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Study of the pseudo-ternary Ag2SAs2S3HgI2 vitreous system
Journal of Solid State Chemistry 199 (2013) 264–270
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Study of the pseudo-ternary Ag2S–As2S3–HgI2 vitreous system R. Boidin a, D. Le Coq a,b,n, A. Cuisset a, F. Hindle a, J.-B. Brubach c, K. Michel d, E. Bychkov a a
LPCA, ULCO Univ. Lille Nord de France, EA 4493, Dunkerque F-59140, France Sciences Chimiques de Rennes, UMR CNRS 6226, Eq. Verres et Ce´ramiques – Univ. de Rennes I, Rennes F-35042, France AILES, Synchrotron SOLEIL, L’Orme des Merisiers Saint-Aubin, Gif-sur-Yvette F-91192, France d BRGM, Direction Eau, Environnement et Ecotechnologies – Unite´ BGE, Orle´ans F-45060, France b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 October 2012 Received in revised form 3 January 2013 Accepted 6 January 2013 Available online 15 January 2013
Chalcogenide alloys in the Ag2S–As2S3–HgI2 pseudo-ternary system were synthesized and their vitreous nature was verified by X-ray diffraction. The glass transition and crystallization temperatures (Tg and Tc), the density (d), and the total electrical conductivity (s) were measured for all samples of three series, A, B, and C corresponding to (Ag2S)50 x/2(As2S3)50 x/2(HgI2)x, (Ag2S)y(As2S3)80 y(HgI2)20 and (Ag2S)z(As2S3)50(HgI2)50 z, respectively. The maximum of Tg was approximately 160 1C for glasses with low HgI2 content whereas the maximum of density (5.75 g cm 3) was obtained for the sample in the B-series with the highest Ag2S concentration (z ¼ 60 mol%). This composition also possesses the highest conductivity at 298 K (s298 K E10 3 S cm 1). Unexpectedly the conductivity of the A-series samples was observed to decrease as a function of the Ag2S content. The far-infrared transmission in the 100–600 cm 1 window range (3.3–18.2 THz, 100–16.6 mm) was also given for a few glass compositions highlighting the strong influence of the HgI2 content. & 2013 Elsevier Inc. All rights reserved.
Keywords: Chalcogenide glasses Ionic-conducting glasses Far-infrared transmission
1. Introduction Due to their broad infrared transmission, high linear and nonlinear refractive index, high photosensitivity, chalcogenide glasses (ChGs) have numerous optical and photonic applications. At present, the near- and mid-infrared radiation domains are very concerned by technology advancement in the ChG field and Refs. there in [1–3]. Curiously, the potential of ChGs for applications in the far-infrared (FIR) or TeraHertz (THz) domain has not been exploited. This domain, even if it is less mature than its neighbouring domains, has already been identified to have great potential for many applications including security screening [4], pharmaceutical, biological or medical science analysis [5,6], wireless communications [7], and environmental monitoring [8], etc. Research into suitable THz materials is now receiving more attention [9] and ChGs can contribute to the significant technological advancements by providing many optical components such as filters, lenses, and waveguides etc. ChGs belong to the family of vitreous semiconductors [10]. Their potential as solid-state electrolytes in sensors and batteries has already been demonstrated [11] and Refs. there in. More recently, application of ChGs to Non Volatile Memories (NVM) has also been the subject of many studies. Among the different NVM,
n Corresponding author at: University of Rennes 1, Sciences chimiques de Rennes, Verres et Ce´ramiques, UMR CNRS 6226, Rennes Eq. 35042, France. E-mail address: [email protected] (D. Le Coq).
0022-4596/$ - see front matter & 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jssc.2013.01.006
the Phase Change Memories (PCM) and the Current Bridging Random Access Memories (CBRAM) are very promising technologies using the intrinsic properties of ChGs [12,13]. The pseudo-binary Ag2S–As2S3 glass system has been intensively studied because it is of interest for both fundamental and application studies [14]. It is vitreous for Ag2S molar fraction between 0% and 40%, and provides glasses with very interesting electrical conductivity behaviour [15,16]. At low content of Ag2S (o5 mol%), the conductivity is typically characteristic of a semiconducting glass, whereas if this content becomes higher than 15 mol%, it is characteristic of an ionic conductor [17,18]. At the same time, numerous experiments have been reported to establish relations between the conductivity and the structure in order to determine the concerned mechanisms [18–23]. The addition of other compounds in this pseudo-binary system has also been investigated essentially as part of ion detection in aqueous media. For example, glasses in CdS(or CdI2)-Ag2S-As2S3 systems have been used to develop membranes for ion-selective electrodes (ISE) devoted to Cd2 þ [24]. In a similar way, membranes for Pb2 þ –ISE have been characterized in the PbS(or PbI2)Ag2S-As2S3 glass systems [25,26]. Chalcogenide glasses in AgBr– Ag2S–As2S3 have been studied to develop membrane of Hg2 þ -ISE by vacuum thermal evaporation [27]. In a development Hg2 þ -ISE and perspective terahertz materials, we have investigated the pseudo-ternary Ag2S–As2S3–HgI2 even if the glass-forming region and structures by means of vibrational spectroscopy have already been reported [28]. The objective of this work is to determine other characteristics such as
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macroscopic properties, conductivity, and far-infrared transmission in order to estimate the potential of these ChGs in both electrical and optical point of views. Consequently, three different series were synthesized to estimate the role of each compound. In A-series, (Ag2S)50 x/2 (As2S3)50 x/2 (HgI2)x, 0r xr50, addition of HgI2 in the Ag2S–As2S3 matrix is carried out. In B-Series, (Ag2S)y (As2S3)80 y (HgI2)20, 5 mol%ryr60 mol%, a substitution of As2S3 by Ag2S with a constant HgI2 content equal to 20 mol% is operated. Finally, the C-series corresponds to the specific composition (Ag2S)z (As2S3)50 (HgI2)50 z, 2.5 mol%rz r50 mol%, meaning that the substitution of HgI2 by Ag2S with a constant As2S3 content of 50 mol% is considered.
