Structure of Glasses in the Pseudobinary System Ga2Se3–GeSe2

Article pubs.acs.org/JPCB

Structure of Glasses in the Pseudobinary System Ga2Se3−GeSe2: Violation of Chemical Order and 8‑N Coordination Rule A. W. Mao,† B. G. Aitken,‡ R. E. Youngman,‡ D. C. Kaseman,† and S. Sen†,* †

Division of Materials Science, University of California at Davis, Davis, California 95616, United States Corning Inc., Corning, New York 14831, United States

‡

ABSTRACT: Structure of glasses in the pseudobinary system Ga2Se3−GeSe2 with Ga2Se3 content ranging from 6.3 to 30 mol % is investigated using a combination of Raman and multinuclear (71Ga, 77Se) solid state nuclear magnetic resonance (NMR) spectroscopy. The results indicate that the structure of these glasses consists primarily of a corner sharing network of (Ge/Ga)Se4 tetrahedra with some fraction of edge-sharing GeSe4 tetrahedra and of ethane-like (Se3)Ge−Ge(Se3) units, in which the Ga, Ge, and Se atoms adopt coordination numbers of 4, 4, and 2, respectively. As expected, the concentration of metal− metal bonds increases with addition of Ga2Se3 as the glass structure becomes too deficient in Se to satisfy the tetrahedral coordination of both Ga and Ge by Se atoms alone. These metal−metal bonds are mostly limited to Ge−Ge homopolar bonds, indicating a violation of chemical order. At relatively high degrees of Se-deficiency, however, spectroscopic evidence suggests the formation of triply coordinated Se atoms as an alternate mechanism to accommodate the tetrahedral coordination of Ga and Ge atoms. This observation indicates a violation of the 8-N coordination rule and is reminiscent of oxygen triclusters in isoelectronic Al2O3−SiO2 glasses. Compositional variation of physical properties such as density, molar volume, optical band gap, glass transition temperature, and fragility are shown to be consistent with the proposed structural model. Ge−Se system.7,8,12−17 Overall, these studies appear to agree that the structure of Ga−Ge−Se glasses tends to follow the typical “chalcogenide” structural scenario exemplified by Ge−X and As−X glasses (where X = S, Se) with continuous alloying between the constituent elements. The structure of glasses in the pseudobinary Ga2Se3−GeSe2 system is thought to be characterized by a primarily tetrahedral network of corner- and edge-shared tetrahedra with the coordination numbers of 4, 4, and 2 for Ga, Ge, and Se, respectively. While Ga does not follow the 8-N rule like Ge and Se, its tetrahedral coordination has been confirmed in selenide glasses on the basis of Raman, extended X-ray absorption fine structure (EXAFS) and X-ray photoelectron (XPS) spectroscopy,7,8 and in sulfide glasses by 71 Ga nuclear magnetic resonance (NMR) spectroscopy.18 Since Ga has only three valence electrons, the fourth Ga−Se bond was proposed to be a dative covalent bond in which a chalcogen atom contributes both electrons in one of its lone pairs to the bond.12 In the pseudobinary system Ga2Se3−GeSe2, the addition of Ga2Se3 effectively introduces a Se-deficiency, since Ga2Se3 lacks one Se atom per formula unit to form GaSe4 tetrahedra. As a result, the formation of metal−metal (M−M) bonds is expected within the GeSe2 network and has been observed with increasing Ga2Se3 content, consistent with the predictions of the chalcogenide structural scenario.

1. INTRODUCTION Chalcogenide glasses are technologically important materials that can be suitably designed to exhibit many interesting physical properties including low phonon energy, significant optical nonlinearity, large photosensitivity, and high ionic conductivity.1−6 The structure−property relationships in these glasses have been described within the framework of “continuous alloying” of the constituent elements that form a covalently bonded network with average connectivity being controlled by the coordination numbers of group IV, V, and VI elements that follow the 8-N rule. Group III elements such as Ga and In, when incorporated into these glasses, are shown to adopt a 4-fold coordination in most cases.7−9 We have recently begun a systematic study of nominally stoichiometric compositions in the BaSe−Ga2Se3−GeSe2 system10,11 to compare and contrast the structures of these chalcogenide glasses with their isoelectronic analogues in the oxide systems (e.g., CaO−Al2O3−SiO2). However, in the investigation of such complex, nominally stoichiometric, pseudoternary glass forming systems it is crucial to understand the compositional evolution of the structure−property relationships in the individual pseudobinary components. Unfortunately, such an investigation is complicated by the fact that the pseudobinary systems BaSe− GeSe2 and BaSe−Ga2Se3 have limited glass-forming ability, at least via the conventional melt-quenching route. The only pseudobinary system that displays glass-forming ability over a significant composition range is Ga2Se3−GeSe2. A number of studies have been reported in the literature concerning structural and topological investigations of glasses in the Ga− © XXXX American Chemical Society

