches are first puri fied chemically, and the other synthesis steps are con ducted under conditions that prevent possible con tamination of the glass with container corrosion products, gaseous impurities, etc. The preparation of highpurity zirconium fluoride by sublimation and distillation from a ZrF4–BaF2 melt in a specially designed induction furnace (Fig. 1, Table 2) was also described in detail by MacFarlane et al. [13]. They proposed a combined technique for analysis for transition metal cation impurities, which comprised ZrF4 dissolution, complexation of transi tion metal impurities, extraction, ZrF4 distillation, and analysis. In Fig. 1, the heater of the induction fur nace and the cooling system of the quartz reactor are omitted. This approach allows one not only to reduce the iron, cobalt, and nickel concentrations but also to lower the OH– and carbon concentrations (Fig. 2, IR spectroscopy data) in purified ZrF4 samples. Newman et al. [14] proposed an analytical proce dure for iron, copper, nickel, and cobalt, which includes zirconium fluoride (glass former) dissolution in a solution of a mixture of boric, nitric, and hydrof INORGANIC MATERIALS
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luoric acids, followed by the complexation of transi tion metal ions with methyl isobutyl ketone in an alka line solution and direct determination of the ions from the mother liquor by atomic absorption spectroscopy in a graphite furnace. The detection limits of this pro cedure for iron, copper, nickel, and cobalt ions are 4, 2, 3, and 4 ppb, respectively. Extraction and chromatographic methods for the removal of transition metals from starting rareearth compounds in the preparation of fluoride glasses were described by Stary [15]. However, the efficiency of the removal of oxide, hydroxide, and oxygendoped triflu oride impurities from rareearth metals is one to three orders of magnitude lower in comparison with zirco nium, hafnium, and thorium fluorides. Thus, it is rea sonable to expect that rareearth fluorides are major contamination sources in fluoride glasses. Properties of fluorination agents. One cause of the formation of phase inhomogeneities in glasses is the presence of poorly soluble impurities consisting of oxides and oxyfluorides in glassforming melts. It fol lows from the correlation between the oxygen content and optical loss that the permissible oxygen content of glasses should not exceed 0.1 ppm (for optical fiber fabrication). To remove oxygencontaining impurities from glass components, wide use is made of fluorina tion agents. In previous work, fluoride glasses were prepared in an ammonium bifluoride (NH4F ⋅ HF) atmosphere by heating a glass batch and holding it at 500°С for 1–2 h. The glass batch was then heated until melting at 800–1000°С. This led to a number of reac tions [16]: 2ZrO2 +7NH4F ⋅ HF → 2(NH4)3ZrF7 + NH3 + 4H2O, NH4F ⋅ HF + NH3 → 2NH4F, M2O3 + 3NH4F ⋅ HF → 2(NH4)3MF6 + 3H2O (M = Sс, Y, Ln, Al, In),
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Table 2. Concentration of transition metal impurities in ZrF4 samples after purification by sublimation and distilla tion [13] Impurities, ppm ZrF4 sample (Morita Kakagu, Japan)
iron (±0.04)
nickel (±0.02)
copper (±0.02)
1.06
0.53
0.18
1.20
0.46
0.22
0.53
0.25
0.10
0.44
0.20
0.06
0.18
0.21
0.10
0.26
0.27
0.11
0.12
0.21
0.12
0.14
0.20
0.10
0.24
0.21
0.12
Commercial
After sublimation
After distillation
rareearth oxyfluorides in fluorides of these metals is 0.1–1 wt %). To reduce the content of oxide and hydroxide impurities in the resultant glassy materials, the glass batch and glass melts are exposed to an atmosphere of nonmetal fluorides (BF3, HF, CF2Cl2, CFCl3, CF4, C2F4, and others [18]) in order to substitute fluorine for oxygen. The efficiency of fluorineforoxygen sub
OH– + F– → [O]2– + HF, etc. With increasing temperature, the ammonium com plexes of zirconium decompose in several steps according to the following scheme [17] (NH4)3ZrF7 357°C
297°C
NH4ZrF5
(NH4)2ZrF6 410°C
1
2
ZrF4.
