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Abstract
It has been established that rare earth elements (REE), as alloying additives, are a significant source of hydrogen and oxygen impurity in glasses of Ge – S and Ge – As – S systems. A method has been developed that includes loading germanium in the form of monosulfide and high-temperature annealing of REE and their compounds in sulfur vapor, reducing content of absorbing impurities in sulfide-germanium glasses. Samples of especially pure glasses of Ge42.5S57.5 and Ge35As10S55 composition doped with praseodymium up to 1450 ppm(at) were prepared. The impurity content of hydrogen and oxygen in the form of SH-groups and Ge–O in the best glass samples was 0.02–0.06 ppm(wt) and <0.1– 0.2 ppm(wt), respectively. The absorption coefficients and the absorption cross section were determined for various energy states of the Pr3+ ion in Ge35As10S55 glass. The luminescence intensity of Pr3+ ions in the spectral ranges 2.0–2.8 and 3.5–5.8 µm in Ge42.5S57.5 glasses is by 3 orders of magnitude higher than in Ge35As10S55 glasses.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Glasses based on germanium sulfides doped with rare earth elements are considered as materials for creating lasers and amplifiers of the mid-IR range [1,2]. Impurities of oxygen and hydrogen in the form of germanium oxides, OH- and SH-groups reduce the transparency of these glasses in the mid-IR range and degrade their emission characteristics due to fast non-radiative ion-ion relaxation of REE [1,3].
Traditionally, glasses based on germanium sulfides are prepared by melting simple highly pure substances in evacuated silica ampoules at temperatures of 750–900°C [4]. Vacuum distillation of the glass-forming melt makes it possible to reduce the impurity content of water, germanium oxides, and heterogeneous impurity inclusions [5,6]. The high distillation temperature (700–900°C) contributes to the entry of hydrogen impurity from the walls of the silica reactor which increases the intensity of the absorption band from SH groups in the region of 3.96 µm [7]. Preparation of doped chalcogenide glasses with low content of absorbing impurities is complicated by the fact that REEs in the form of simple substances and most of the compounds have low volatility and cannot be subjected to vacuum distillation purification. The main REE reagents, which are used in the preparation of doped chalcogenide glasses, are simple substances (metals), their chalcogenides and halides [3,8]. The impurity content of oxygen and hydrogen in these reagents in the form of oxides, hydroxides, oxo- and hydroxocarbonates of different composition, as a rule, is not controlled. REE sulfides are predominantly produced by the interaction of oxides and hydroxides with hydrogen sulfide or carbon disulfide at elevated temperatures [9]. This is due to the additional content of impurities of oxygen, hydrogen and carbon in sulfides. The use of such reagents as doping additives can lead to significant contamination of chalcogenide glasses with absorbing impurities and degradation of their optical and luminescent properties. An additional source of impurities is the interaction of REE reagents with the atmosphere of air during storage and loading into the reactor.
In the present work, in order to reduce the content of absorbing impurities in chalcogenide glasses, it has been proposed to subject them to high-temperature treatment in sulfur vapor before loading REE into the batch. This approach is based on the assumption that the reactions of thermal decomposition of rare-earth oxycarbonates and the interaction of their oxides, hydroxides and adsorbed OH-groups with sulfur vapors proceed. The resulting volatile compounds (SO2, H2O, CO2, etc.) can be removed from the reaction zone when heated. Theoretically and experimentally, the interaction of sulfur with REE oxides was studied in [10,11] during sulfidation leading to the formation of Ln2O2S, Ln2O2S2, Ln2S3 and Ln2S4. In [10], on the basis of thermodynamic calculations, a phase diagram of the Ce – O – S system at 700°C was constructed. From this diagram, it follows that in an excess of sulfur vapor S2 (∼ 1 atm) and the disappearing low partial pressure of oxygen (<10−10 atm.), the main thermodynamically stable phase is cerium sulfide Ce2S3. A similar behavior can be expected for oxides of other REEs, including praseodymium and thulium.