Received: October 8, 2013 Revised: November 26, 2013

A

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2.3. NMR Spectroscopy. 77Se magic angle spinning (MAS) NMR spectra of all GGS glasses were collected on a Bruker Avance 500 spectrometer operating at 11.7 T (77Se resonance frequency = 95.4 MHz). Crushed samples were packed into ZrO2 rotors and spun at ∼14 kHz in a Bruker 4 mm tripleresonance MAS probe. Spectra were collected using a Hahn− echo pulse sequence with a 2.6 μs π/2 pulse length and 300 s recycle delay, and the echo maximum was shifted out by one rotor period. A total of ∼900 scans were averaged and Fourier transformed to obtain each spectrum. The 77Se spectra of the crystalline compounds GaSe and Ga2Se3 were collected on a Varian spectrometer operating at 4.7 T (77Se resonance frequency =38.3 MHz) and a 5 mm MAS probe. Samples were spun at 6 to 8 kHz in ZrO2 rotors and spectra were collected using a Hahn−echo pulse sequence with a 2.6 μs π/2 pulse length and 300 s recycle delay. Approximately 500 to 1000 scans were averaged and Fourier transformed to obtain each spectrum. The chemical shifts of all 77Se spectra were referenced to the 77Se isotropic shift (δiso = 1040.2 ppm) of the crystalline compound (NH4)2SeO4.23 71 Ga NMR spectra of GGS glasses were collected using the Bruker Avance 500 spectrometer mentioned above (71Ga resonance frequency =152.5 MHz). Samples were packed into 2.5 mm ZrO2 rotors and spun at ∼35 kHz. Spectra were collected using a Hahn−echo pulse sequence with a 0.6 μs π/2 pulse length and 0.2 s recycle delay. The echo maximum was shifted out by three rotor periods. The displayed spectra are the results of averaging and Fourier transforming ∼32 000 scans. 71 Ga spectra were referenced to a 1 M aqueous solution of gallium nitrate. 2.4. Density Measurements. Densities of all glass samples were measured using a Micromeritics AccuPyc II gas expansion pycnometer under a helium environment (6 N purity). Samples (∼1.5 to 2 g) were taken in a 1 cm3 measuring cup. Reported densities are averages of 10 consecutive measurements taken at 20 °C. Errors of all averaged measurements are within 0.5 mg/ cm3. 2.5. Optical Absorption Spectroscopy. Optical absorption spectra were measured using a Cary 50 Bio UV−vis spectrophotometer in the frequency range of 400−900 nm, on glass samples that were ground to a thickness of ∼300 to 500 μm using SiC grinding paper and optically polished using diamond suspension. The optical transmission near the absorption edge of a glass, assuming an indirect band gap, can be expressed as follows:10

Nevertheless, several key questions regarding the structures of these glasses remain unanswered. The first question involves the identities of the M−M bonds. While the formation of such bonds is more or less undisputed, there is little evidence concerning their specific identities as homopolar Ga(Ge)− Ga(Ge) bonds vs mixed Ga−Ge bonds. This discrepancy arises mostly from the inability of Raman spectroscopy to differentiate between Ga and Ge atoms, because of their similarity in both atomic mass and bonding character. This drawback of Raman spectroscopy also brings into question the issue of Ga coordination. Although a tetrahedral coordination is consistent with previous studies, there is relatively little direct evidence of 4-fold coordinated Ga atoms.7 The third question concerns the coordination of Se atoms. All of the data in the literature support a network dominated by corner and edge-shared GeSe2 tetrahedra, in which all Se exists in 2-fold coordination, regardless of composition. However, it is possible that for high degrees of Se-deficiency in Ga2Se3-rich glasses, besides the formation of M−M bonds, some Se atoms are forced into higher (e.g., 3-fold) coordination states. 3-fold coordinated Se atoms are indeed present in the structures of crystalline Ga2Se319 and GaSe.20 This is reminiscent of 3-coordinated oxygen atoms (i.e., oxygen triclusters) proposed to exist in the analogous, isoelectronic oxide glasses in the system Al2O3− SiO221 where the formation of such overcoordinated oxygen atoms serves as a means to help alleviate the deficiency in O atoms resulting from Al atoms adopting 4-fold coordination. Here, we present the results of a combined Raman and multinuclear (71Ga, 77Se) NMR spectroscopic study of the structure of Ga2Se3−GeSe2 glasses in an attempt to address the above-mentioned issues. The resulting structural model is used to draw structure−property relationships based on measurements of density, molar volume, optical band gap, glass transition temperature, and fragility.