One advantage of this process is the possibility of using readily available reagents, and its serious draw back is the formation of water vapor in the additional fluorination steps. This approach ensures a relatively low degree of removal of oxygen impurities from glass batches (the content of zirconium, hafnium, or INORGANIC MATERIALS
Transmission
F– + H2O(g) → OH– + HF(g),
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5000 4500 4000 3500 3000 2500 2000 1500 Wavenumber, cm–1
Fig. 2. IR transmission spectra of (1) commercial ZrF4 (Morita Kakagu) and (2) purified ZrF4.
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wt % oxygen
10–1
200°C
200°C
200°C
10–2
10–3
300°C 500°C 400°C
XeF2
300°C 500°C 400°C
ClF3
300°C
500°C 400°C
BrF3
Fig. 3. Removal of oxygen impurities from rareearth, zir conium, hafnium, and thorium fluorides at elevated tem peratures by xenon fluoride (XeF2), chlorine trifluoride (ClF3), and bromine trifluoride (BrF3).
stitution reactions is, however, insufficient for obtain ing IRtransparent materials. In connection with this, attempts have been made to use more reactive fluo rinecontaining reagents. For example, considerable research effort has been devoted to reactions of various compounds, including rareearth oxides, with elemental fluorine, chlorine trifluoride, bromine trifluoride [19], and krypton dif luoride [20]. The rareearth metals were found to form tri or tetrafluorides. The particular character of these processes is, however, such that oxygenfree rareearth fluorides are difficult to obtain, and this requires pro longed holding at the synthesis temperature. In partic ular, the oxyfluoride content of LnF3 prepared by reaction with F2 ranges from a few tenths of a weight percent to several weight percent. Complete fluorine substitution for oxygen in rareearth oxides via low temperature (0–20°C) fluorination with krypton dif luoride takes several days [20]. Nabiev et al. [21] inves tigated the fluorination properties of krypton diflu oride in the anhydrous systems KrF2–HF and KrF2–BrF5. Given that the fluorides of the Group III and IV elements tend to experience pyrohydrolysis, we pro posed using volatile inorganic fluoride oxidants based on nonmetal fluorides (xenon, chlorine, and bromine fluorides) in fluoride systems in the preparation of glasses containing no absorption bands of OH– groups in the IR spectral region. It is known that, in organic chemistry, bromine trifluoride and bromine pentaflu oride are used in the fluorine oxidation of olefins and nitriles [22–24], and xenon difluoride, in the fluo rine oxidation of ketones and aromatic compounds [25, 26]. The use of reactive fluoride oxidants involves a number of difficulties related to the possible corrosion
of the apparatus and some side processes, including the formation of anion fluoro complexes, such as M2XeIVF6, MIXFx, and MII(XFx)2 (where MI is an alkali metal, MII is an alkalineearth metal, and X is a halogen) [27–29]. Tetravalent fluoroxenates may form with the participation of both XeF4 and XeF2. As shown by Kiselev and Goryachenkov [30], xenon dif luoride partially disproportionates according to the scheme 2XeF2 ↔ XeF4 + Xe. At temperatures above 350°С, the degree of con version is about 10 mol %. It is worth pointing out that the formation of anion fluoro complexes of nonmetals generally should not impede the preparation of fluo ride glasses, because M2XeIVF6 and MIXFx decompose at temperatures as low as 200–600°С [31] to give vol atile gaseous nonmetal fluorides and crystalline MIF or MIIF2. These compounds are known to be compo nents of batches for fluoride glass preparation. Form ing directly in the system, in subsequent sintering and melting processes they are capable of forming the same glasses as the alkali and alkalineearth fluorides present in the glass batch. Studies of the chemical transformations of rare earth oxide compounds and zirconium, hafnium, and thorium oxides with the participation of xenon, chlo rine, and bromine fluorides as oxidants provided insight into the conditions of the formation of binary and mixed fluorides and molecular oxygen release [32]. The fluorination of oxides occurs in several steps, with LnOF oxyfluorides as reaction intermediates: Ln2O3 + XeF2 → 2LnOF + Xe + 0.5O2, LnOF + XeF2 → LnF3 + Xe +0.5O2. In most cases, reaction between Ln2O3 and gaseous XeF2 occurs at t 350°C. Tetravalent metal oxides react with XeF2 according to the following scheme: IV