2. Experimental 2.1. Glass preparation The preparation of various compositions was carried out by performing the prior synthesis of Ag2S and As2S3 and by using commercial HgI2 (Sigma-Aldrich 99.999%). HgI2 powder was used without any additional purification. As2S3 and Ag2S were synthesized from arsenic (Cerac 99.9999%) and silver (Neyco 99.999%) pieces with the required amount of sulphur pellets (Acros Organics 99.999%). The high vapour pressure contaminants (oxides) in As and Ag were removed by evaporation at 320 and 640 1C respectively under vacuum. The sulphur was heated at 130 1C under vacuum then was sealed and distilled at 430 1C. The synthesis of As2S3 and Ag2S was performed at 800 and 950 1C in a rocking furnace respectively before quenching in cold water. The appropriate proportions of the base compounds HgI2, Ag2S and As2S3 were heated to prepare the Ag2S–As2S3–HgI2 alloys. The mixing of the compounds was homogenized in a rocking furnace at 800 1C for 24 h. The melt was cooled down to 650 1C before quenching in air. The samples had masses varying between 2 and 10 g. 2.2. Macroscopic properties measurements The measurement of the sample density d was performed by a hydrostatic method using the toluene as immersion fluid and the germanium as standard (5.323 g cm 3). A Sartorius YDK 01-0D density kit was used for the measurements, the sample mass in air varied between 0.4 and 2.5 g. The amorphous nature of the samples was verified at room temperature by x-ray diffraction, XRD, technique using a Bruker
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D8 Advance Da Vinci diffractometer equipped with a cobalt tube and a Linex eye detector. A Bruker D5000 was also used with a cobalt tube and a scintillation detector. The scattering intensities were measured over an angular range of 41r2y r841 with a step-size of D(2y)¼0.021 and a count time of 1 s per step. The Bragg peaks in diffraction patterns of glassy/crystalline samples were identified and indexed using the JCPDS database. Differential Scanning Calorimetry analyses using a TA instruments Q200 were performed to determine the glass transition (Tg), the crystallization (Tc) and melting (Tm). The samples of 3–15 mg were hermetically sealed in standard aluminium pans and were heated at a rate of 10 K min 1 from 10 to 400 1C in high purity nitrogen gas environment. 2.3. Impedance measurements Total electrical conductivity of the samples was measured using a Hewlett Packard 4194 A impedance meter. The impedance modulus Z and the phase angle y were obtained in the frequency range from 100 Hz to 15 MHz from room temperature up to 383 K, corresponding to a temperature below Tg for all the glass samples. The quenched samples, prepared as rectangular plates, were polished using SiC powder (9.3 mm grain size). The sample sides were ground parallel and gold was deposited on opposite sides to form electrodes, meaning that the electrochemical cell for conductivity measurements was Au9glass9Au. The typical sample thickness was 1.0 mm (70.2 mm) with a typical area of 7 mm2 (71 mm2). The temperature dependence on the conductivity was studied over several cycles. Each cycle consisted of a heating step followed by a cooling step, in order to investigate a possible hysteresis. No significant hysteresis was observed for any of the samples. 2.4. Far-infrared window FIR transmissions measurements performed on the AILES (Advanced Infrared Line Exploited for Spectroscopy) beamline [29] at the French national synchrotron facility SOLEIL. Using the Bruker IFS 125 interferometer, the spectra were recorded in the 10–1000 cm 1 spectral region with a resolution of 2 cm 1 by coadding the Fourier transform of 200 interferograms. The synchrotron radiation was focused on a 1.5 mm entrance aperture of the interferometer containing a Mylar 6 mm multilayer beamsplitter suitable for the FIR/THz spectral ranges. The interferometer was installed in a vacuum chamber allowing an operational pressure
Fig. 1. Glass-forming region in the Ag2S–As2S3–HgI2 pseudo-ternary system with the three investigated series; A (red online): (Ag2S)50 x/2 (As2S3)50 x/2 (HgI2)x, B (blue online): (Ag2S)y (As2S3)80 y (HgI2)20, C (green online): (Ag2S)z (As2S3)50 (HgI2)50 z. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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of 10 6 mbar to be achieved to eliminate the absorption due to atmospheric compounds. Thanks to the use of a 4 K heliumcooled silicon bolometer equipped with an optical band-pass filter and to the high brightness of the synchrotron FIR light (100–1000 times greater than thermal source) high signal/noise ratio levels were obtained over the spectral range 10–1000 cm 1. The transmission spectra were obtained on glass samples 400 mm thick by dividing the signal transmitted through the ChG disk by the signal measured in the absence of a sample but identical in all other conditions. Interesting results are only found in the windows between 100 and 600 cm 1.
3. Results 3.1. Glass-forming region The glass-forming region of the pseudo-ternary system Ag2S–As2S3–HgI2 obtained in our conditions of synthesis is presented in Fig. 1 appears to be wider than that reported earlier [28] and spreads over almost half of the diagram. The vitreous domain is preferentially located in the rich-As2S3 part and a minimum of 20 mol% of As2S3 is required. The maximum solubility of Ag2S and HgI2 are approximately 65 and 45 mol%, respectively. The XRD patterns of four different samples belonging to the A-series are shown in Fig. 2. For this series, the first crystallization peaks appears when the HgI2 content is higher than 40 mol%. The indexation of the peaks by using JCPDS database reveals that the crystallites correspond to AgI (00-009-0374) and Ag2HgI4 (00-0321016) crystalline phases suggesting that the following exchange reactions could take place during the homogenization step:
(1) Ag2SþHgI222AgI þHgS (2) 2AgI þHgI22Ag2HgI4
Table 1 Density and characteristic temperatures (glass transition (Tg) and crystallization temperature (Tc)) of samples belonging to the three investigated series: A: (Ag2S)50 x/2 (As2S3)50 x/2 (HgI2)x, B: (Ag2S)y (As2S3)80 y (HgI2)20, C: (Ag2S)z (As2S3)50 (HgI2)50 z. The partially crystallized samples are noted with an asterisk. Sample Density (g cm 3) Tg (1C) (7 2 1C) Tc (1C) ( 72 1C) DT (1C) (7 4 1C) A-series: x ¼0 x ¼10 x ¼15 x ¼20 x ¼25 x ¼30 x ¼35 x ¼40 x ¼45n x ¼50n
(Ag2S)50 x/2 (As2S3)50 x/2 (HgI2)x 4.642 7 0.009 162 4.8057 0.004 144 4.934 7 0.003 137 5.0297 0.004 134 5.0757 0.011 126 5.146 7 0.011 120 5.263 7 0.002 102 5.264 7 0.019 92 5.411 7 0.007 76 5.5047 0.008 76
238 215 264 227 230 198 190 162 152 138
76 71 127 93 104 78 88 70 76 62
B-series: y¼ 5 y¼ 20 y¼ 30 y¼ 40 y¼ 50 y¼ 60
(Ag2S)y (As2S3)80-y 3.838 7 0.011 4.3057 0.010 4.617 7 0.012 5.0297 0.003 5.325 7 0.013 5.752 7 0.003
170 277 230 227 156 185
32 138 99 93 21 57
C-series: z ¼2.5n z ¼5n z ¼7.5n z ¼10 z ¼12.5 z ¼15 z ¼17.5 z ¼22.5 z ¼25 z ¼30 z ¼35 z ¼40 z ¼45 z ¼50
(Ag2S)z (As2S3)50 (HgI2)50 z 4.5507 0.009 216 4.646 7 0.011 137 4.5607 0.002 131 4.548 7 0.007 114 4.563 7 0.013 101 4.583 7 0.005 101 4.611 7 0.015 108 4.631 7 0.019 120 4.616 7 0.002 121 4.617 7 0.012 131 4.655 7 0.020 138 4.568 7 0.033 141 4.618 7 0.012 151 4.642 7 0.009 162
383 241 170 390 150 171 / / 336 230 260 263 215 238
167 104 39 254 49 70 / / 223 99 122 122 64 76
(HgI2)20 138 139 131 134 135 128
3.2. Density and thermal properties The sample density of the three series A, B, and C following the Ag2S content are given in Table 1 and plotted in Fig. 3. A decrease of density following the Ag2S content is observed from 5.504 to 4.642 g cm 3 in the A-series. On the contrary, in the
Fig. 3. Evolution of the density versus the Ag2S mol% for the three investigated A-, B-, and C-series corresponding to (Ag2S)50 x/2 (As2S3)50 x/2 (HgI2)x, B: (Ag2S)y (As2S3)80 y (HgI2)20, C: (Ag2S)z (As2S3)50 (HgI2)50 z, respectively. The empty symbols correspond to lightly crystallized samples.