2. EXPERIMENTAL METHODS 2.1. Synthesis. Glasses in the system (Ga2Se3)x(GeSe2)100‑x (hereafter referred to as GGS glasses) are synthesized using conventional melt-quench techniques, with x = 6.3, 21.1, 25, and 30 (in mol %). Glasses were made in 10 g batches. The constituent elements were placed into fused silica ampules, which were then evacuated to ∼10−5 Torr and flame-sealed. Batches were melted in a rocking furnace at 900 °C for ∼3 days and ampules were quenched in water. The crystalline reference β-Ga2Se3 was synthesized using a method similar to that described by Finkman et al.22 A 5g batch of the constituent elements was placed into a fused silica ampule, evacuated, and flame-sealed. The batch was slowly heated in a rocking furnace up to 1100 °C, homogenized for ∼20 h, subsequently slowly cooled in the furnace to 900 °C, and held at this temperature for 24 h. Finally, the batch was cooled to 850 °C and held for another 48 h before cooling to room temperature by shutting off the furnace. The GaSe crystal was commercially obtained from Alfa Aesar. The crystal structures of these phases were confirmed by powder X-ray diffraction. 2.2. Raman Spectroscopy. Unpolarized Raman spectra were collected at ambient temperature and in backscattering geometry using a Bruker RFS 100/S Fourier transform (FT) Raman spectrometer equipped with a Nd:YAG laser operating at 1064 nm. The power level was set to 30 mW and the resolution was configured to 1 cm−1. Approximately 32 scans were collected and averaged to obtain each Raman spectrum.

2 ⎞ ⎛ − B (E photon − Eg ) ⎜ d⎟⎟ T = A exp⎜ Ephoton ⎝ ⎠

(1)

In the above equation, T is the transmittance (I/I0), Ephoton is the energy of the incident photon, Eg is the optical band gap, and d is the sample thickness. The parameter A is equal to (1 − R)2, where R is the reflectance. The values for the optical band gap Eg of these BGGS glasses were obtained by fitting eq 1 to the high energy region of the corresponding optical transmission curves, where A, B, and Eg were used as fitting parameters that were assumed to remain constant for each sample over the narrow energy range examined. 2.6. Differential Scanning Calorimetry (DSC). Conventional DSC scans were measured using a Mettler-Toledo DSC1 calorimeter. Measurements were carried out in a flowing nitrogen environment on 15 to 25 mg samples taken in 40 μL Al crucibles. Scans were taken using a three-step program to B

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frequency band in region iii encompasses antisymmetric stretching modes of the GeSe4/2 tetrahedra near 300 cm−1, in addition to the out-of-phase stretching mode of edge-sharing GeSe4/2 tetrahedra in a 4-membered ring near ∼245 cm−1.8,30 As Ga2Se3 is added to the system, the relative intensity of the shoulder at ∼215 cm−1 decreases in intensity, while that of the shoulder at ∼175 cm−1 increases. Additionally, another shoulder appears near 160 cm−1 at x ≈ 21.1 and grows with Ga content (Figure 1). Minimal changes are observed in regions i and iii, although the band near 245 cm−1 appears to lose definition with the addition of Ga, consistent with the concomitant lowering of intensity of the 215 cm−1 band. The Raman spectrum of crystalline Ga2Se3 is shown in Figure 1. Both α and β crystalline polymorphs of Ga2Se3 exhibit a zincblende-type structure with vacancies at the Ga site, although the Ga vacancies are ordered in the β variant.19 All of the Ga atoms in crystalline Ga2Se3 are 4-fold coordinated, while 2- and 3-coordinated Se atoms (henceforth denoted as Se1/2 and Se1/3, respectively) exist in a 1:2 ratio.19,22 The primary peak observed at ∼152 cm−1 in the Raman spectrum of crystalline Ga2Se3 has been assigned to the localized breathing motion of Se1/2 and Se1/3 in the β phase.31,32 3.2. NMR Spectra. The 77Se MAS NMR spectra of the GGS glasses are shown in Figure 2. The 77Se MAS NMR

