ISSN 00201685, Inorganic Materials, 2014, Vol. 50, No. 9, pp. 924–929. © Pleiades Publishing, Ltd., 2014. Original Russian Text © M.N. Brekhovskikh, V.A. Fedorov, 2014, published in Neorganicheskie Materialy, 2014, Vol. 50, No. 9, pp. 1000–1005.
ON THE OCCASION OF THE EIGHTYFIFTH BIRTHDAY OF ACADEMICIAN V.B. LAZAREV
IRTransparent Glassy Materials Based on Halides of Group I–IV Elements 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 March 25, 2014
Abstract—This paper addresses the preparation, purification (removal of oxygencontaining impurities), and applications of fluoride glasses, with particular attention to the preparation of modified glasses for extending their transmission window. DOI: 10.1134/S0020168514090027
INTRODUCTION The development of laser engineering and fiber optics is impossible without novel transparent materi als for transmitting optical radiation. Rareearth doped optical halide materials with a lowfrequency phonon spectrum are of great interest for midIR laser development. Halide single crystals with a broad IR transmission window, which might be used as laser hosts, have not found wide application for a number of reasons, in particular because of their hygroscopicity and hydration in air, incongruent melting behavior in a number of cases, and the small isomorphous capac ity for rareearth dopants. It is for this reason that halide glasses, free of these drawbacks, may be poten tially attractive as laser materials. Fluoride glasses. Glass formation in fluoride sys tems was first detected by A.V. Novoselova in studies of beryllium fluorides. [BeF4]2 tetrahedra were shown to act as glassformers in fluoroberyllate glasses [1]. The higher bond ionicity in fluoride glasses compared to oxide glasses leads to a higher degree of order in their structural network and, accordingly, to unique optical properties: minimum refractive indices, minimum dispersion, and narrow luminescence bands of dopant ions. In 1974, the group headed by Prof. Lucas (Univer sity of Rennes, France) discovered a large family of new glasses, which are now known as heavy metal flu oride glasses [2]. The studies of fluoride glasses in the 1980s–1990s have been the subject of several review papers [3–7]. The many fluoride glasses obtained since the preparation of the first fluorozirconate glass can be divided into distinct groups according to the type of glassformer (Table 1). Also indicated in Table 1 are the glass transition temperature (tg), crys tallization temperature (tx), and melting point (tm) for glassforming compositions.
Table 2 lists the main glassforming fluoride sys tems, the compositions of some glasses, and their physicochemical properties [3–7]. One important advantage of these glasses over quartz glass is that they have a considerably broader transmission window: from the nearUV to midIR spectral region (0.295–7.5 μm). The longwavelength limit of transmission in the IR spectral region and the low Rayleigh scattering level suggest that the intrinsic attenuation in the fluoride glasses may be on the level of 0.01 dB/km. Electronic absorption, Rayleigh scat tering (due to density and composition inhomogene ities), and vibrational absorption are thought of as intrinsic types of optical loss because they are inherent in the material per se (Fig. 1) [8]. This limit can only be approached by identifying and controlling extrinsic loss sources. Hydroxyl ions captured by optical fibers from starting materials or in the glass preparation pro cess strongly absorb IR radiation. Estimates suggest that the presence of 1 ppm of hydroxyl ions may lead to an attenuation near 104 dB/km at a wavelength of 2.9 μm. Figure 2 shows transmission spectra of various types of glasses [5]. The highest transmission is offered by the fluoride glasses containing heavy cations, and the lowest transmission, by the fluoroaluminate glasses. At a thickness of 2 mm, the absorption edge of the ZBLAN fluorozirconate glasses lies at ~7 μm. The absorption edge of the BaInGa glasses is shifted to longer wavelengths by 1.5 μm, and that of the ZnSrBa glasses, by 2 μm [5, 9]. Despite the lowfrequency phonon spectrum of fluoroindate glasses and glasses based on divalent cations, considerable research effort is still concentrated on the ZBLAN glasses. The mid1990s saw the advent of silica glass fiber amplifiers, which made it possible to create longhaul communication links. At the same time, immense sci entific and technological advances in the preparation