Previously, the loading efficiency of germanium loading in the reactor in the form of its monochalcogenides to reduce the impurity content in glasses of Ge – S – I and Ge – Se systems was demonstrated in [12,13]. This approach allowed us to significantly (by 10–20 times) reduce the content of hydrogen and oxygen in the form of SH-, SeH-groups and Ge–O. In the present work, this method is modified to prepare chalcogenide glasses doped with praseodymium. The modification consists in passing sulfur vapors over a mixture of germanium and REE in the form of a simple substance or sulfide. This allows the loading of germanium in the form of GeS and the purification of REE from oxygen- and hydrogen-containing impurities in one stage.
The aim of the work was to develop a method for producing highly pure chalcogenide glasses based on germanium sulfides doped with REE, with a low content of hydrogen and oxygen impurities. The objects of study were the glasses of Ge42.5S57.5 and Ge35As10S55 composition doped with praseodymium. The choice of objects is due to the fact that sulfide-germanium glasses have a pronounced ability to dissolve REE [14]. The disadvantage of these glasses is a high tendency to crystallization. Ge42.5S57.5 glass corresponds to the eutectic composition on the Ge – S system state diagram [15] which contributes to an increase in the glass-forming ability. The addition of arsenic to sulfide-germanium glasses significantly increases their stability against crystallization. Previously, optical fibers with an intense luminescence in the range 1.6–2.2 µm were made from Ge35As10S55 bismuth-doped glasses [16].
2. Experimental
To evaluate the efficiency of removing impurities from the surface of rare-earth elements and germanium by high-temperature treatment in sulfur vapor, we simulated the thermodynamic equilibrium of Ge – S, C – S, GeO – S, H2O – S и C – GeO – H2O – S systems. The equilibrium composition of the systems was calculated in the IVTANTERMO software [17] in the temperature range of 500–750 K, pressure of 1013.25 Pa, and initial impurity content from 0.01 to 10 mol. % The results of the calculations were the degree of conversion of carbon-containing impurities, water and germanium monoxide.
For the synthesis of glasses the granulated germanium of 6N brand, sulfur and arsenic of 5N grade were used. Sulfur was subjected to additional purification according to the method described in [7]. To reduce the content of the oxygen impurity, we performed double vacuum sublimation of arsenic. Germanium granules were preliminarily calcinated at 700°C in vacuum to remove the film of monoxide and adsorbed water from its surface. The purity of metallic praseodymium was 3N grade.
A scheme of set-up from silica glass for loading the batch components into the reactor is shown in Fig. 1. The set-up consists of sulfur evaporator (1), an ampoule with germanium, arsenic and praseodymium (2), a receiver (3) with a forming tube. The exact weights of simple substances required to prepare 20–30 g of glass were placed in section 2. After evacuating the system to the residual pressure of 10−3 Pa and warming up the reactor to remove adsorbed water, the sublimation loading of arsenic into receiver 3 was carried out at an evaporator temperature of 50°C. Then, section 2 was heated to 580–600°C, the glass partition 7 was broken with a magnetic hammer 8 and sulfur was evaporated from the loading ampoule 1. The formed germanium monosulfide evaporated from section 2 and condensed in the reactor 3. After germanium was completely loaded as a monosulfide, the temperature section 2 was increased to 700°C for more efficient removal of impurities from the surface of REE due to the interaction with sulfur vapor. At the end of the evaporation of sulfur, the set-up was sealed along A and C waists. Praseodymium was mechanically moved to section 3 and the reactor was sealed off by B waist.
Fig. 1. Scheme of set-up for loading the batch and synthesis of chalcogenide glasses based on germanium sulfide doped with praseodymium: 1 - the sulfur evaporator; 2 - the section with germanium, arsenic and praseodymium; 3 - the receiver with a forming tube, 4, 5, 6 - the resistance furnaces; 7 - the crashing partition; 8 - the magnetic hammer; 9, 10, 11 - the wired heaters; A, B, C - the waists for sealing-off.
