Network vs Molecular Structural Characteristics of Ge-Doped Arsenic

Mar 24, 2009 - The germanium−arsenic sulfide glasses reported in this study were synthesized by melting 40 g mixtures of the constituent elements Ge...
0 downloads 0 Views 701KB Size
Download PDF
J. Phys. Chem. C 2009, 113, 6231–6242

6231

Network vs Molecular Structural Characteristics of Ge-Doped Arsenic Sulfide Glasses: A Combined Neutron/X-ray Diffraction, Extended X-ray Absorption Fine Structure, and Raman Spectroscopic Study S. Soyer-Uzun,† S. Sen,*,† and B. G. Aitken‡ Chemical Engineering & Materials Science, UniVersity of California, DaVis, One Shields AVenue, DaVis, California 95616, and Corning Inc., Corning, New York 14831 ReceiVed: NoVember 27, 2008; ReVised Manuscript ReceiVed: February 13, 2009

Combined neutron/X-ray diffraction, Ge and As K-edge extended X-ray absorption fine structure analysis, and Raman spectroscopy are employed to study the compositional dependence of the short- and intermediaterange structures of As-rich GexAsyS100-x-y glasses with a constant Ge:As atomic ratio of 1:17.3. The structures of glasses with compositions near stoichiometry (35 e x + y e 43) are dominated by the presence of a predominantly heteropolar-bonded As2S3 network. However, an increasing metal content (x + y ) 55) results in a novel glass consisting predominantly of As4S3 molecules, held together by van der Waals forces. The formation of this “molecular” glass implies an apparently anomalous situation of near-zero connectivity and dimensionality with increasing average coordination number. A further increase in metal content (60 e x + y e 65) results in the formation of As-As homopolar-bonded structural regions that coexist with As4S3 molecules. Such unusual evolution of intermediate-range order is shown to be closely related to the compositional variation of thermophysical properties and density fluctuation in these glasses. Introduction Chalcogenide glasses are of wide-ranging importance in a variety of technological applications in the areas of photonics and telecommunication.1-6 The unique compositional flexibility of these glasses in the form of continuous alloying enables tuning of optical, electronic, thermomechanical, and other properties via compositional “engineering”. The compositiondependent structural characteristics such as formation of homopolar bonds and of molecular and other low-dimensional structural units and violation of chemical order are expected to control a wide range of physical properties relevant to various technological applications of these materials. A number of studies of the short- and intermediate-range atomic structure of simple binary chalcogenide glasses in AsX, PX, and GeX (X ) S, Se) systems have been reported in the literature over the past several decades.7-19 However, in contrast, structural studies of complex glasses in the ternary and quaternary Ge-As-S/Se/Te systems had been rather limited and had focused mostly on short-range order only.20-24 This is primarily due to the fact that direct experimental studies of structural characteristics beyond the nearest-neighbor length scale are particularly difficult with spectroscopic techniques alone. A number of previous studies have investigated the compositional evolution of the length scale of intermediate-range order in ternary Ge-As-S glasses along the join GeS2-As2S3 in a phenomenological fashion using Raman spectroscopy.25-27 These studies have used the Ioffe-Regel criterion to relate the position of the Boson peak in the Raman spectra to a length scale of intermediate-range order that increased with increasing GeS2 content and ranged from a few angstrom to 20 Å in GeS2-As2S3 glasses. On the other hand, a recent Raman spectroscopic study of ternary (Ge2S3)x(As2S3)1-x glasses has indicated possible “nanoscale phase separation” of small † ‡

University of California, Davis. Corning Inc.