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Table 1. Main glassforming fluoride systems, compositions of the glasses, and their physicochemical properties Temperature, °C Glassforming system
Characteristic composition Fluorozirconate systems [3, 5, 6] 53ZrF4 ⋅ 20BaF2 ⋅ 4LaF3 ⋅ 3AlF3 ⋅ 20NaF
ZrF4–BaF2–LaF3–AlF3–NaF (ZBLAN) ZrF4–BaF2–LaF3–AlF3 (ZBLA) 57ZrF4 ⋅ 34BaF2 ⋅ 5LaF3 ⋅ 4AlF3 Fluoroaluminate systems [7] 30.2AlF3 ⋅ 10.6BaF2 ⋅ 20.2CaF2 ⋅ 8.3YF3 ⋅ 3.5MgF2 ⋅ AlF3–BaF2–CaF2–YF3 3.8NaF ⋅ 13.2SrF2 ⋅ 10.2ZrF4 Fluoroindate systems [4] 50InF3 ⋅ 10BaF2 ⋅ 40YF3 InF3–BaF2–YF3 40GaF3 ⋅ 20InF3 ⋅ 40BaF2 BaF2–InF3–GaF3 (BIG) 15InF3 ⋅ 10YF3 ⋅ 20GaF2 ⋅ 25PbF2 ⋅ 15CaF2 ⋅ 15ZnF2 40InF3 ⋅ 20ZnF2 ⋅ 20BaF2 ⋅ 10GdF3 ⋅ 10SrF2 40InF3 ⋅ 20ZnF2 ⋅ 30BaF2 ⋅ 10GdF3 Systems based on divalentmetal fluorides [4, 5] 36PbF2 ⋅ 24ZnF2 ⋅ 35GaF3 ⋅ 5YF3 ⋅ 2AlF3 PbF2–ZnF2–GaF3 (PZG) 47ZnF2 ⋅ 5CdF2 ⋅ 6InF3 ⋅ 4GaF3 ⋅ 2LaF3 ⋅ 26SrF2 ⋅ 10BaF2 ZnF2–SrF2–BaF2
tg
tx
tg–tx
tm
258
363
105
455
307
392
85
388
482
94
673
333 364 270 300 309
416 438 390 405 392
83 74 120 105 83
582 573
270 306
325 380
55 74
547 615
tg, tx, and tm are the glasstransition, crystallization, and melting temperatures, respectively, for the glassforming compositions.
Table 2. Physicochemical properties of fluoride glasses [3] Property
Range
Glass transition temperature tg, °C
260–400
Difference between the crystallization and glass transition temperatures tx – tg, °C
20–150
Critical cooling rate Rc (ZBLAN glass), °C/min Density d,
1
g/cm3
3.7–5.7
Microhardness, kg/mm2
230–370
Refractive index nd
1.48–1.67
Thermal expansion coefficient α ×
10–7,
K–1
140–200
Working IR range, μm
2–7 –1–10–3
Theoretical minimum of intrinsic optical loss, dB/km
10
Wavelength corresponding to the minimum of intrinsic optical loss, dB/km
2.5–3.5
Viscosity of softened glass at Tm, P
105–106
of highpurity fluorides of Group I–IV elements and fluoride glasses initiated studies concerned with the application of these materials in other areas, such as laser engineering (fiber lasers and amplifiers, planar waveguides from rareearthdoped fluoride glasses), scintillators, and IR optical components, including passive optical fibers [10]. Despite the large number of glassforming fluoride systems, considerable research effort is concentrated on the ZBLAN glasses (system formed by zirconium, barium, lanthanum, aluminum, INORGANIC MATERIALS
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and sodium fluorides). These glasses are the most resistant to crystallization, which allows one to pro duce optical fibers of high optical quality and bulk glass samples. Table 3 presents the theoretical loss in oxide, chal cogenide, and fluoride glasses and the minimal optical loss in optical fibers [11, 12]. It is seen that the theo retical loss in the fluoride glasses at a wavelength of 2.5 μm, corresponding to the theoretical loss mini mum, is an order of magnitude lower than that in
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BREKHOVSKIKH, FEDOROV 100
Infrared
Visible
Percent transmission
Ultra violet
Electronic absorption
Optical loss
Vibrational absorption
Optical loss minimum
80 60
ZBLAN BIG
ZnSB
40 20 AIBCMSY 0 5
3
7 9 11 Wavelength, μm
13
15
Rayleigh scattering
Fig. 2. IR transmission spectra of fluoride glasses in differ ent glassforming systems [5]: (ZBLAN) ZrF4–BaF2– LaF3–AlF3–NaF, (BIG) BaF2–InF3–GdF3, (AlBC MSY) AlF3–BaF2–CaF2–MgF2–SrF2–YF3, (ZnSB) ZnF2–SrF2–BaF2.