The batch in section 3 was melted in the rocking muffle furnace at a temperature of 850°C for 5 hours. Next, the chalcogenide melt was cooled down to 700°C, quenched in air and the resulting glass was annealed at a temperature Tg+20°C (Tg is the glass transition temperature) for 1 h. Glass samples were prepared in the form of cylinders up to 60 mm long and 11 mm in diameter. The ends of the samples were ground and polished for IR spectroscopic measurements. The transparency of glasses in the range of 800–7500 cm-1 (1.3–12.5 µm) was studied on Tensor 27 spectrometer (Bruker). Thermal analysis of the prepared glasses was carried out by the method of differential scanning calorimetry (DSC) on Netzsch 409 microcalorimeter at a heating rate of 10 degrees/min. Preliminary calibration of the calorimeter provided temperature measurement with an accuracy of ± 0.5°C.
Pr3+ luminescence spectra in glasses were obtained on IFS-113v spectrometer at a resolution of 8 cm-1 using InSb detector cooled with liquid nitrogen. For pumping, a single-mode 2W fiber thulium laser with a wavelength of 1975nm was used.
3. Results
According to the results of the simulation of the thermodynamic equilibrium of the «C, GeO, H2O – sulfur» systems, the main products of impurity transformation are carbon disulfide, germanium monosulfide and sulfur oxides. Chemical transformations in these systems can be simplified by the following reactions:
Fig. 2. Degree of transformation of impurities C(–), H2O(-·-) and GeO(—) with an impurity content (mol.%) = 0.1 (8), 0.05 (1), 0.01 (2, 5), 0.001 (3, 6, 10), 0.0001 (4, 7, 9).
The legend of the prepared glass samples, their chemical compositions and conditions for their preparation are given in Tables 1–2. GS1 glass is prepared by loading components in air; GS3 glass by vacuum loading of germanium and sulfur in the form of monosulfide without treatment of praseodymium with sulfur vapor.
Table 1. Composition of glasses and their legend.
Table 2. Content of oxygen and hydrogen impurities in the form of SH-groups and Ge–O in the prepared glasses.
A characteristic glass transition interval (Tg = 351°C) and two intense exothermic peaks with the maxima at 450°C and 490°C (Fig. 3) appear on the DSC curve for heating the glass of Ge42.5S57.5 composition. These peaks correspond to the crystallization of monosulfide and germanium disulfide, respectively. The difference between the beginning of the first crystallization peak (443°C) and the glass transition temperature ΔT = Tx – Tg is 92°C. This is significantly less than the generally accepted minimum value of ΔT (120°C) which determines the possibility of manufacturing optical fibers. Tg of Ge35As10S55 glass is 415°C. In the investigated temperature range (150–550°C), the crystallization of this sample was not recorded which indicates its rather high stability against crystallization.
Figures 4 and 5 show the absorption spectra of glasses in the range of 1.3–11.5 µm. The absorption coefficient ɛ(λ) (y-axis) was calculated by the equation
where T(λ) is the sample transparency at the wavelength λ with regard to the background; l is the sample thickness, cm. Spectra show absorption bands from impurities of hydrogen, oxygen and carbon in the form of SH-groups (3.96 µm), H2O (2.75, 2.83, 6.30 µm), Ge–O (7.6 µm), COS (4.93 µm), CS2 (6.63 µm), the absorption bands of the matrix of glass GeS4/2 (8.7, 9.3 µm) [6,18,19]. In glasses doped with praseodymium, the absorption bands of the Pr3+ ion at 1.48, 1.59, 2.02, 2.35 µm and a series of bands in the 3.7–5 µm regions appear [14]. Using the known absorption coefficients [20,21], the concentration of impurities of hydrogen and oxygen in the form of SH-groups and Ge–O was calculated with relative error 10–30% (Table 2).Fig. 5. Absorption spectra of glass samples in the region of the long-wavelength transmission edge: a - GS1 (1), GS3 (2), GS4 (3), GS2 (4); b - GAS1 (1), GAS2 (2), GAS3 (3), GAS4 (4), GAS5 (5).