concentrations of As4S4 and As4S3 molecular units at low S deficiency and of ethane-like Ge2(S1/2)6 and distorted rock-saltlike GeS structural units at high S deficiency from the heteropolar-bonded GeS2-As2S3 network.28 Other recent studies have employed neutron diffraction to study short- and intermediate-range order in ternary Ge-As-S glasses with Ge:As ) 1:1 and glasses of composition Gex(As/Sb)40-x(S/Se)60 based on radial distribution function (RDF) and first sharp diffraction peak analyses.29,30 However, structural interpretation of the diffraction data for multicomponent glasses becomes strongly model-dependent and nonunique due to the convolution of a large number of pair-correlation functions in the RDF. Combined neutron and X-ray diffraction, employing the contrast resulting from the differences in neutron and X-ray weighting factors to decipher the contributions from various paircorrelation functions in the RDF, has recently been used by us to study the short- and intermediate-range-order atomic structure of ternary GexAsxS100-2x glasses.31 The results of these studies have shown that the structure of sulfide glasses with Ge:As ) 1:1 at the short and intermediate range is controlled by a mixed GeS2 and As2S3 network for compositions near stoichiometry. An initial increase in S deficiency results in As-As homopolar bonding and, hence, in the coexistence of the GeS2 network and As clusters. A further increase in S deficiency (e45 atom % S) results in the formation of large Ge-As metal-rich regions. A recent study has shown that this somewhat intuitive scenario of the compositional evolution of structure in ternary Ge-As-S glasses that is dominated by a network of heteropolar Ge/As-S bonds near stoichiometry and by a Ge/As-Ge/As bonded network for strongly S-deficient compositions breaks down in As-rich Ge-As-S glasses.32 Specifically, the Raman spectra of As-rich, S-deficient glasses with an As:Ge atomic ratio g10:1 have been found to display strong intramolecular vibrational signatures of As4S3 molecules.32 The cagelike As4S3 molecules consist of a three-membered As3 ring surmounted

10.1021/jp810446g CCC: $40.75  2009 American Chemical Society Published on Web 03/24/2009

6232 J. Phys. Chem. C, Vol. 113, No. 15, 2009

Soyer-Uzun et al.

TABLE 1: Compositions, Tg Values, Coefficient of Thermal Expansion (CTE), and Average Coordination Numbers (〈r〉) of Glasses Used in This Study CTE chemical composition % S in excess of ((0.1 atom %) stoichiometry Tg (°C) (ppm/°C) Ge1.91As33.09S65 Ge2.16As37.41S60.43 Ge2.35As40.65S57 Ge3As52S45 Ge3.27As56.73S40 Ge3.41As59.09S37.5 Ge3.55As61.45S35 a

21.6 0.0 -13.21 -46.43 -56.35 -60.71 -64.74

170 173 176 36 61 73 88

25.3 20.8 28.3 96.7 57.8 47.5 NDa

〈r〉 2.37 2.42 2.45 2.58 2.63 2.66 2.69

Not determined.

by an AsS3 pyramid, with each atom in the As3 ring being bonded to one of the S atoms in the pyramid. These glasses display fragile viscosity vs temperature behavior, large thermal expansion coefficients, and low glass transition temperatures (Tg), all consistent with their structure being dominated by large concentrations of As4S3 molecules.32 The formation of such molecules in S-deficient GexAsyS1-x-y glasses results in a structural scenario that is rather counterintuitive. The Ge, As, and S atoms in a GexAsyS1-x-y glass are 4-, 3-, and 2-coordinated, respectively. Therefore, the average coordination number (〈r〉) for such a glass can be written as 〈r〉 ) 4x + 3y + 2(1 x - y). An increasing S deficiency would therefore be expected to result in an increase in 〈r〉 and consequently in an increase in the dimensionality and connectivity of the structure. However, the formation of predominantly molecular glasses consisting of As4S3 molecules in the As-rich, S-deficient part of the Ge-As-S ternary system would in fact imply decreasing connectivity and dimensionality with increasing 〈r〉! Such unusual structural characteristics warrant systematic and quantitative studies of the short- and intermediate-range structure and compositional variation in the molecular vs network character of the structure of these glasses that remain completely lacking to date. In this paper we report the results of a combined neutron/X-ray diffraction, Ge and As K-edge extended X-ray absorption fine structure (EXAFS), and Raman spectroscopic study of compositional evolution of the structural characteristics of a series of GexAsyS100-x-y chalcogenide glasses with x:y ) 1:17.3 and S content ranging between 35 and 65 atom %. Experimental Section Sample Synthesis and Thermophysical Characterization. The germanium-arsenic sulfide glasses reported in this study were synthesized by melting 40 g mixtures of the constituent elements Ge, As, and S with g99.9995% purity (metals basis) in evacuated (10-5 Torr) and flame-sealed fused silica ampoules at 800 °C for 24 h in a rocking furnace and subsequently quenching the ampoules into water. A more detailed description of the preparation method can be found elsewhere.32 The glass transition temperatures (Tg) were determined to (2 °C by differential scanning calorimetry using a heating rate of 10 °C/ min. Thermal expansion coefficients were measured to (0.1 ppm/°C up to Tg by dilatometry, using Al2O3 as a standard. The compositions, Tg values, thermal expansion coefficients, and nominal average coordination numbers of these glasses are listed in Table 1. The compositions of these glasses are compared in Figure 1 with ternary Ge-As-S compositions whose structures have been previously studied by us and other authors and with the stable glass-forming region in this system as reported in the literature by Savage and Nielsen.33 It is clear that the glasses investigated in this study extend the previously known range