M O 2 + 2XeF 2 → MF 4 + 2Xe IV
+ O 2 ( M = Ce, Zr, Hf, Th ). Based on our results, we have developed a method ological approach to the preparation of fluoride glasses which includes the treatment of a glass batch in chem ically active media (fluorine, xenon difluoride, chlo rine trifluoride, and bromine trifluoride) (Fig. 3) and allows one to obtain glasses with concentrations of oxygencontaining impurities two orders of magni tude lower (within 10–3 wt %) than those in the start ing fluorides (10–1 wt %). Such glasses do not have absorption bands of OH– groups or bridging oxygen and possess a broad transmission window, from the nearUV to the midIR spectral region (0.295 to 8 μm) [32]. INORGANIC MATERIALS
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In addition, as illustrated by the example of Ba(BrF4)2, in the region of their thermal instability the anion fluoro complexes of nonmetals are capable of further fluorinating oxide impurities in an inert filler. The use of such crystalline nonvolatile fluoride oxi dants as reagents for additional fluorination agents of glass batches has considerable potential for the removal of oxygencontaining impurities from fluo ride glasses [34]. Thus, we have considered processes for the ultra purification of starting materials for the preparation of fluoride glasses with the lowest possible impurity con tent. Based on the results of physicochemical studies of reactions between fluorination agents and compo nents of fluoride glasses, we have proposed novel tech niques for the preparation of fluorides and glasses free of oxygencontaining impurities. ACKNOWLEDGMENTS This work was supported in part by the Russian Foundation for Basic Research, grant no. 1203 00531. REFERENCES 1. Poulain, M., Poulain, M., Lucas, J., and Brun, P., Verres fluores au tetrafluorure de zirconium: proprietes optiques d’un verre dope au Nd3+, Mater. Res. Bull., 1975, vol. 10, no. 4, pp. 243–246. 2. Dianov, E.M., Dmitruk, L.N., Plotnichenko, V.G., and Churbanov, M.F., Optical fibers based on highpurity fluoride glasses, Vysokochist. Veshchestva, 1987, no. 3, pp. 10–34. 3. Sakharov, V.V., Baskov, P.B., Akimova, O.V., Berikash vili, V.Sh., and Lebedev, G.F., Fluoride glasses for mul tifunctional multifiber systems, Inorg. Mater., 2008, vol. 44, no. 12, pp. 1386–1392. 4. Katsuyama, T. and Matsumura, H., Infrared Optical Fibres, Bristol: Adam Hilger, 1989. 5. Adam, J.L., Nonoxide glasses and their applica tions in optics, J. NonCryst. Solids, 2001, vol. 287, pp. 401–404. 6. Fedorov, P.P., Crystallochemical aspects of fluoride glass formation, Crystallogr. Rep., 1997, vol. 42, no. 6, pp. 1064–1075. 7. Sanghera, J.S. and Aggarwal, I.D., Infrared Fiber Optics, Boca Raton: CRC, 1998. 8. Poignant, H., Role of impurities in halide glasses, in Halide Glasses for Infrared Fiberoptics, Amsterdam: Martinus Nijhoff, 1987, pp. 35–36. 9. Fedorov, V.D., Sakharov, V.V., Baskov, P.B., Pro vorova, A.M., Churbanov, M.F., Plotnichenko, V.G., Ioahim, P.H., Poulain, M., Kirhof, J., and Kobelka, J., Development of highpurity fluoride glasses and fibers for instrument engineering, Ross. Khim. Zh., 2001, vol. 45, nos. 5–6, pp. 51–57.
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Translated by O. Tsarev
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