erase thermal history, where each sample was (1) heated from about 80 degrees below the glass transition temperature (Tg − 80 K) up to about Tg + 50 K (avoiding crystallization) at the desired scanning rate +q, (2) cooled back to the starting temperature at the rate −q, and (3) reheated past Tg at the rate +q. Values of Tg were taken, using the heating scan obtained during segment 3, as the onset of the glass transition region for q = 5, 10, 20, and 50 K/min. Kinetic fragility (m) is traditionally defined according to Angell as the slope of the viscosity vs normalized 1/T curve at Tg; that is, m = [d(log η)/d(Tg/T)]Tg, where η is the viscosity and Tg is the glass transition temperature.24 However, values of m can be estimated using the heating rate dependence of the glass transition temperature.25−27 Specifically, Tg is related to the heating rate and kinetic fragility parameter according to eq 2, where q is the heating rate (in K/min), and Tg is the glass transition temperature (in K). ⎛ d(ln q) ⎞ 1 ⎟⎟ m = ⎜⎜ − d(1/ T ) (ln 10) Tg ⎝ g ⎠

(2)

3. RESULTS 3.1. Raman Spectra. In general, the Raman spectra for all GGS glasses (Figure 1) closely resemble those of amorphous

Figure 2. 77Se NMR spectra of GGS glasses. Glass compositions, in terms of Ga2Se3 content, are listed alongside each spectrum. The inset shows the difference between the spectra corresponding to the 30% and 6.25% Ga2Se3 compositions. Figure 1. Raman spectra of GGS glasses. The spectrum of crystalline Ga2Se3 is included at the bottom. Glass compositions, in terms of Ga2Se3 content, are listed alongside each spectrum. Region ii is magnified in the inset. Dashed, vertical lines indicate the locations of the bands near ∼175 and ∼160 cm−1.

spectrum of the GGS glass with x = 6.25 is characterized by a single broad peak centered at ∼400 ppm and extending from about 700 to 0 ppm. This signal can be assigned, on the basis of previous studies, primarily to the Ge−Se-Ge environments in corner-shared GeSe4/2 tetrahedra along with some contributions from (i) Ge−Se-Ge environments in edge-shared GeSe4/2 tetrahedra and (ii) Se3−Ge−Ge−Se3 structural units that are characterized by 77Se isotropic chemical shifts of ∼600 and 200 ppm, respectively. With the addition of Ga2Se3, the high-field edge of the line shape increases in intensity near ∼0 ppm and extends to nearly −250 ppm. While a peak near 0 ppm is not observed in the 77Se NMR spectra of Ge−Se glasses,28,29 the 77 Se MAS NMR spectrum of crystalline Ga2Se3 (Figure 3) is characterized by two resonances centered at ∼200 and ∼10 ppm corresponding to two Se sites with a ratio of 1:2. On the basis of this ratio and the known crystal structure of Ga2Se3, the peaks at ∼200 and ∼10 ppm can be readily assigned to twocoordinated Se1/2 and three-coordinated Se1/3 sites, respectively. As noted above, these Se sites correspond to cornershared GaSe4 tetrahedra. The 77Se MAS NMR spectrum of

GeSe2 and other Ge-rich Ge−Se glasses.28,29 These spectra exhibit three distinct regions: (i) a broad band extending from about 50 to 150 cm−1, (ii) a strong band at ∼200 cm−1 with shoulders near ∼175 cm−1 and ∼215 cm−1, and (iii) a broad, low-intensity band extending from around 230 to 350 cm−1. Previous studies on the Ge−Se system have assigned the primary peak near 200 cm−1 to the symmetric A1 breathing mode of corner-sharing (CS) GeSe4/2 tetrahedra. Here, the subscript 4/2 indicates that Ge is connected to 4 Se atoms that are each 2-fold coordinated in turn. The shoulders near 175 and 215 cm−1 are attributed to stretching of Ge−Ge bonds in (Se3/2)Ge−Ge(Se3/2) ethane-like units and to the symmetric, in-phase stretching of edge-sharing GeSe4/2 tetrahedra in a 4membered ring, respectively.30 The broad, low-frequency band in region i contains various bending modes while the highC

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Figure 3. Overlay of the 77Se spectra of crystalline Ga2Se3 (solid line) and GaSe (dotted line).