Figure 1. Ge-As-S ternary composition diagram showing the location of the glass compositions ([) studied in this paper. The dotted line outlines the region of phase separation and/or As4S4 crystallization,32 while the dashed line outlines the As-rich limit of the glassforming region.28,33 Compositional tie lines along the joins GeS2-As2S3 and Ge2S3-As2S3 and those along constant As:Ge ratios of 2:1, 1:1, and 1:2.5 represent ternary compositions in this system whose structures at the short and intermediate range have been studied previously by us and others using diffraction and spectroscopic techniques.23,25-31

of glass formation in the Ge-As-S system toward more Asrich and S-deficient compositions. Raman Spectroscopy. Raman spectra of GexAsyS100-x-y glasses were collected using a Bruker RFS 100/S Fouriertransform (FT) Raman spectrometer. Glass samples were irradiated with a frequency-doubled Nd:YAG laser operating at a wavelength of 1064 nm and at a power level of 30 mW. The scattered signal was collected using a 180° backscattering geometry. EXAFS Spectroscopy. Ge and As K-edge EXAFS spectra were collected for a single composition (Ge3As52S45) in this series to obtain an independent check on the accuracy of the Ge and As nearest-neighbor coordination environments as derived from the results of diffraction experiments. The EXAFS spectra were collected at beamline X10C at the National Synchrotron Light Source at Brookhaven National Laboratory. A Si(220) monochromator was used with the focusing mirror tuned to reject higher harmonics. The glass sample was finely ground into a powder that was mounted on a Mylar tape on the sample holder of a liquid nitrogen cooled cryostat. The sample temperature was maintained within a range of 80-90 K to lower the effect of the thermal Debye-Waller factor on the signal. The EXAFS data were collected in the transmission mode in energy steps of 2 eV. Ionization chambers were used as detectors for measuring the incident and transmitted X-ray beam intensities. The Ge and As K-edge EXAFS data have been analyzed using the standard software packages EXBROOK and EXCURV98 developed by the Daresbury Laboratory.34 The EXBROOK package has been used to subtract a linear background in the pre-edge region and a quadratic background in the postedge region from the raw absorption spectra. The EXAFS oscillations are subsequently k3-weighted and fitted using the nonlinear least-squares fitting routine in EXCURV98, based on the curved-wave theory of EXAFS.35 The three structural parameters that are varied to obtain the best fit are (i) the radial distance (R) of the neighboring atoms around the central photoexcited Ge or As atom, (ii) the number (N) of neighboring atoms around the central atom within a shell of radius R, and (iii) the Debye-Waller factor (2σ2). The calculated electron scattering phase shifts for As-S, Ge-S, and Ge-As atom pairs are tested by fitting the Ge and As EXAFS spectra of model