Wavelength
Fig. 1. Attenuation curve of optical fibre [8].
quartz glass. It is for this reason that there is immense interest in fluoride glasses as materials for optical fiber communication systems, especially for longhaul communication links. Since extensive research efforts have been focused on the technology of novel, highperformance optical media for lasers, amplifiers, and nonlinear frequency converters in the IR spectral region and also for IRto visible light converters capable of improving the effi ciency of solar cells and exciting photocatalysts [13], we proposed a study aimed at developing new, crystal lizationresistant rareearthdoped glasses in which substitution of heavier cations, hafnium and indium, for zirconium and aluminum and partial chlorine and bromine substitutions for fluorine reduce the maxi mum phonon energy in optical media in comparison with unmodified glasses. The purpose of this work was to investigate modi fied ZrF4–BaF2–LaF3–AlF3–NaF (ZBLAN) fluo ride glassforming systems with enhanced resistance to crystallization by introducing heavy ions (Hf4+, In3+, Pb2+, Gd3+, and Cl–) in order to extend their IR transmission window, reduce the relaxation loss in rareearthdoped glasses, and assess their spectro scopic properties.
EXPERIMENTAL PROCEDURE AND DISCUSSION We studied glass formation in the hafnium fluoride based system HBLAN, similar in cation stoichiometry to the ZBLAN fluorozirconate glasses, with partial or complete substitutions of heavier cations (In3+, Gd3+, and Pb2+) and chlorine anions for the Al3+, La3+, and Ba2+ cations and fluorine anions. In our experiments, we used glass with the composition 57HfF4 ⋅ 20BaF2 ⋅ 3LaF3 ⋅ 3AlF3 ⋅ 17NaF. The following partial or com plete substitutions were performed (the substituents are indicated in parentheses): BaF2(PbF2)– AlF3(InF3); NaF(NaCl)–BaF2(PbCl2); AlF3(InF3)– BaF2(BaCl2), and AlF3(InF3)– BaF2(BaCl2)–NaF(NaCl). The ratio of cations of the same valence was main tained constant, corresponding to the above formula (Fig. 3). To remove oxygencontaining impurities, we used, for the first time, xenon, chlorine, and bromine fluorides as volatile inorganic fluorooxidizers, as was suggested by studies of the mechanisms of the oxida tion of oxides of Group III and IV elements by these fluorination agents [14]. Figure 4 shows IR spectra of ZBLAN glass with the composition 55.8ZrF4 ⋅
Table 3. Theoretical losses in oxide, chalcogenide, and fluoride glasses and real optical losses in optical fibers [11, 12] Theoretical loss, dB/km
λi, μm
SiO2
0.14
1.55
0.2–0.35
1.55
Fluorozirconate glass in the ZBLAN system
0.022
2.55
0.45–0.15
2.35
SiO2–Na2O–CaO
4.5
0.85
–
–
As2S3
0.08
5
12
2
18
4.8
Material
Real loss in fibers, dB/km
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927 1
3400–3450
ZBLAN 55.8ZrF4 · 14.4BaF2 · 5.8LaF3 · 3.8AlF3 · 20.2NaF Absorption
HBLAN
BaF2–PbF2 ⇒ AlF3–InF3 LaF3–GdF3
2
NaF–NaCl BaF2–BaCl2
3
4
Fig. 3. ZBLAN glass composition modification for extending the IR transmission window and reducing the relaxation loss of rareearthdoped glasses.