Fig. 2. XRD patterns of four samples of the A-series (Ag2S)50 x/2 (As2S3)50 x/2 (HgI2)x, x ¼0, 20, 45, and 40 mol%.
B-series, the density increases from 3.838 to 5.752 g cm 3. Density in the C-series remains almost constant around 4.6 g cm 3. The thermal parameters were extracted from the DSC trace. Typical curves of the three A, B and C-series are presented in Fig. 4. Each analyzed sample shows a single Tg, which is
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characteristic of an homogeneous glass in macroscopic and mesoscopic scales. The evolution of Tg for the three investigated series are plotted in Fig. 5 following the Ag2S content. The A-series is characterized by an increase of Tg from 76 1C72 1C (x¼ 50 mol%) up to 162 1C72 1C (x¼ 0 mol%) meaning that HgI2 addition in the (Ag2S)50 (As2S3)50 matrix is harmful in a thermal point of view. The Tg of the B-series is relatively constant since it varies between 128 1C72 1C (y¼60 mol%) and 139 1C72 1C (y¼20 mol%). At last, the evolution of Tg in C-series shows two distinct behaviours. Firstly, the substitution of HgI2 by Ag2S is characterized by a large decrease of Tg from 216 1C72 1C (z¼2.5 mol%) down to 114 1C72 1C (z¼10 mol%) but as mentioned in Fig. 5, these data correspond to partially crystallized samples. When the samples are completely vitreous, Tg weakly decreases down to 101 1C72 1C before linearly increasing with the Ag2S content to reach 162 1C for the z¼50 mol%.
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3.3. Electrical conductivity Typical complex impedance plots or Cole–Cole diagrams of the investigated glasses are shown in Fig. 6. In the high-frequency range, the diagrams represent distorted semi-circles typical for glassy ionic conductors [30]. A low-frequency polarization is observed for the samples measured using an electrochemical cell with blocking Au electrodes. This polarization characterizes difficulties of the charge transfer at the Au/glass interface and indicates a predominantly ionic conductivity in the sample, most probably caused by Ag þ ions. Temperature dependence on the conductivity of some samples is shown in Fig. 7. These data obey the Arrhenius law: s Es ð1Þ s ¼ 0 exp T kT where s0 is the pre-exponential factor, Es the activation energy, k the Boltzmann constant and T the temperature. The conductivity at 298 K (s298 K), s0, and Es were calculated from a least-square fit of the data to the Arrhenius law (Eq. (1)) and their evolution as a function of Ag2S content is plotted in Fig. 8. The pre-exponential factor remains almost constant for each series at approximately 3 105 S K cm 1. Consequently, the activation energy and conductivity at 298 K exhibit opposite trends. When s298 K increases from 7.8 10 8 to 6.5 10 4 S cm 1, the activation energy decreases from 0.6 to 0.3 eV. Conductivity evolution as a function of Ag2S content is different for each series. The conductivity decreases in series A while it increases in series B and remains nearly invariant in series C (except for a very low level of Ag2S). 3.4. Far-infrared transmission
Fig. 4. Typical DSC curves of glasses belonging to the A-, B-, and C-series. Insets for the A- and B-series are given to emphasize the glass transition phenomenon.
The glasses of this pseudo-ternary system possess a relatively interesting infrared transmission as exhibited in Fig. 9. If the HgI2 content of the A-series increases the glasses show better transmission and also a lower shift of the wavenumber cut-off. The maximum of transmission in the 100–600 cm 1 window goes from about 26% for x ¼0 up to more than 60% for x¼45 mol%. Correspondingly, the wavenumber cut-off decreases by about 30 cm 1. The FIR transmission of two other glasses with a HgI2
Fig. 5. Evolution of the glass transition temperature (Tg) following the Ag2S content (mol%) for the three investigated series: A (red online): (Ag2S)50 x/2 (As2S3)50 x/2 (HgI2)x, B (blue online): (Ag2S)y (As2S3)80 y (HgI2)20, C (green online): (Ag2S)z (As2S3)50 (HgI2)50 z. The empty symbols correspond to lightly crystallized samples. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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rate of 20 mol% belonging to B- and C-series are also plotted, (Ag2S)5(As2S3)75(HgI2)20 and (Ag2S)30(As2S3)50(HgI2)20, respectively. Analogy with the A-series glass containing 20 mol% of HgI2 is observed since the wavenumber cut-off is more or less equivalent ( 410 cm 1). However significant differences in the transparency window are observed. For example, absorption around 490 cm 1 is seen in glasses of B- and C-series contrary to the A-series.
4. Discussion 4.1. Density and thermal properties In the A-series, the density monotonically decreases as a direct function of x (Fig. 3). This behaviour is expected since the density of mercury iodide (dHgI2 ¼6.36 g cm 3) is higher than that of the host glass (dAs2 S3 2Ag2 S ¼4.64 g cm 3). A similar explanation may be applied to the density in the B-series. Density in the B-series
increases with y because the density of vitreous Ag2S (dAg2 S g ¼7.17 g cm 3) is higher than the density of the vitreous As2S3 (dAs2 S3 g ¼3.68 g cm 3). In the C-series, the difference of density between vitreous Ag2S and HgI2 is probably too low to generate a variation. As previously mentioned, Tg increases following the rate of Ag2S in A- and C-series (Fig. 5). This feature is likely to be directly correlated to the content of HgI2 in the As2S3–Ag2S. The structure of glasses in the Ag–As–S system has been widely studied in the past. The glass containing the same molar concentration of Ag2S and As2S3, that is to say our host matrix, is composed of a majority of edge or corner sharing trigonal pyramids AgS3 [31] and of As3S6 rings [28] that forming a highly connected network. Consequently, the addition of HgI2 in the composition probably induces terminal iodine and is responsible of a depolymerization of the vitreous network, and higher its concentration is lower the Tg is. This is in good accordance with the results obtained for the B-series since the substitution of As2S3 by Ag2S with a constant HgI2 content has no consequence in the evolution of Tg that remains almost constant. Also previous investigations on the pseudo-binary Ag2S–As2S3 for the Ag-rich phase [19] have shown a similar constancy in Tg vs. the composition. The only difference concerns the value of the mean Tg for each system since in our case the Tg is around 135 1C whereas in the Ag2S–As2S3 system Tg is close to 165 1C, which is consistent with the fact that the addition of HgI2 on the Ag2S–As2S3 matrix is followed by a decrease of Tg. Some particular compositions do not exhibit Tx. In the other cases, the values of DT vary between 21 and 254 1C. No systematic evolution of DT can find in the three investigated series meaning that the stability of these glasses against crystallization is very sensitive to the compositions.