Ge-Doped Arsenic Sulfide Glasses

J. Phys. Chem. C, Vol. 113, No. 15, 2009 6233

compounds and proved to be adequate without further refinement. It is important to note that the Debye-Waller factor and the coordination number are correlated quantities in EXAFS data analysis and the quality of the fit can be kept unchanged by varying both the quantities simultaneously over a limited range. The related uncertainties in the coordination numbers of Ge and As atoms in these glasses are found to be on the order of (0.2 or less. Neutron and X-ray Diffraction. Neutron diffraction experiments were carried out using the glass, liquid, and amorphous materials diffractometer (GLAD) at the Intense Pulsed Neutron Source (IPNS) at Argonne National Laboratory.36 Crushed glass samples were taken in 9.5 mm diameter cylindrical vanadium cans, and data were collected over a Q range of 0.3-40 Å-1. The beam size was 4 × 4 cm2, and the duration of the data collection for each sample was ∼18 h. High-energy X-ray diffraction experiments were carried out on the 11-IDC beamline at the Advanced Photon Source (APS) at Argonne National Laboratory. Crushed samples were taken in 2.5 mm diameter cylindrical SiO2 capillaries (wall thickness of 100 µm). Data were collected over a Q range of 0.3-22 Å-1 using a MAR 345 imaging plate detector. An incident photon energy of 115.07 keV (λ ) 0.10788 Å) was used. The beam size was 0.6 × 0.6 mm2, and the duration of the data collection for each sample was ∼5 h. Neutron diffraction data were corrected using standard procedures for container and background scattering, absorption, multiple scattering, and inelasticity effects and were normalized to an absolute scale with the isotropic incoherent scattering from vanadium. The resulting total scattering structure factor [FN(Q)] is related to the total neutron static structure factor [SN(Q)] via the following expression:

SN(Q) )

FN(Q) (

∑ i cibi)

1

)

2

(

∑ i cibi)2

∑ cibicjbj(Sij(Q) - 1) i,j

(1) where ci is the atomic concentration and bi is the neutron scattering length of species i ) As, Ge, and S and Sij(Q) are the Faber-Ziman partial structure factors.37,38 A Q range of 0.3-22 Å-1 was used along with the Lorch modification function for the Fourier transformation of the SN(Q) data.39 The high-energy X-ray data were reduced using standard procedures and the software analysis package FIT2D.40 X-ray structure factors SX(Q) were obtained up to Q ) 22 Å-1 using the program PDFGETX2 by applying standard corrections.41 In X-ray diffraction, within the independent atom approximation, total X-ray static structure factors SX(Q) can be expressed as

SX(Q) )

IX(Q) (

∑

(

N(r) ) 4πr2F0g(r)

(3)

where F0 is the total number density and g(r) is a weighted average of the partial pair distribution functions [gij(r)] for the atom pair i and j and is given by

1

g(r) )

〈b〉2

∑ cicjbibjgij(r) ij

for neutron diffraction and

g(r) )

cicjfi(Q) fj(Q) gij(r) 〈f(Q)〉2

∑ ij

for X-ray diffraction, where 〈b〉2 is (∑icibi)2 and 〈f(Q)〉2 is [∑icifi(Q)]2. The quantities ((2 - ∂)cicjbibj)/〈b〉2 and ((2 - ∂)cicjfi(Q) fj(Q))/〈f(Q)〉2 (where ∂ ) 1 for i ) j and ∂ ) 0 for i * j) are Faber-Ziman weighting factors wNij and wXij for neutron and X-ray, respectively, and are tabulated in Table 2a for all glass compositions studied here. The double counting of the weighting factors for i * j takes into account the fact that contributions from partial pair distribution functions gij(r) and gji(r) are identical. The quantity wXij has been approximated to be independent of Q in the subsequent discussion. N(r) dr has a direct physical interpretation as the number of atoms that are present within a range (r, r + dr) from any given atom.43,44 A weighted average coordination number for a peak in neutron (X-ray) N(r) extending from r1 to r2 can be described as

C N(X) )

∫rr NN(X)(r) dr 2

1

(4)