Figure 5. 71Ga MAS NMR spectrum of crystalline Ga2Se3.

crystalline GaSe, on the other hand, exhibits only one peak near −85 ppm (Figure 3). Crystalline GaSe is composed of layers of GaSe3/3 pyramids that are interconnected through Ga−Ga bonds.20,33 Thus, the peak near −85 ppm in the 77Se MAS NMR spectrum of crystalline GaSe is assigned to a Se1/3 site connected to a Ga−Ga homopolar bond. The 77Se chemical shift of the 3-coordinated Se site in the Ga−Se compounds therefore appears to undergo a shift of ∼100 ppm toward lower frequency when an adjacent Ga−Se bond is replaced with a Ga−Ga bond. The 71Ga MAS NMR spectra of select GGS glasses are shown in Figure 4. All of these spectra exhibit only one peak at

Figure 6. Density and molar volume of GGS glasses as a function of Ga2Se3 content. Filled, black squares denote experimental data from this study (Table 1). Data taken from Giridhar and Mahadevan13,15 (open, red circles) are shown for reference. Error bars for the density measurements are within the data markers.

Figure 4. 71Ga MAS NMR spectra of select GGS glasses. Spinning sidebands are marked with asterisks. Glass compositions, in terms of Ga2Se3 content, are listed alongside each spectrum.

Ga2Se3) yields a value of 4.27 g/cm3, which is close to the density of GeSe2 (4.26 g/cm3) reported by Yang et al.34 3.4. Optical Band Gap. Over the entire range of compositions examined, the optical band gap Eg,opt changes by no more than 0.05 eV, which is nearly within error bars of the measurements themselves (Figure 7). Nevertheless, the optical band gap drops sharply from the value of glassy GeSe2 (1.92 eV at x = 0) down to about 1.85 eV at x = 5 and decreases very slightly with further addition of Ga2Se3. Since the optical band gap is related to the electronic band gap, the red-shift observed with the addition of Ga2Se3 may point to an increasingly metallic bonding character. 3.5. Glass Transition Temperature and Fragility. The compositional variation of the glass transition temperature Tg of these GGS glasses is shown in Figure 8 for the four heating rates investigated. As expected, the value of Tg increases with increasing heating rate due to the kinetic nature of the glass transition. In general, the compositional trend in Tg is in good

∼140 ppm that is independent of composition. Spectra collected at multiple spinning speeds confirm the absence of any other 71Ga signal with significant intensity that could be hidden underneath the spinning sideband manifold. In comparison, the 71Ga spectrum of crystalline Ga2Se3 (Figure 5) also shows one peak near 120 ppm that can be attributed to tetrahedrally coordinated Ga atoms in GaSe4 environments. Therefore, the 71Ga MAS NMR signal observed in the GGS glasses is likely associated with a similar tetrahedral Ga environment. 3.3. Density. The compositional variation of density and molar volume of these glasses are shown in Figure 6. The addition of Ga2Se3 causes the density (molar volume) of these glasses to increase (decrease) monotonically and linearly, and the values agree well with those previously reported by Mahadevan and Giridhar on similar glasses.13,15 Furthermore, linear extrapolation of the density to x = 0 (i.e., 0 mol % D

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value of m = 25 at x = 0. Overall, the range of fragility values lies within 40 < m < 60. These values are slightly higher than the Se-deficient compositions in the Ge−Se system, which have fragilities ranging from about 30 < m < 50.29,36