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4000
3000
2000
1000
Wavenumber, cm–1 Fig. 4. IR transmission spectra of ZBLAN glass: prepara tion from commercially available 99.5 wt % pure fluorides (1) with pretreatment of the glass batch by xenon difluoride (2), chlorine trifluoride (3), and bromine triflu oride (4).
glasses prepared after treatment of the glass batch with the fluorooxidizers and subsequent CCl4 bubbling through the melt strongly suggests that the proposed procedure for the removal of oxygen from fluorides and chlorides is effective and allows one to obtain materials of high optical quality. As laser hosts, the fluorochloride glasses prepared in this study differ advantageously from their fluo rozirconate analogs by the broader IR transmission window and, as a consequence, by the lower relaxation loss. Another of their advantages is the possibility of
10–1 wt % oxygen
14.4BaF2 ⋅ 5.8LaF3 ⋅ 3.8AlF3 ⋅ 20.2NaF before and after exposure to the fluorooxidizers. As a result of the fluoride oxidation of the glass batch, the broad, asymmetric absorption band with a maximum at λ = 3400–3450 cm–1 (3.0–2.9 μm) and arising from OH– stretching vibrations completely disappeared. In addition, the content of oxygencontaining impurities in the glass samples dropped to the 10–3 wt % level (Fig. 5) [15, 16]. To remove these impurities from the chlorides in the glass batch, we developed a process for the prepa ration of fluorochloride glasses which involved CCl4 bubbling through a melt at a temperature of 1000°С. After these steps, the glass was prepared at tempera tures from 770 to 820°С (depending on glass compo sition) over a period of 30–40 min in an Ar atmo sphere containing 2–3 vol % CCl4. Glass samples 4– 5 mm in thickness were obtained by melt casting into a brass mold [17]. The region of lead fluoride substitution for BaF2 and gadolinium fluoride substitution for LaF3 in HBLAN glasses is limited to about 6 and 2 mol %, respectively. Complete InF3 substitution for AlF3 markedly extends the IR transmission window of the glass (by ~0.2 μm), but the glass transition tempera ture slightly decreases. Chlorine substitution for fluo rine also extends the lowloss window. Figure 6 shows transmission spectra of 57HfF4 ⋅ 20BaF2 ⋅ 3LaF3 ⋅ 3AlF3 ⋅ 17NaF glass in which BaF2 and AlF3 are com pletely replaced by BaCl2 and InF3, respectively. The transmission window is markedly extended (to 8.5 μm) when both AlF3 and BaF2 are completely replaced by InF3 and BaCl2 , respectively. In this case, the IR region extends with increasing Cl : F ratio. At the same time, the UV absorption edge of the fluorochloride glasses is slightly shifted to longer wavelengths (Fig. 7) [17]. A characteristic feature of the glasses with a large Cl : F ratio is that their transmission spectra contain a weak absorption band in the range 7–8 μm, which typically arises from Me–O bond vibrations in the glasses. The fact that the oxygenrelated absorption band is missing in the transmission spectra of the
200°
200°
200°
10–2 300°
300°
10–3
300° 400°500°
XeF2
400°500°
ClF3
400° 500°
BrF3
Fig. 5. Conditions of oxygen impurity removal from rare earth, zirconium, hafnium, and thorium fluorides at ele vated temperatures by xenon, chlorine, and bromine fluo rides (XeF2, ClF3, and BrF3).