In a neutron diffraction experiment CN can be related to the partial coordination numbers [Ci(j)] expressing the average number of j atoms around an atom i within this r range:

CN )

1 〈b〉2

∑ cibibjCi(j)

(5)

ij

For the Ge-As-S ternary system eq 5 becomes

c f (Q))2 i i i 1

)

range (0.3-22 Å-1) as those used for the Fourier transformation of the SN(Q) data. The total neutron (X-ray) radial distribution function N(r) is given by

∑ i cifi(Q))

2

∑ cifi(Q) cjfj(Q) (Sij(Q) - 1) (2) i,j

where IX(Q) is the distinct scattering and fi(Q) are the Qdependent atomic form factors.42 The total structure factor SN(X)(Q) is related to the neutron (X-ray) total radial distribution function N(r) via Fourier transformation. The Fourier transformation of SX(Q) data was carried out using the same Lorch modification function and Q

bGe2 bGebAs C (Ge) + 2cGe CGe(As) + 〈b〉2 Ge 〈b〉2 bGebS bAs2 bAsbS 2cGe C (S) + c As 〈 〉2 CAs(As) + 2cAs 〈 〉2 CAs(S) + 〈b〉2 Ge b b bS2 cS 2 CS(S) (6) 〈b〉

C N ) cGe

Equation 6 can be written as

6234 J. Phys. Chem. C, Vol. 113, No. 15, 2009

Soyer-Uzun et al.

N N C N ) W NGeGeCGe(Ge) + W GeAs CGe(As) + W GeS CGe(S) + N CAs(As) W AsAs

+

N W AsS CAs(S)

+

N W SS CS(S)

(7)

where WNij are the neutron weighting factors representing the contribution of each partial Ci(j) to CN.43 In an X-ray diffraction experiment, the relation between CX and the partial coordination numbers Ci(j) can be given as

CX )

1 〈f(0)〉2

∑ ciZiZjCi(j)

(8)

j

where 〈f(0)〉2 is the average neutral atom X-ray form factor at Q ) 0 Å-1 and is given by (∑ιciZi)2, where Zi is the number of electrons of species i ) As, Ge, and S (33, 32, and 16, respectively). For the Ge-As-S system, eq 8 becomes

ZGe2 ZGeZAs C X ) cGe C (Ge) + 2cGe C (As) + 〈f(0)〉2 Ge 〈f(0)〉2 Ge ZGeZS ZAs2 2cGe C (S) + c C (As) + As 〈 〈f(0)〉2 Ge f(0)〉2 As ZAsZS ZS2 2cAs C (S) + c C (S) (9) S〈 〈f(0)〉2 As f(0)〉2 S In eqs 6 and 9 contributions from Ci(j) terms for heteroatom pairs where i * j have been counted twice to take into account

the contributions from both the terms Ci(j) and Cj(i). Equation 9 can be written in terms of X-ray weighting factors as X X C X ) W XGeGeCGe(Ge) + W GeAs CGe(As) + W GeS CGe(S) + X X X CAs(As) + W AsS CAs(S) + W SS CS(S) (10) W AsAs

where WXij are the X-ray weighting factors indicating the contribution of each of the partials Ci(j) to CX.43 Neutron and X-ray diffraction can be used as complementary techniques due to the contrast resulting from the differences in the neutron and X-ray weighting factors of various partials Ci(j). These weighting factors for all glasses are shown in Table 2b. A combination of eqs 7 and 10 is used to solve for the nearest-neighbor coordination environments in the GexAsyS100-x-y glasses. Results Raman Spectroscopy. Raman spectra of S-excess and stoichiometric glasses (x + y ) 35 and 39.57) are dominated by a broad band spanning the region between 300 and 400 cm-1 that peaks near ∼345 cm-1 (Figure 2). This band can be readily assigned, on the basis of the results of previous studies, to the symmetric stretching modes of GeS4/2 tetrahedra and AsS3/2 pyramids.23 Since the concentration of Ge is very low in these glasses, this band can be considered to arise mainly from the stretching of AsS3/2 pyramids. A weak but clear band at ∼490 cm-1 appears only in the spectrum of S-excess glass, indicating the expected presence of some S-S bonds.23 With decreasing S content, new and significantly narrower bands appear, being