4. DISCUSSION When taken together, the Raman and 77Se NMR spectroscopic results indicate that the structure of GGS glasses consists primarily of a network of corner-sharing GeSe4/2 and GaSe4/2 tetrahedra along with some fraction of edge-sharing GeSe4 tetrahedra and of (Se3/2)Ge−Ge(Se3/2) units with Ge−Ge homopolar bonds. The relative fraction of the edge-sharing tetrahedra decreases and that of the (Se3/2)Ge−Ge(Se3/2) units increases, with increasing Ga2Se3 content. The coordination numbers of Ge, Ga and Se atoms are predominantly 4, 4, and 2, respectively, irrespective of the glass composition. Such conclusions are fully consistent with previous studies on this system,7,8,12−17 which indicate that the structure of GGS glasses could be described using the continuously alloyed scenario that is characteristic of chalcogenide glasses. It is possible to rationalize the composition-dependent trends in the Raman spectra in Figure 1 on the basis of this structural scenario. As the Se-deficiency increases with the addition of Ga2Se3, the content of edge-sharing GeSe4/2 tetrahedra, estimated by the relative intensity of the band at ∼215 cm−1, decreases. This observation indicates that the probability of finding edge-sharing tetrahedra is the highest at the chemical threshold and decreases with either the addition or removal of Sea trend that is well-documented in the literature for Ge− Se glasses.8,28,29 On the other hand, the concentration of M−M bonds in the GGS glasses, estimated by the relative intensity of the band at ∼175 cm−1, increases with Ga2Se3 content since the 4-fold coordination of Ga and 2-fold coordination of Se requires the deficiency in Se atoms to be accommodated in the glass structure through the formation of M−M bonds between metal atoms. In fact, it is possible to further deduce that these M−M bonds are predominantly between two Ge atoms,37,38 since the 71Ga MAS NMR spectra of these glasses (Figure 4) show evidence for the presence of GaSe4/2 tetrahedra only, and no 71Ga NMR signal is observed in these spectra near 1200 ppm that is characteristic of the Ga atoms in (Se)3Ga−Ga(Se)3 structural environments.11 The preferential formation of Ge− Ge bonds over Ga−Ga and mixed Ga−Ge bonds implies a violation of chemical order and can be explained on the basis of electronegativities and bond strengths. Since the Pauling electronegativities of Ga, Ge, and Se atoms are 1.81, 2.01, and 2.55 respectively, the larger electronegativity difference between Ga and Se (=0.71) results in a stronger heteropolar bond compared to that between Ge and Se (=0.54).39 Therefore, in Se-deficient GGS glasses, one would expect an energetic preference for the formation of Ga−Se bonds over that for the Ge−Se bonds upon addition of Ga2Se3. As discussed above, besides the formation of M−M bonds in GGS glasses with tetrahedrally coordinated Ga and Ge atoms, another structural mechanism for accommodation of Se deficiency is to force the Se atoms to adopt coordination numbers higher than two, similar to oxygen atoms forming triclusters in analogous Al2O3−SiO2 glasses. Although 3-fold coordinated Se atoms are indeed present in the structure of crystalline Ga2Se3, there has been little direct evidence to date of the presence of such environments in chalcogenide glasses.40,41 In fact, a recent study has argued against the formation of Se1/3 sites on the basis of EXAFS and XPS

Figure 7. Optical band gap of GGS glasses as a function of Ga2Se3 content. Filled, black squares denote experimental data from this study (Table 1). The open, red diamond denotes the value of GeSe2.44

Figure 8. Glass transition temperatures (±2 °C) of GGS glasses as a function of Ga2Se3 content. Filled markers represent experimental data taken at 5 (■), 10 (▼), 20 (●), and 50 K/min (▲). Open, red squares represent literature values from Giridhar and Mahadevan taken at 10 K/min.13,15 The open, red diamond represents the Tg of GeSe2 as reported in Petri et al.35

agreement with that reported in the literature for similar glasses.13 Considering our results together with the literature data, Tg is seen to drop sharply from the value characteristic of GeSe2 (x = 0)35 upon addition of about 5 mol % Ga2Se3, followed by a slower drop with increasing x until an apparent minimum in Tg is reached near x = 20 before slightly increasing again up to the highest Ga concentration studied here (x = 30). In contrast with Tg, the fragility m (Figure 9) appears to exhibit an upward trend with the addition of Ga2Se3 from the

Figure 9. Kinetic fragility of GGS glasses as a function of Ga2Se3 content. The open, red diamond represents the fragility of GeSe2 as calculated by Gupta and Mauro.49 E

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results.7 Nevertheless, a careful consideration of the Raman and 77 Se NMR spectroscopic data presented here strongly indicates that the formation of Se1/3 sites cannot be completely ruled out, especially in GGS glasses with high Ga concentration and consequently with high Se deficiency. The first such indication supporting the formation of Se1/3 sites may be seen in the Raman band near ∼160 cm−1 that appears in the GGS glass with x = 21.1 and increases in intensity with increasing Ga concentration (see Figure 1 inset). A strong shoulder at this position was indeed observed in previous studies, in the Raman spectra of Se-deficient Ge−Se glasses with ≥40 atom % Ge that grew in intensity with increasing Se-deficiency28,41 and was attributed to the appearance of local order in the glass structure resembling that of crystalline GeSe with 3-fold coordinated Ge and Se atoms. It may be noted that this Raman band is distinct from that near ∼175 cm−1 that corresponds to the Ge−Ge homopolar bonds. Rather, the frequency of the Raman band at ∼160 cm−1 is comparable to the ∼152 cm−1 band in the Raman spectrum of crystalline Ga2Se3 (Figure 1) that was associated with the motion of Se1/2 and Se1/3 atoms around Ga vacancies. In fact, Ohmura et al. predicted the presence of a peak at 160 cm−1 in the spectrum of β-Ga2Se3, which was attributed to the localized breathing motion of Se1/3 atoms.31 Furthermore, similar to the suggestion of 3-fold coordinated Se atoms in Sedeficient Ge−Se glasses, the presence of analogous 3-fold coordinated S atoms has also been proposed in Ge-rich Ge−S glasses with ≥36 atom % Ge, in a recent structural study based on combined X-ray and neutron diffraction.42 However, the most convincing evidence for the presence of higher coordinated Se1/3 sites in GGS glasses comes from the 77 Se MAS NMR spectra (Figure 2). Specifically, a comparison between the 77Se MAS NMR spectra of the GGS glasses and that of crystalline Ga2Se3 clearly reveals the presence of a signal near ∼10 ppm (Figure 2) that rapidly increases in intensity with increasing Ga content. As discussed in the previous section, this 77Se isotropic chemical shift can be assigned to the presence of Se1/3 sites in these glasses with Se atoms shared by three GaSe4/2 tetrahedra as in crystalline Ga2Se3. A schematic diagram illustrating the presence of these and other structural moieties and their connectivities is shown in Figure 10. This structural model of GGS glasses can be used to attempt an interpretation of the physical property data presented here to establish structure−property relationships. As shown in