GS1 sample for which preparation the germanium and sulfur were loaded into the reactor in air contains the maximum amount of impurities of hydrogen and oxygen (1.1 and 10 ppm(wt)). The use of sublimation loading of pre-calcinated germanium through GeS and distillation loading of sulfur (GS2 glass) reduces the content of these impurities by 18 and 100 times, respectively (Table 2). The introduction of praseodymium into the batch in the form of a simple substance leads to contamination of the glass with impurities of hydrogen and oxygen in the form of SH-groups and Ge-O to the level of 0.79 ppm(wt) and 2.7 ppm(wt) (GS3 glass). Sublimation loading of the batch with transport of sulfur vapor over praseodymium (GS4 glass) allows one to significantly reduce the content of these impurities to 0.3 and 0.4 ppm(wt), respectively. These regularities are also applied to Ge – As – S system. Sublimation loading of arsenic, germanium sulfide and distillation loading of sulfur into the reactor makes it possible to prepare glasses with hydrogen content in the form of SH groups at the level of 0.02 ppm(wt) and oxygen in the form of Ge–O 0.1 ppm(wt). Adding praseodymium in these glasses leads to a regular increase in the content of hydrogen and oxygen impurities. Increased oxygen content in GAS2 glass with 250 ppm(wt) of Pr may be due to incomplete distillation of arsenic oxide during batch loading.
Figure 6a shows the spectra of Ge35As10S55 glasses in the region of praseodymium absorption with the designation of the corresponding terms of Pr3+ ion. From the dependence of the integrated intensity of the absorption bands on the concentration of praseodymium (Fig. 6b) for each term, with the exception of 3H6, the Pr3+ ion absorption coefficients in cm-2/ppm(at) were calculated by regression analysis (Table 3). Figure 7 shows the absorption cross section spectrum of Pr3+ ${\sigma _{abs.}}(\lambda )$ ground state calculated by the equation
Fig. 6. Absorption spectra of glasses GAS1 (1), GAS2 (2), GAS3 (3), GAS4 (4), GAS5 (5) (a) and the dependence of the integrated intensity of the absorption bands Pr3+ on its content in glass (b).
Table 3. Absorption coefficients and absorption cross sections for Pr3+ ion terms.
Figure 8 shows the luminescence spectra of praseodymium ions for samples of GS4 and GAS4 glasses. The observed luminescence bands with the maxima in the 2.4 and 5.0 µm region have a complex structure and, according to [14], include more than 10 different transitions, the most intense of which are 4F3 → 3H6 (4.63 µm), 3H5 → 3H4 (4.74 µm), 3H6 → 3H4 (4.05 µm). The absolute luminescence intensity of Pr3+ in Ge42.5S57 glasses is significantly (by 3 orders of magnitude) higher than in Ge35As10S55 glasses (in Fig. 8, spectrum 2 is multiplied by 100 times).
Fig. 8. Luminescence spectra of praseodymium ions in samples of GS4 (1) and GAS4 (2) glasses. Spectrum GAS4 is multiplied by 100.