TABLE 2: (a) Normalized Faber-Ziman Neutron (N) and X-ray (X) Weighting Factors (wij) for GexAsyS100-x-y Glassesa and (b) Neutron and X-ray Weighting Factors (Wij) Defining the Contribution of Each of the Partials Ci(j) to CN and CX through Eqs 7 and 10 chemical composition Ge1.91As33.09S65 (x + y ) 35) Ge2.16As37.41S60.43 (x + y ) 39.57) Ge2.35As40.65S57 (x + y ) 43) Ge3As52S45 (x + y ) 55) Ge3.27As56.73S40 (x + y ) 60) Ge3.41As59.09S37.5 (x + y ) 62.5) Ge3.55As61.45S35 (x + y ) 65)

Ge1.91As33.09S65 (x + y ) 35) Ge2.16As37.41S60.43 (x + y ) 39.57) Ge2.35As40.65S57 (x + y ) 43) Ge3As52S45 (x + y ) 55) Ge3.27As56.73S40 (x + y ) 60) Ge3.41As59.09S37.5 (x + y ) 62.5) Ge3.55As61.45S35 (x + y ) 65) a

Ge-Ge 0.035 0.020 0.036 0.021 0.037 0.022 0.038 0.023 0.039 0.023 0.039 0.023 0.039 0.023

Ge-As

Ge-S

As-As

As-S

S-S

0.396 0.377 0.406 0.389 0.413 0.397 0.431 0.418 0.436 0.425 0.439 0.427 0.441 0.430

0.342 0.366 0.351 0.377 0.357 0.385 0.373 0.405 0.378 0.412 0.380 0.415 0.382 0.417

0.145 0.174 0.123 0.148 0.108 0.131 0.070 0.085 0.058 0.070 0.052 0.064 0.047 0.058

N X N X N X N X N X N X N X

of Ci(j) to CN and CX 0.051 0.818 0.041 0.749 0.053 0.853 0.043 0.790 0.054 0.873 0.044 0.816 0.057 0.919 0.048 0.883 0.058 0.930 0.049 0.902 0.058 0.934 0.049 0.910 0.058 0.936 0.050 0.917

0.708 0.727 0.738 0.766 0.755 0.792 0.796 0.857 0.805 0.875 0.808 0.882 0.810 0.889

0.301 0.346 0.258 0.300 0.229 0.269 0.149 0.180 0.123 0.150 0.111 0.136 0.100 0.123

N X N X N X N X N X N X N X

(a) wij for GexAsyS100-x-y Glasses 0.057 0.025 0.042 0.020 0.058 0.025 0.044 0.021 0.059 0.026 0.044 0.022 0.062 0.027 0.047 0.023 0.063 0.027 0.047 0.023 0.063 0.027 0.048 0.023 0.063 0.027 0.048 0.023

(b) Wij Defining the Contribution 0.073 0.118 0.041 0.084 0.076 0.122 0.043 0.088 0.078 0.126 0.044 0.092 0.082 0.132 0.048 0.099 0.083 0.133 0.049 0.101 0.083 0.134 0.049 0.102 0.084 0.135 0.050 0.103

X-ray values have some Q dependence and were only calculated at Q ) 0 Å-1.

Ge-Doped Arsenic Sulfide Glasses

Figure 2. Raman spectra of GexAsyS100-x-y glasses with x:y ) 1:17.3 as a function of the metal content (x + y).