Figure 6, the density of GGS glasses undergoes a linear increase with the addition of Ga2Se3. The simultaneous decrease in molar volume suggests that the packing of the glassy network becomes more efficient with the addition of Ga. Furthermore, examination of the density as a function of the degree of Se deficiency (Figure 11) shows that GGS glasses are denser than

Figure 11. Density of GGS and Ge−Se glasses as a function of the percent Se deficiency. Here the % Se deficiency in a glass is calculated as the fractional difference (expressed as percentage) between the number of Se atoms required to exactly satisfy 4-fold coordination of all Ga and Ge atoms and the number of Se atoms actually present in the formula unit representing the glass composition. Filled, black squares represent experimental data for GGS glasses from this study. Open, red circles represent experimental data for Ge−Se glasses taken from Yang et al.34

Ge−Se glasses for comparable levels of Se deficiency, despite the substitution of Ge by lighter Ga atoms.13,15,34 Given that both Ga and Ge are in 4-fold coordination for all compositions, these observations are not surprising, because increasing Ga concentration in these glasses results in a decreasing fraction of the inefficiently packed edge-sharing GeSe4/2 tetrahedra. Such observations are also consistent with an increased degree of connectivity due to the formation of 3-fold coordinated Se atoms. The structural model of GGS glasses proposed here also provides a rationale for the trend observed in the optical band gaps (Figure 7). According to a model proposed by Tronc et al.,43 the total absorption in Ge−Se glasses with excess Se was described as a linear combination of the absorption properties of the end member constituents GeSe2 and Se. The optical band gaps of bulk, glassy GeSe2 and β-Ga2Se3 are about 1.9244 and 2.56 eV45 respectively; hence, a linear combination of these two end member components predicts an increase in the optical band gap with addition of Ga2Se3 instead of the observed decrease. Furthermore, the addition of Ga2Se3 is expected to increase the optical band gap via the formation of dative bonds to Ga that stabilize the lone pair states of Se which are thought to constitute the upper valence band states in amorphous chalcogenides.12 The discrepancy between these predictions and the observed trend seems to imply that the rapid decrease of the optical band gap of GGS glasses with initial addition of Ga2Se3 to GeSe2 is due to the formation of homopolar Ge−Ge bonds with strong metallic character. Subsequent flattening of the trend with further addition of Ga2Se3 beyond ∼15 mol % (Figure 7) is consistent with the leveling off of the concentration of Ge−Ge bonds and the appearance of 3-fold coordinated Se atoms that accommodate the increasing Se deficiency.

Figure 10. Schematic diagram of a structural fragment showing the presence of (I) corner-sharing (Ga/Ge)Se4 tetrahedra, (II) edgesharing GeSe4 tetrahedra, (III) ethane-like (Se3)Ge−Ge(Se3) units, and (IV) 3-fold coordinated Se. Larger green (blue) circles represent Ga (Ge) atoms, while smaller white (red) circles represent 2-fold (3fold) coordinated Se atoms. F

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Table 1. Density (ρ), Molar Volume (V), Optical Band Gap (Eg,opt), Glass Transition Temperature (Tg), Kinetic Fragility (m), and Average Coordination Number ⟨r⟩ for Each of the Pseudo-Binary Compositions Studieda composition 6.25% Ga2Se3 21.1% Ga2Se3 25% Ga2Se3 30% Ga2Se3 a