4. Discussion of the results
According to the results obtained in this work, REEs introduced into chalcogenide glasses as a luminescent additive are an essential source of absorbing impurities in these materials. Increase of praseodymium content in Ge35As10S55 glasses from 0 to 1450 ppm(at). leads to an increase in the content of hydrogen impurities in the form of SH-groups from 0.02 to 0.75 ppm(wt), oxygen in the form of Ge–O from 0.1 to 1.9 ppm(wt). This is due to surface contamination of REEs in the form of simple substances and sulfides by impurities of oxygen- and hydrogen-containing compounds (oxides, carbonates, oxo- and hydroxo-carbonates, sulfide oxides, etc.). The presence of these impurities is due to the active interaction of praseodymium with oxygen, moisture and carbon dioxide in the air. Additional sources of these impurities are the starting materials that are used to prepare praseodymium chalcogenides (Pr6O11, Pr2O3, Pr(OH)3). Preliminary high-temperature treatment of metallic praseodymium in sulfur vapor can significantly reduce the content of absorbing impurities in glasses. This is due to the chemical interaction of praseodymium compounds with sulfur and its impurities, for example, by the reactions:
It follows from the absorption spectra of the prepared glasses (Fig. 5) that the impurity content of germanium oxide affects the intensity and shape of the absorption bands in the region of the long-wave transparency edge (7.5–10 µm). The nature of these bands is not discussed in detail in the literature. With a decrease in the Ge–O content recorded by absorption at 7.6 µm, the band with the maximum at 8.9 µm splits into two “shoulders”: more intense in the region of 9.1 µm and less intense in the region of 8.7 µm. This splitting is a consequence of a decrease in the intensity of the shortwave component. With an impurity content of germanium oxide <0.2 ppm(wt), the intensity of the band in the region of 7.6 µm becomes comparable to the background level, and the intensity of the “shoulder” at 8.7 µm does not change significantly. This suggests that in the range of 8–10 µm there are three absorption bands, one of which corresponds to Ge–O impurity, and the other two to the vibrations of GeS4/2 tetrahedra. From the analysis of the data available in the literature on IR and Raman spectra of glasses based on sulfide and germanium oxide [22,23], it can be assumed that these bands correspond to the overtone of the deformation vibrations of Ge – O – Ge bond (2×570 cm-1 = 1140 cm-1 = 8.77 µm), an overtone of triply degenerate GeS4/2 v3(F2) vibration (3 × 375 cm-1 = 1125 cm-1 = 8.89 µm) and a composite vibration v3(F2) with “breathing” full-symmetry vibration of the GeS4/2 tetrahedra connected by vertices (2×375 cm-1 + 345 cm-1 = 1095 cm-1 = 9.13 µm). Due to the position of the absorption bands of germanium oxide in the region of the long-wavelength transmission edge of sulfide-germanium glasses, this impurity can have a significant impact on the luminescent characteristics of activated glasses by increasing the phonon energy (multi-phonon relaxation).
According to the literature data, the dissolution of REEs in chalcogenide glasses is promoted by the presence of ions with low polarizing ability [24], i.e. with a smaller charge and a larger ionic radius. In Ge – S and Ge – As – S systems, this is the Ge2+ ion which formally corresponds to germanium monosulfide (r(Ge2+) = 0.073 nm, r(Ge4+) = 0.053 nm, r(As3+) = 0.058 nm [25]). This can determine the high luminescence intensity of praseodymium in Ge42.5S57.5 glass as compared with samples of Ge35As10S55 composition. An additional factor contributing to the luminescence of REE in Ge42.5S57.5 glasses is the presence of the microcrystalline phase of germanium monosulfide [24] which existence is registered in sulfide-germanium glasses by Raman spectroscopy [26]. The search for optimal glass compositions in Ge - As - S system, which provide intensive luminescence of REE ions and possessing high stability against crystallization, requires separate studies.
5. Conclusions
A technique has been developed for preparing high-purity glasses of Ge – S and Ge – As – S systems doped with praseodymium. This technique includes vacuum loading of germanium into the reactor in the form of monosulfide and high-temperature annealing of praseodymium and its compounds in sulfur vapor. According to the results of thermodynamic modeling, this approach is efficient for reducing the content of oxygen, hydrogen and carbon in the batch. Glass samples of Ge42.5S57.5 and Ge35As10S55 composition doped with praseodymium up to 1450 ppm(at) have been prepared. The impurity content of hydrogen and oxygen in the form of SH-groups and Ge – O in the best glass samples was 0.02–0.06 ppm(wt) and <0.1– 0.2 ppm(wt). It is shown that the impurity of germanium oxide affects the intensity and shape of the absorption bands in the region of the long-wavelength transparency edge of glasses (7.5–10 µm). The absolute luminescence intensity of Pr3+ ions in the spectral ranges 2.0–2.8 and 3.5–5.8 in Ge42.5S57.5 glasses is by 3 orders of magnitude higher than in Ge35As10S55 glasses. This behavior of Pr3+ can be interpreted by particular features of the structural grid of glass and in the framework of the theory of ion polarization.
Funding
Russian Science Foundation (RSF) (15-12-20040).
Acknowledgments
This work was supported by the Russian Science Foundation (Grant No. 15-12-20040).
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