superimposed on the broad band at 345 cm-1 and located at ∼344 and 362 cm-1, as well as in the lower frequency region, primarily between ∼180 and 250 cm-1, with the two most prominent bands at ∼188 and 221 cm-1. These narrow bands correspond well with the intramolecular vibrational modes of As4S4 molecules as observed in the Raman spectrum of crystalline R-As4S4 and indicate the formation of such molecules in the slightly S-deficient glass with x + y ) 43.45 On the other hand, the continued strong presence of the 345 cm-1 band in the Raman spectrum of this glass indicates that the structure is still dominated by the As-S network. A further decrease in S content results in a drastic change in the Raman spectra of these glasses. Most notably the spectrum of the glass with x + y ) 55 is somewhat uniquely characterized by the sudden appearance of a sharp and intense band centered at 273 cm-1 and a series of relatively sharp bands at 180, 200, 220, 357, and 375 cm-1 that can be readily associated with various intramolecular vibrational modes of the As4S3 molecule.32,46 The 273 cm-1 band corresponds to the breathing mode of the basal As3 ring in the As4S3 molecule that is represented by a similarly sharp and intense band at 275 cm-1 in the Raman spectrum of crystalline As4S3.32,46 The relative intensity of the 273 cm-1 band decreases systematically with a further decrease in the S content in the system (55 e x + y e 65), indicating a corresponding decrease in the concentration of As4S3 molecules in the glass structure. Moreover, bands in the region between 200 and 250 cm-1 corresponding to nonmolecular As-As homopolar bonds become stronger with decreasing S content in this composition range. The relative intensity of the 273 cm-1 band above a smoothly varying local background in the Raman spectra of these glasses and Tg are plotted in Figure 3 as a function of the metal content (x + y). Clearly, the intensity of the 273 cm-1 peak is zero in glasses with 35 e x + y e 43, which also have the highest Tg, and the intensity suddenly jumps to a maximum at x + y ) 55 accompanied by a remarkable drop in Tg. With a further increase in the total metal content, the intensity of the 273 cm-1 peak monotonically decreases while the Tg linearly increases (Figure 3). Ge and As K-Edge EXAFS Spectroscopy. The k3-weighted experimental Ge and As K-edge EXAFS spectra and the fitted curves for the Ge3As52S45 glass are shown in Figure 4. The corresponding Fourier transforms are shown in Figure 5. Two peaks are present in the Fourier transforms of the Ge and As

J. Phys. Chem. C, Vol. 113, No. 15, 2009 6235

Figure 3. Glass transition temperatures Tg and relative intensity of the band at 273 cm-1 in the Raman spectra of GexAsyS100-x-y glasses with x:y ) 1:17.3 as a function of the metal content (x + y).

Figure 4. k3-weighted (a) Ge and (b) As K-edge EXAFS spectra of the Ge3As52S45 glass. Solid lines represent experimental data, and dashed lines correspond to least-squares fits obtained using EXCURV98.

EXAFS spectra. The Ge EXAFS spectrum has been fitted using a nearest-neighbor shell of 4 S atoms with an average Ge-S distance of ∼2.20 ( 0.02 Å and a second shell of ∼1.1 Ge/As next-nearest neighbors at an average distance of ∼3.50 ( 0.05 Å, with Debye-Waller factors for the first and second shells being ∼0.012 and 0.020 Å2, respectively (Figures 4 and 5). On the other hand, the As EXAFS spectrum has been fitted using a nearest-neighbor shell of ∼1.5 S and 1.5 As atoms at 2.22 and 2.48 Å, respectively, and a second shell of ∼1.0 As neighbor at an average distance of ∼3.60 ( 0.03 Å, with Debye-Waller factors for the first and second shells being ∼0.008 and 0.015 Å2, respectively (Figures 4 and 5). It should be noted here that

6236 J. Phys. Chem. C, Vol. 113, No. 15, 2009

Soyer-Uzun et al.

Figure 5. Magnitudes of the Fourier transform of the k3-weighted (a) Ge and (b) As K-edge EXAFS spectra of the Ge3As52S45 glass. Solid lines represent experimental data, and dashed lines correspond to leastsquares fits obtained using EXCURV98.