4.2. Ion transport in the HgI2–Ag2S–As2S3 glasses
Fig. 6. Cole–Cole diagrams of the (Ag2S)37.5(As2S3)37.5(HgI2)25 glass collected at 298, 325, 348, and 374 K. The plots corresponding to 348 and 374 K are also given in the inset to a better visibility. Arrows show the increasing measurement frequency o.
A qualitative analysis of the Cole–Cole complex impedance diagrams has shown that the investigated Ag2S–As2S3–HgI2 ternary glasses seem to be ionic conductors. A detailed study using silver tracer diffusion is in progress but it should be noted that the 110mAg tracer diffusion and Wagner d.c. Polarization measurements
Fig. 7. Evolution of the total electric conductivity as a function of 1000/T (K 1) in the three studied series: A (red online): (Ag2S)50 x/2 (As2S3)50 x/2 (HgI2)x, B (blue online): (Ag2S)y (As2S3)80 y (HgI2)20, C (green online): (Ag2S)z (As2S3)50 (HgI2)50 z. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 9. Far-infrared transparency window of some ChGs of the Ag2S–As2S3–HgI2 system. Other compositions, As2S3, As20Se80 and Te20Se80 are also plotted to give comparative data.
Fig. 8. Influence of the Ag2S content on the (a) Pre-exponential factor s0, (b) activation energy Es and (c) conductivity at 298 K s298 K for the three investigated series. þ revealed the silver ion transport number tAg E1 for binary Ag2S– As2S3 parents with silver concentration above 1 at% Ag [17,32]. In other words, the above hypothesis appears to be reasonable. Assuming the ternary Ag2S–As2S3–HgI2 glasses to be Ag þ ion conductors, the composition dependences of the room temperature conductivity s298 K and activation energy Es for the B-series glasses, (Ag2S)y (As2S3)80 y (HgI2)20, are consistent with the expected trend: s298 K increases and Es decreases with increasing silver content (Fig. 8). In contrast, the A-series glasses, (Ag2S)50 x/ 2 (As2S3)50 x/2 (HgI2)x, exhibit a counter-intuitive tendency: s298 K increases and Es decreases with decreasing silver content. It indicates that the mercury iodide additions increase the Ag þ ion transport with simultaneous decrease of the Ag2S concentration. Three possible scenarios might be responsible for the observed phenomenon: (i) a change in the conductivity mechanism from
predominantly ionic to electronic transport; (ii) an exchange reaction in the glass-forming melt and formation of intrinsic conductivity pathways in the glass network characterizing by high Ag þ ion mobility; and (iii) the appearance of additional extrinsic conductivity pathways related to HgI2. In this last scenario, hypothesis on the enlargement of the traditional conduction pathways due to the addition of another entity has to take into account. Consequently, the ion mobility could be more pronounced. The change in the conductivity mechanism seems to be unrealistic. Mercury iodide is a wide-gap insulator [33,34] confirmed indirectly by colour changes in the ternary glasses; from nearly black binary parents to deep red ternary glassy alloys. These colour changes suggest widening of the optical gap and consequently decrease of the electronic conductivity with increasing HgI2 content. The two other scenarios are more likely. The glassy/crystalline ternary alloys contain both AgI and Ag2HgI4 crystalline phases, indicating exchange reactions in the glass-forming melt. As a result, at least part of silver ions in the glass has an iodide environment giving rise to the formation of intrinsic conductivity pathways formed by silver iodide. The Ag þ ions appear to be highly mobile within these pathways evidenced by the numerous experiments on AgI-containing superionic glasses (see, for example, Refs. [35,36]). In addition, it was also found that heavy metal iodides increase Ag þ and Cu þ ion conductivity in superionic glasses: CuI–PbI2–As2Se3, CuI–SbI3–As2Se3, AgI–SbI3–Sb2S3, AgI–PbI2–Sb2S3 [37,38], giving rise to a hypothesis of extrinsic conductivity pathways [39]. We are planning to verify the two approaches using a quantitative modelling of the exchange reactions. The C-series glasses, (Ag2S)z (As2S3)50 (HgI2)50 z, in which mercury iodide is substituted by Ag2S, appears to be an intermediate case, when the two opposite trends related to Ag2S and HgI2 compete in controlling the Ag þ ion transport. 4.3. Far-infrared transparence window The A-series glasses of this pseudo-ternary system exhibit an expected trend since addition of HgI2, corresponding to the heavier compound in the glass composition, is characterized by a decrease of the wavenumber cut-off (Fig. 9). The IR measurements on ChGs in the literature are essentially carried out in the 400–4000 cm 1 range although important applications are possible at much lower wavenumbers corresponding to the THz domain. One of the challenges for applications in the THz domain is the development of new classes of materials transparent in this waveband. The addition of heavy
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elements in the ChGs composition was implemented. This approach is hindered by the high number of species resulting in a large number of potential vibrations, meaning that the multi-phonon absorption will be very significant. As shown in Fig. 9, the two glass compositions Te20Se80 and As20Se80 offer very interesting properties in the THz region since a transparency window (almost 20% of transmission) appears between 140 and 190 cm 1 and 140 and 205 cm 1, respectively. These specific compositions are very Se-rich that involves a low reticulation of the structural network. Complementary experiments are scheduled on many binary systems in order to give valuable information in order to synthesize glasses to be used as filters in the far-infrared domain.
5. Conclusion In this paper, we have investigated the pseudo-ternary Ag2S– As2S3–HgI2 system on the base of three glass series. The influence of the HgI2 and Ag2S content in the glass composition has been highlighted with the evolution of glass transition and crystallization temperatures and the density. Following the glass composition, the room temperature conductivity varies between 10 3 and 10 8 S cm 1 for these glasses. HgI2 addition in (Ag2S)50(As2S3)50 glass matrix has revealed an unexpected behaviour since the conductivity increases although the content of Ag þ is decreasing. The hypothesis on the ionic conductivity mechanism has been proposed on the base of intrinsic or extrinsic conductivity pathways related to Ag þ ion mobility and HgI2, respectively. The positive effect of HgI2 on the far-infrared transmission has also been underlined since the wavenumber cut-off is decreased down to 400 cm 1.
Acknowledgments This work was supported by both IRENI and the European Commission within the Interreg IVA (CleanTech project) programme. The authors acknowledge SOLEIL for provision of synchrotron radiation facilities. The team of AILES group (Synchrotron SOLEIL) and Ve´ronique Jean-Prost (BRGM) are also gratefully thanked for their valuable assistance in the FTIR measurements and the XRD experiments, respectively.