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Percent transmission
3 80 2 1
60 40
0 6.0
6.5
7.0
7.5
8.0
8.5
Wavelength, µm Fig. 6. IR transmission edge of glasses (4mmthick samples) with the compositions (1) 57HfF4 ⋅ 20BaF2 ⋅ 3LaF3 ⋅ 3AlF3 ⋅ 17NaF, (2) 57HfF4 ⋅ 20BaCl2 ⋅ 3LaF3 ⋅ 3InF3 ⋅ 17NaF, and (3) 57HfF4 ⋅ 20BaCl2 ⋅ 3LaF3 ⋅ 3InF3 ⋅ 17NaF (glass prepared after additional fluorination of the glass batch with XeF2 and CCl4 bubbling through the melt).
introducing a larger amount of rareearth dopants. Using the proposed technique, we prepared glasses doped with rare earths (Nd, Er, Dy, and Tm) to a con centration of 8 at %. At total LnF3 concentrations above 5 mol %, the fluoride glasses exhibited strong lateral scattering. In the fluorozirconate glasses with similar cation stoichiometry, the maximum rareearth concentration at which light scattering was detected was 5 at %, that is, a factor of 1.5 lower.
100
Percent transmission
1 2
60 40 20 0 200
2 0.6
1
0.4 0.2
20
80
0.8
300
400
500
600
700
800
900
Wavelength, µm Fig. 7. UV transmission edge of fluorohafnate glasses with the compositions (1) 57HfF4 ⋅ 20BaF2 ⋅ 3LaF3 ⋅ 3AlF3 ⋅ 17NaF and (2) 57HfF4 ⋅ 20BaCl2 ⋅ 3LaF3 ⋅ 3InF3 ⋅ 17NaF.
1400 1450 1500 1550 1600 1650 1700 Wavelength, nm
Fig. 8. Luminescence spectra of Er3+ in (1) 57HfF4 ⋅ 20BaF2 ⋅ 3LaF3 ⋅ 3AlF3 ⋅ 17NaF fluoride glass and (2) 57HfF4 ⋅ 20BaCl2 ⋅ 3LaF3 ⋅ 3InF3 ⋅ 17NaF fluorochlo ride glass, both doped with 1 at % Er3+. Excitation by a diode laser (λ = 975 nm).
We studied the Er3+ luminescence in the 1.55μm range in the fluoride and fluorochloride glasses under excitation by a diode laser (λ = 975 nm) (Fig. 8). Comparison of spectral lines demonstrates that the incorporation of chlorine into the glass network leads to a considerable shift of the luminescence line to longer wavelengths and a slight broadening [18]. CONCLUSIONS Crystallizationresistant modified HBLAN glasses with parallel substitutions of several heavier cations (In3+, Pb2+, and Gd3+) for lighter ones (Al3+, Ba2+, and La3+) and Cl– for F– have been prepared for the first time, and the maximum possible dopant concen trations have been determined. The results demon strate that these glasses have a broader IR transmission window and accommodate larger amounts of rareearth dopants than do ZBLAN fluorozirconate glasses. We have developed general methodological approaches and apparatus for the preparation of fluo ride and fluorochloride glasses in chemically active media (fluorine, xenon difluoride, chlorine trifluo ride, bromine trifluoride, and carbon tetrachloride), which ensure a decrease in oxygen concentration in the glasses by two orders of magnitude (down to 10–3 wt %) relative to the oxygen concentration in the starting flu orides (10–1 wt %). The new fluoride glasses are anticipated to be attractive materials for IR, visible, and UV lasers and for planar and fiber amplifiers. A promising area of research is the development of transparent glassceramics con sisting of a fluoride glass matrix with nanocrystalline INORGANIC MATERIALS
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