ρ (g/cm3) ± 0.5 mg/cm3

V (cm3) ± 0.002 cm3

Eg,opt (eV) ± 0.02 eV

Tg5 (°C) ± 2 °C

Tg10 (°C) ± 2 °C

Tg20 (°C) ± 2 °C

Tg50 (°C) ± 2 °C

m±5

⟨r⟩

4.3053

17.813

1.85

367

370

377

382

41

2.68

4.3903

17.393

1.82

358

362

366

371

47

2.71

4.4116 4.4433

17.292 17.147

1.82 1.83

363 361

365 366

368 369

372 374

58 48

2.71 2.72

The glass transition temperatures taken at 5, 10, 20, and 50 K/min are denoted as Tg5, Tg10, Tg20, and Tg50, respectively.

bonds via ethane-like (Se3)Ge−Ge(Se3). The lack of any spectroscopic evidence for homopolar Ga−Ga or mixed Ga− Ge bonds indicates a violation of chemical order. Besides homopolar Ge−Ge bonding, increasing Se deficiency appears to be accommodated by the formation of 3-fold coordinated Se atoms shared by three neighboring (Ga/Ge)Se4 tetrahedra, similar to oxygen triclusters proposed for peraluminous oxide glasses. This structural scenario is consistent with the experimentally observed nonlinear compositional variation of Tg and optical band gap in GGS glasses, that is hypothesized to result from the counteracting effects of homopolar Ge−Ge bonds and Se triclusters on these properties.

The glass transition temperature is typically an indicator of network connectivity and bond strength in glasses. While the slight upward trend in Tg for x > 20 may be attributed to the formation of Se1/3 and a corresponding increase in connectivity, the initial rapid decrease in Tg with the addition of Ga2Se3 for x < 20 could be attributed to the lowering of mean bond strength due to the formation of Ge−Ge bonds. The bond energy of Ge−Ge bonds (∼157 kJ/mol) is substantially lower than that of the Ge−Se bonds (∼215 kJ/mol)13,34,39 and hence replacement of the latter by the former upon addition of Ga2Se3 to GeSe2 lowers the mean bond strength of the GGS glasses. The two counteracting effects of decreasing bond strength and increasing connectivity on Tg results in a minimum around x = 20 to 25 (Figure 7). In contrast to the optical band gap and Tg, there is no straightforward structural interpretation for fragility, as it depends on the number of structural configurations dynamically available to a supercooled liquid, i.e., configurational entropy.25,36,46 Nevertheless, compositional trends in fragility of supercooled liquids within a single system can often be related to the static structures of corresponding quenched glasses. In modified and charge-compensated network oxide liquids, an increase in fragility is observed to correlate with the formation of both nonbridging oxygen atoms and higher coordination states of the conditional network-forming atoms such as Al, resulting in a fragility minimum at the composition corresponding to a fully charge-compensated network.47 In chalcogenide glasses, a decrease in m is typically associated with the formation of heteropolar bonded and corner-shared tetrahedral and/or pyramidal coordination polyhedra, while the formation of homopolar bonds and edge-sharing polyhedra tend to increase m.29,36,48 When taken together, the oxide and chalcogenide structural models of fragility seem to suggest that the observed increase in kinetic fragility in the GGS glasses is consistent with the formation of both homopolar Ge−Ge bonds and triply coordinated Se atoms.

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AUTHOR INFORMATION

Corresponding Author

*(S.S.) Telephone: 1-530-754-8397. Fax: 1-530-752-1031. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was funded by a grant to S.S. from the National Science Foundation (NSF DMR 1104869). The authors thank Steve Currie for help with the glass synthesis.

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5. CONCLUSIONS The structure of (Ga2Se3)x(GeSe2)100‑x glasses consists primarily of a network of Ge/Ga-centric tetrahedra, where the coordination numbers of Ga, Ge, and Se are 4, 4, and 2, respectively. The majority of these tetrahedra are cornersharing (Ga/Ge)Se4 structural units. Edge-sharing GeSe4 tetrahedra exist at all compositions, but their relative concentration decreases as the glass composition deviates from pure GeSe2. The structure of these glasses, with progressive addition of Ga2Se3, becomes increasingly deficient in Se required to satisfy the tetrahedral coordination of Ga and Ge atoms. This Se deficiency and the requirement for tetrahedral coordination of Ga and Ge are accommodated in the structure via preferential formation of homopolar Ge−Ge G

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