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jssc.2013.01.006.
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Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc
Study of the pseudo-ternary Ag2S–As2S3–HgI2 vitreous system R. Boidin a, D. Le Coq a,b,n, A. Cuisset a, F. Hindle a, J.-B. Brubach c, K. Michel d, E. Bychkov a a
LPCA, ULCO Univ. Lille Nord de France, EA 4493, Dunkerque F-59140, France Sciences Chimiques de Rennes, UMR CNRS 6226, Eq. Verres et Ce´ramiques – Univ. de Rennes I, Rennes F-35042, France AILES, Synchrotron SOLEIL, L’Orme des Merisiers Saint-Aubin, Gif-sur-Yvette F-91192, France d BRGM, Direction Eau, Environnement et Ecotechnologies – Unite´ BGE, Orle´ans F-45060, France b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 October 2012 Received in revised form 3 January 2013 Accepted 6 January 2013 Available online 15 January 2013
Chalcogenide alloys in the Ag2S–As2S3–HgI2 pseudo-ternary system were synthesized and their vitreous nature was verified by X-ray diffraction. The glass transition and crystallization temperatures (Tg and Tc), the density (d), and the total electrical conductivity (s) were measured for all samples of three series, A, B, and C corresponding to (Ag2S)50 x/2(As2S3)50 x/2(HgI2)x, (Ag2S)y(As2S3)80 y(HgI2)20 and (Ag2S)z(As2S3)50(HgI2)50 z, respectively. The maximum of Tg was approximately 160 1C for glasses with low HgI2 content whereas the maximum of density (5.75 g cm 3) was obtained for the sample in the B-series with the highest Ag2S concentration (z ¼ 60 mol%). This composition also possesses the highest conductivity at 298 K (s298 K E10 3 S cm 1). Unexpectedly the conductivity of the A-series samples was observed to decrease as a function of the Ag2S content. The far-infrared transmission in the 100–600 cm 1 window range (3.3–18.2 THz, 100–16.6 mm) was also given for a few glass compositions highlighting the strong influence of the HgI2 content. & 2013 Elsevier Inc. All rights reserved.
Keywords: Chalcogenide glasses Ionic-conducting glasses Far-infrared transmission
1. Introduction Due to their broad infrared transmission, high linear and nonlinear refractive index, high photosensitivity, chalcogenide glasses (ChGs) have numerous optical and photonic applications. At present, the near- and mid-infrared radiation domains are very concerned by technology advancement in the ChG field and Refs. there in [1–3]. Curiously, the potential of ChGs for applications in the far-infrared (FIR) or TeraHertz (THz) domain has not been exploited. This domain, even if it is less mature than its neighbouring domains, has already been identified to have great potential for many applications including security screening [4], pharmaceutical, biological or medical science analysis [5,6], wireless communications [7], and environmental monitoring [8], etc. Research into suitable THz materials is now receiving more attention [9] and ChGs can contribute to the significant technological advancements by providing many optical components such as filters, lenses, and waveguides etc. ChGs belong to the family of vitreous semiconductors [10]. Their potential as solid-state electrolytes in sensors and batteries has already been demonstrated [11] and Refs. there in. More recently, application of ChGs to Non Volatile Memories (NVM) has also been the subject of many studies. Among the different NVM,
n Corresponding author at: University of Rennes 1, Sciences chimiques de Rennes, Verres et Ce´ramiques, UMR CNRS 6226, Rennes Eq. 35042, France. E-mail address: [email protected] (D. Le Coq).
0022-4596/$ - see front matter & 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jssc.2013.01.006
the Phase Change Memories (PCM) and the Current Bridging Random Access Memories (CBRAM) are very promising technologies using the intrinsic properties of ChGs [12,13]. The pseudo-binary Ag2S–As2S3 glass system has been intensively studied because it is of interest for both fundamental and application studies [14]. It is vitreous for Ag2S molar fraction between 0% and 40%, and provides glasses with very interesting electrical conductivity behaviour [15,16]. At low content of Ag2S (o5 mol%), the conductivity is typically characteristic of a semiconducting glass, whereas if this content becomes higher than 15 mol%, it is characteristic of an ionic conductor [17,18]. At the same time, numerous experiments have been reported to establish relations between the conductivity and the structure in order to determine the concerned mechanisms [18–23]. The addition of other compounds in this pseudo-binary system has also been investigated essentially as part of ion detection in aqueous media. For example, glasses in CdS(or CdI2)-Ag2S-As2S3 systems have been used to develop membranes for ion-selective electrodes (ISE) devoted to Cd2 þ [24]. In a similar way, membranes for Pb2 þ –ISE have been characterized in the PbS(or PbI2)Ag2S-As2S3 glass systems [25,26]. Chalcogenide glasses in AgBr– Ag2S–As2S3 have been studied to develop membrane of Hg2 þ -ISE by vacuum thermal evaporation [27]. In a development Hg2 þ -ISE and perspective terahertz materials, we have investigated the pseudo-ternary Ag2S–As2S3–HgI2 even if the glass-forming region and structures by means of vibrational spectroscopy have already been reported [28]. The objective of this work is to determine other characteristics such as
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macroscopic properties, conductivity, and far-infrared transmission in order to estimate the potential of these ChGs in both electrical and optical point of views. Consequently, three different series were synthesized to estimate the role of each compound. In A-series, (Ag2S)50 x/2 (As2S3)50 x/2 (HgI2)x, 0r xr50, addition of HgI2 in the Ag2S–As2S3 matrix is carried out. In B-Series, (Ag2S)y (As2S3)80 y (HgI2)20, 5 mol%ryr60 mol%, a substitution of As2S3 by Ag2S with a constant HgI2 content equal to 20 mol% is operated. Finally, the C-series corresponds to the specific composition (Ag2S)z (As2S3)50 (HgI2)50 z, 2.5 mol%rz r50 mol%, meaning that the substitution of HgI2 by Ag2S with a constant As2S3 content of 50 mol% is considered.
2. Experimental 2.1. Glass preparation The preparation of various compositions was carried out by performing the prior synthesis of Ag2S and As2S3 and by using commercial HgI2 (Sigma-Aldrich 99.999%). HgI2 powder was used without any additional purification. As2S3 and Ag2S were synthesized from arsenic (Cerac 99.9999%) and silver (Neyco 99.999%) pieces with the required amount of sulphur pellets (Acros Organics 99.999%). The high vapour pressure contaminants (oxides) in As and Ag were removed by evaporation at 320 and 640 1C respectively under vacuum. The sulphur was heated at 130 1C under vacuum then was sealed and distilled at 430 1C. The synthesis of As2S3 and Ag2S was performed at 800 and 950 1C in a rocking furnace respectively before quenching in cold water. The appropriate proportions of the base compounds HgI2, Ag2S and As2S3 were heated to prepare the Ag2S–As2S3–HgI2 alloys. The mixing of the compounds was homogenized in a rocking furnace at 800 1C for 24 h. The melt was cooled down to 650 1C before quenching in air. The samples had masses varying between 2 and 10 g. 2.2. Macroscopic properties measurements The measurement of the sample density d was performed by a hydrostatic method using the toluene as immersion fluid and the germanium as standard (5.323 g cm 3). A Sartorius YDK 01-0D density kit was used for the measurements, the sample mass in air varied between 0.4 and 2.5 g. The amorphous nature of the samples was verified at room temperature by x-ray diffraction, XRD, technique using a Bruker
265
D8 Advance Da Vinci diffractometer equipped with a cobalt tube and a Linex eye detector. A Bruker D5000 was also used with a cobalt tube and a scintillation detector. The scattering intensities were measured over an angular range of 41r2y r841 with a step-size of D(2y)¼0.021 and a count time of 1 s per step. The Bragg peaks in diffraction patterns of glassy/crystalline samples were identified and indexed using the JCPDS database. Differential Scanning Calorimetry analyses using a TA instruments Q200 were performed to determine the glass transition (Tg), the crystallization (Tc) and melting (Tm). The samples of 3–15 mg were hermetically sealed in standard aluminium pans and were heated at a rate of 10 K min 1 from 10 to 400 1C in high purity nitrogen gas environment. 2.3. Impedance measurements Total electrical conductivity of the samples was measured using a Hewlett Packard 4194 A impedance meter. The impedance modulus Z and the phase angle y were obtained in the frequency range from 100 Hz to 15 MHz from room temperature up to 383 K, corresponding to a temperature below Tg for all the glass samples. The quenched samples, prepared as rectangular plates, were polished using SiC powder (9.3 mm grain size). The sample sides were ground parallel and gold was deposited on opposite sides to form electrodes, meaning that the electrochemical cell for conductivity measurements was Au9glass9Au. The typical sample thickness was 1.0 mm (70.2 mm) with a typical area of 7 mm2 (71 mm2). The temperature dependence on the conductivity was studied over several cycles. Each cycle consisted of a heating step followed by a cooling step, in order to investigate a possible hysteresis. No significant hysteresis was observed for any of the samples. 2.4. Far-infrared window FIR transmissions measurements performed on the AILES (Advanced Infrared Line Exploited for Spectroscopy) beamline [29] at the French national synchrotron facility SOLEIL. Using the Bruker IFS 125 interferometer, the spectra were recorded in the 10–1000 cm 1 spectral region with a resolution of 2 cm 1 by coadding the Fourier transform of 200 interferograms. The synchrotron radiation was focused on a 1.5 mm entrance aperture of the interferometer containing a Mylar 6 mm multilayer beamsplitter suitable for the FIR/THz spectral ranges. The interferometer was installed in a vacuum chamber allowing an operational pressure
Fig. 1. Glass-forming region in the Ag2S–As2S3–HgI2 pseudo-ternary system with the three investigated series; A (red online): (Ag2S)50 x/2 (As2S3)50 x/2 (HgI2)x, B (blue online): (Ag2S)y (As2S3)80 y (HgI2)20, C (green online): (Ag2S)z (As2S3)50 (HgI2)50 z. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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of 10 6 mbar to be achieved to eliminate the absorption due to atmospheric compounds. Thanks to the use of a 4 K heliumcooled silicon bolometer equipped with an optical band-pass filter and to the high brightness of the synchrotron FIR light (100–1000 times greater than thermal source) high signal/noise ratio levels were obtained over the spectral range 10–1000 cm 1. The transmission spectra were obtained on glass samples 400 mm thick by dividing the signal transmitted through the ChG disk by the signal measured in the absence of a sample but identical in all other conditions. Interesting results are only found in the windows between 100 and 600 cm 1.
3. Results 3.1. Glass-forming region The glass-forming region of the pseudo-ternary system Ag2S–As2S3–HgI2 obtained in our conditions of synthesis is presented in Fig. 1 appears to be wider than that reported earlier [28] and spreads over almost half of the diagram. The vitreous domain is preferentially located in the rich-As2S3 part and a minimum of 20 mol% of As2S3 is required. The maximum solubility of Ag2S and HgI2 are approximately 65 and 45 mol%, respectively. The XRD patterns of four different samples belonging to the A-series are shown in Fig. 2. For this series, the first crystallization peaks appears when the HgI2 content is higher than 40 mol%. The indexation of the peaks by using JCPDS database reveals that the crystallites correspond to AgI (00-009-0374) and Ag2HgI4 (00-0321016) crystalline phases suggesting that the following exchange reactions could take place during the homogenization step:
(1) Ag2SþHgI222AgI þHgS (2) 2AgI þHgI22Ag2HgI4
Table 1 Density and characteristic temperatures (glass transition (Tg) and crystallization temperature (Tc)) of samples belonging to the three investigated series: A: (Ag2S)50 x/2 (As2S3)50 x/2 (HgI2)x, B: (Ag2S)y (As2S3)80 y (HgI2)20, C: (Ag2S)z (As2S3)50 (HgI2)50 z. The partially crystallized samples are noted with an asterisk. Sample Density (g cm 3) Tg (1C) (7 2 1C) Tc (1C) ( 72 1C) DT (1C) (7 4 1C) A-series: x ¼0 x ¼10 x ¼15 x ¼20 x ¼25 x ¼30 x ¼35 x ¼40 x ¼45n x ¼50n
(Ag2S)50 x/2 (As2S3)50 x/2 (HgI2)x 4.642 7 0.009 162 4.8057 0.004 144 4.934 7 0.003 137 5.0297 0.004 134 5.0757 0.011 126 5.146 7 0.011 120 5.263 7 0.002 102 5.264 7 0.019 92 5.411 7 0.007 76 5.5047 0.008 76
238 215 264 227 230 198 190 162 152 138
76 71 127 93 104 78 88 70 76 62
B-series: y¼ 5 y¼ 20 y¼ 30 y¼ 40 y¼ 50 y¼ 60
(Ag2S)y (As2S3)80-y 3.838 7 0.011 4.3057 0.010 4.617 7 0.012 5.0297 0.003 5.325 7 0.013 5.752 7 0.003
170 277 230 227 156 185
32 138 99 93 21 57
C-series: z ¼2.5n z ¼5n z ¼7.5n z ¼10 z ¼12.5 z ¼15 z ¼17.5 z ¼22.5 z ¼25 z ¼30 z ¼35 z ¼40 z ¼45 z ¼50
(Ag2S)z (As2S3)50 (HgI2)50 z 4.5507 0.009 216 4.646 7 0.011 137 4.5607 0.002 131 4.548 7 0.007 114 4.563 7 0.013 101 4.583 7 0.005 101 4.611 7 0.015 108 4.631 7 0.019 120 4.616 7 0.002 121 4.617 7 0.012 131 4.655 7 0.020 138 4.568 7 0.033 141 4.618 7 0.012 151 4.642 7 0.009 162
383 241 170 390 150 171 / / 336 230 260 263 215 238
167 104 39 254 49 70 / / 223 99 122 122 64 76
(HgI2)20 138 139 131 134 135 128
3.2. Density and thermal properties The sample density of the three series A, B, and C following the Ag2S content are given in Table 1 and plotted in Fig. 3. A decrease of density following the Ag2S content is observed from 5.504 to 4.642 g cm 3 in the A-series. On the contrary, in the
Fig. 3. Evolution of the density versus the Ag2S mol% for the three investigated A-, B-, and C-series corresponding to (Ag2S)50 x/2 (As2S3)50 x/2 (HgI2)x, B: (Ag2S)y (As2S3)80 y (HgI2)20, C: (Ag2S)z (As2S3)50 (HgI2)50 z, respectively. The empty symbols correspond to lightly crystallized samples.
Fig. 2. XRD patterns of four samples of the A-series (Ag2S)50 x/2 (As2S3)50 x/2 (HgI2)x, x ¼0, 20, 45, and 40 mol%.
B-series, the density increases from 3.838 to 5.752 g cm 3. Density in the C-series remains almost constant around 4.6 g cm 3. The thermal parameters were extracted from the DSC trace. Typical curves of the three A, B and C-series are presented in Fig. 4. Each analyzed sample shows a single Tg, which is
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characteristic of an homogeneous glass in macroscopic and mesoscopic scales. The evolution of Tg for the three investigated series are plotted in Fig. 5 following the Ag2S content. The A-series is characterized by an increase of Tg from 76 1C72 1C (x¼ 50 mol%) up to 162 1C72 1C (x¼ 0 mol%) meaning that HgI2 addition in the (Ag2S)50 (As2S3)50 matrix is harmful in a thermal point of view. The Tg of the B-series is relatively constant since it varies between 128 1C72 1C (y¼60 mol%) and 139 1C72 1C (y¼20 mol%). At last, the evolution of Tg in C-series shows two distinct behaviours. Firstly, the substitution of HgI2 by Ag2S is characterized by a large decrease of Tg from 216 1C72 1C (z¼2.5 mol%) down to 114 1C72 1C (z¼10 mol%) but as mentioned in Fig. 5, these data correspond to partially crystallized samples. When the samples are completely vitreous, Tg weakly decreases down to 101 1C72 1C before linearly increasing with the Ag2S content to reach 162 1C for the z¼50 mol%.
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3.3. Electrical conductivity Typical complex impedance plots or Cole–Cole diagrams of the investigated glasses are shown in Fig. 6. In the high-frequency range, the diagrams represent distorted semi-circles typical for glassy ionic conductors [30]. A low-frequency polarization is observed for the samples measured using an electrochemical cell with blocking Au electrodes. This polarization characterizes difficulties of the charge transfer at the Au/glass interface and indicates a predominantly ionic conductivity in the sample, most probably caused by Ag þ ions. Temperature dependence on the conductivity of some samples is shown in Fig. 7. These data obey the Arrhenius law: s Es ð1Þ s ¼ 0 exp T kT where s0 is the pre-exponential factor, Es the activation energy, k the Boltzmann constant and T the temperature. The conductivity at 298 K (s298 K), s0, and Es were calculated from a least-square fit of the data to the Arrhenius law (Eq. (1)) and their evolution as a function of Ag2S content is plotted in Fig. 8. The pre-exponential factor remains almost constant for each series at approximately 3 105 S K cm 1. Consequently, the activation energy and conductivity at 298 K exhibit opposite trends. When s298 K increases from 7.8 10 8 to 6.5 10 4 S cm 1, the activation energy decreases from 0.6 to 0.3 eV. Conductivity evolution as a function of Ag2S content is different for each series. The conductivity decreases in series A while it increases in series B and remains nearly invariant in series C (except for a very low level of Ag2S). 3.4. Far-infrared transmission
Fig. 4. Typical DSC curves of glasses belonging to the A-, B-, and C-series. Insets for the A- and B-series are given to emphasize the glass transition phenomenon.
The glasses of this pseudo-ternary system possess a relatively interesting infrared transmission as exhibited in Fig. 9. If the HgI2 content of the A-series increases the glasses show better transmission and also a lower shift of the wavenumber cut-off. The maximum of transmission in the 100–600 cm 1 window goes from about 26% for x ¼0 up to more than 60% for x¼45 mol%. Correspondingly, the wavenumber cut-off decreases by about 30 cm 1. The FIR transmission of two other glasses with a HgI2
Fig. 5. Evolution of the glass transition temperature (Tg) following the Ag2S content (mol%) for the three investigated series: A (red online): (Ag2S)50 x/2 (As2S3)50 x/2 (HgI2)x, B (blue online): (Ag2S)y (As2S3)80 y (HgI2)20, C (green online): (Ag2S)z (As2S3)50 (HgI2)50 z. The empty symbols correspond to lightly crystallized samples. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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rate of 20 mol% belonging to B- and C-series are also plotted, (Ag2S)5(As2S3)75(HgI2)20 and (Ag2S)30(As2S3)50(HgI2)20, respectively. Analogy with the A-series glass containing 20 mol% of HgI2 is observed since the wavenumber cut-off is more or less equivalent ( 410 cm 1). However significant differences in the transparency window are observed. For example, absorption around 490 cm 1 is seen in glasses of B- and C-series contrary to the A-series.
4. Discussion 4.1. Density and thermal properties In the A-series, the density monotonically decreases as a direct function of x (Fig. 3). This behaviour is expected since the density of mercury iodide (dHgI2 ¼6.36 g cm 3) is higher than that of the host glass (dAs2 S3 2Ag2 S ¼4.64 g cm 3). A similar explanation may be applied to the density in the B-series. Density in the B-series
increases with y because the density of vitreous Ag2S (dAg2 S g ¼7.17 g cm 3) is higher than the density of the vitreous As2S3 (dAs2 S3 g ¼3.68 g cm 3). In the C-series, the difference of density between vitreous Ag2S and HgI2 is probably too low to generate a variation. As previously mentioned, Tg increases following the rate of Ag2S in A- and C-series (Fig. 5). This feature is likely to be directly correlated to the content of HgI2 in the As2S3–Ag2S. The structure of glasses in the Ag–As–S system has been widely studied in the past. The glass containing the same molar concentration of Ag2S and As2S3, that is to say our host matrix, is composed of a majority of edge or corner sharing trigonal pyramids AgS3 [31] and of As3S6 rings [28] that forming a highly connected network. Consequently, the addition of HgI2 in the composition probably induces terminal iodine and is responsible of a depolymerization of the vitreous network, and higher its concentration is lower the Tg is. This is in good accordance with the results obtained for the B-series since the substitution of As2S3 by Ag2S with a constant HgI2 content has no consequence in the evolution of Tg that remains almost constant. Also previous investigations on the pseudo-binary Ag2S–As2S3 for the Ag-rich phase [19] have shown a similar constancy in Tg vs. the composition. The only difference concerns the value of the mean Tg for each system since in our case the Tg is around 135 1C whereas in the Ag2S–As2S3 system Tg is close to 165 1C, which is consistent with the fact that the addition of HgI2 on the Ag2S–As2S3 matrix is followed by a decrease of Tg. Some particular compositions do not exhibit Tx. In the other cases, the values of DT vary between 21 and 254 1C. No systematic evolution of DT can find in the three investigated series meaning that the stability of these glasses against crystallization is very sensitive to the compositions.
4.2. Ion transport in the HgI2–Ag2S–As2S3 glasses
Fig. 6. Cole–Cole diagrams of the (Ag2S)37.5(As2S3)37.5(HgI2)25 glass collected at 298, 325, 348, and 374 K. The plots corresponding to 348 and 374 K are also given in the inset to a better visibility. Arrows show the increasing measurement frequency o.
A qualitative analysis of the Cole–Cole complex impedance diagrams has shown that the investigated Ag2S–As2S3–HgI2 ternary glasses seem to be ionic conductors. A detailed study using silver tracer diffusion is in progress but it should be noted that the 110mAg tracer diffusion and Wagner d.c. Polarization measurements
Fig. 7. Evolution of the total electric conductivity as a function of 1000/T (K 1) in the three studied series: A (red online): (Ag2S)50 x/2 (As2S3)50 x/2 (HgI2)x, B (blue online): (Ag2S)y (As2S3)80 y (HgI2)20, C (green online): (Ag2S)z (As2S3)50 (HgI2)50 z. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 9. Far-infrared transparency window of some ChGs of the Ag2S–As2S3–HgI2 system. Other compositions, As2S3, As20Se80 and Te20Se80 are also plotted to give comparative data.
Fig. 8. Influence of the Ag2S content on the (a) Pre-exponential factor s0, (b) activation energy Es and (c) conductivity at 298 K s298 K for the three investigated series. þ revealed the silver ion transport number tAg E1 for binary Ag2S– As2S3 parents with silver concentration above 1 at% Ag [17,32]. In other words, the above hypothesis appears to be reasonable. Assuming the ternary Ag2S–As2S3–HgI2 glasses to be Ag þ ion conductors, the composition dependences of the room temperature conductivity s298 K and activation energy Es for the B-series glasses, (Ag2S)y (As2S3)80 y (HgI2)20, are consistent with the expected trend: s298 K increases and Es decreases with increasing silver content (Fig. 8). In contrast, the A-series glasses, (Ag2S)50 x/ 2 (As2S3)50 x/2 (HgI2)x, exhibit a counter-intuitive tendency: s298 K increases and Es decreases with decreasing silver content. It indicates that the mercury iodide additions increase the Ag þ ion transport with simultaneous decrease of the Ag2S concentration. Three possible scenarios might be responsible for the observed phenomenon: (i) a change in the conductivity mechanism from
predominantly ionic to electronic transport; (ii) an exchange reaction in the glass-forming melt and formation of intrinsic conductivity pathways in the glass network characterizing by high Ag þ ion mobility; and (iii) the appearance of additional extrinsic conductivity pathways related to HgI2. In this last scenario, hypothesis on the enlargement of the traditional conduction pathways due to the addition of another entity has to take into account. Consequently, the ion mobility could be more pronounced. The change in the conductivity mechanism seems to be unrealistic. Mercury iodide is a wide-gap insulator [33,34] confirmed indirectly by colour changes in the ternary glasses; from nearly black binary parents to deep red ternary glassy alloys. These colour changes suggest widening of the optical gap and consequently decrease of the electronic conductivity with increasing HgI2 content. The two other scenarios are more likely. The glassy/crystalline ternary alloys contain both AgI and Ag2HgI4 crystalline phases, indicating exchange reactions in the glass-forming melt. As a result, at least part of silver ions in the glass has an iodide environment giving rise to the formation of intrinsic conductivity pathways formed by silver iodide. The Ag þ ions appear to be highly mobile within these pathways evidenced by the numerous experiments on AgI-containing superionic glasses (see, for example, Refs. [35,36]). In addition, it was also found that heavy metal iodides increase Ag þ and Cu þ ion conductivity in superionic glasses: CuI–PbI2–As2Se3, CuI–SbI3–As2Se3, AgI–SbI3–Sb2S3, AgI–PbI2–Sb2S3 [37,38], giving rise to a hypothesis of extrinsic conductivity pathways [39]. We are planning to verify the two approaches using a quantitative modelling of the exchange reactions. The C-series glasses, (Ag2S)z (As2S3)50 (HgI2)50 z, in which mercury iodide is substituted by Ag2S, appears to be an intermediate case, when the two opposite trends related to Ag2S and HgI2 compete in controlling the Ag þ ion transport. 4.3. Far-infrared transparence window The A-series glasses of this pseudo-ternary system exhibit an expected trend since addition of HgI2, corresponding to the heavier compound in the glass composition, is characterized by a decrease of the wavenumber cut-off (Fig. 9). The IR measurements on ChGs in the literature are essentially carried out in the 400–4000 cm 1 range although important applications are possible at much lower wavenumbers corresponding to the THz domain. One of the challenges for applications in the THz domain is the development of new classes of materials transparent in this waveband. The addition of heavy
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elements in the ChGs composition was implemented. This approach is hindered by the high number of species resulting in a large number of potential vibrations, meaning that the multi-phonon absorption will be very significant. As shown in Fig. 9, the two glass compositions Te20Se80 and As20Se80 offer very interesting properties in the THz region since a transparency window (almost 20% of transmission) appears between 140 and 190 cm 1 and 140 and 205 cm 1, respectively. These specific compositions are very Se-rich that involves a low reticulation of the structural network. Complementary experiments are scheduled on many binary systems in order to give valuable information in order to synthesize glasses to be used as filters in the far-infrared domain.
5. Conclusion In this paper, we have investigated the pseudo-ternary Ag2S– As2S3–HgI2 system on the base of three glass series. The influence of the HgI2 and Ag2S content in the glass composition has been highlighted with the evolution of glass transition and crystallization temperatures and the density. Following the glass composition, the room temperature conductivity varies between 10 3 and 10 8 S cm 1 for these glasses. HgI2 addition in (Ag2S)50(As2S3)50 glass matrix has revealed an unexpected behaviour since the conductivity increases although the content of Ag þ is decreasing. The hypothesis on the ionic conductivity mechanism has been proposed on the base of intrinsic or extrinsic conductivity pathways related to Ag þ ion mobility and HgI2, respectively. The positive effect of HgI2 on the far-infrared transmission has also been underlined since the wavenumber cut-off is decreased down to 400 cm 1.
Acknowledgments This work was supported by both IRENI and the European Commission within the Interreg IVA (CleanTech project) programme. The authors acknowledge SOLEIL for provision of synchrotron radiation facilities. The team of AILES group (Synchrotron SOLEIL) and Ve´ronique Jean-Prost (BRGM) are also gratefully thanked for their valuable assistance in the FTIR measurements and the XRD experiments, respectively.
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jssc.2013.01.006.
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