Figure 6. (a) Neutron total structure factors SN(Q) and (b) X-ray total structure factors SX(Q) for GexAsyS100-x-y glasses with x:y ) 1:17.3. The curves are vertically displaced for clarity. Corresponding metal contents (x + y) are shown alongside each curve.

the Ge and As backscatterers cannot be directly distinguished by EXAFS due to the similarity in their electronic structures. However, the large As:Ge ratio in this glass implies that the majority of the backscatterers must be As atoms, at least in the case of As EXAFS. The As-S and As-As nearest-neighbor and (apical) As-(basal) As next-nearest-neighbor distances in the R-As4S3 molecular crystal are 2.22, 2.45, and 3.53 Å, respectively.47 Therefore, the nearest- and next-nearest-neighbor coordination environments of As atoms in this glass, as obtained from As K-edge EXAFS spectroscopy, are fully consistent with the expected average coordination environment for As atoms in As4S3 molecules. Neutron and X-ray Diffraction. The experimental neutron and X-ray structure factors SN(Q) and SX(Q) and corresponding radial distribution functions N(r) for all glasses are shown in Figures 6 and 7. It is clear from a visual inspection of Figure 6 that SN(Q) and SX(Q) display a marked difference between glasses with 55 e x + y e 65 and those with 35 e x + y e 43. First Sharp Diffraction Peak. The low-Q parts of the SN(Q) and SX(Q), especially the position and intensity of the first sharp diffraction peak (FSDP), exhibit significant changes with glass composition (Figure 6). The local background underneath the FSDP in SN(Q) and SX(Q) was approximated with a linear function and was subtracted. Subsequently the FSDPs were fitted with Gaussian functions to extract their position, width, and intensity, and the compositional variations of these parameters are shown in Figures 8 and 9. Neutron and X-ray FSDP

parameters exhibit similar trends; however, the positions of X-ray FSDPs are located at slightly higher Q values compared to those of the neutron FSDPs (Figure 8, Table 3). On the other hand, the intensities and widths of X-ray FSDPs are positioned at slightly lower values than those of neutron FSDPs (Figures 8 and 9). These minor but systematic differences between the X-ray and neutron FSDP parameters possibly arise from the differences in the neutron and X-ray weighting factors of various partials Ci(j) as discussed previously. The positions of the FSDPs monotonically shift to lower Q values with increasing metal content (Figure 8, Table 3). On the other hand, their intensities rise systematically with increasing metal content until they reach a maximum at the glass composition with x + y ) 55 and subsequently decrease rapidly with a further increase in metal content (Figure 8). The widths of the FSDPs exhibit a decreasing trend in the compositional region of 35 e x + y e 55 until they attain a minimum at x + y ) 55 and subsequently increase slightly with increasing metal content in the compositional region of 55 e x + y e 65 (Figure 9). The lowest Q part of SN(Q) for these glasses displays smallangle scattering (Figure 6). Since the neutron scattering data reported here do not extend far into the small-angle region, the magnitude of small-angle scattering has been approximated as the difference in intensity of SN(Q) at Q ) 0.2 and 0.5 Å-1 as the scattering intensity in this low-Q range is not significantly affected by the tail of the FSDP. The magnitude of small-angle neutron scattering remains somewhat constant in the composi-

Ge-Doped Arsenic Sulfide Glasses

J. Phys. Chem. C, Vol. 113, No. 15, 2009 6237

Figure 9. Variation in the width of the FSDP and the coherence length of intermediate-range order (2π/∆QFSDP) in GexAsyS100-x-y glasses with x:y ) 1:17.3 as a function of the metal content (x + y). Widths and coherence lengths related to the neutron (X-ray) FSDP are represented by filled and open squares (circles), respectively.

Figure 7. (a) Neutron radial distribution functions and (b) X-ray radial distribution functions for GexAsyS100-x-y glasses with x:y ) 1:17.3. These functions are obtained by the Fourier transformation of the SN(Q) and SX(Q) data shown in Figure 6. The curves are vertically displaced for clarity. The corresponding metal contents (x + y) are shown alongside each curve.

Figure 8. Variation in the position and intensity of the neutron and X-ray FSDPs in GexAsyS100-x-y glasses with x:y ) 1:17.3 as a function of the metal content (x + y). Neutron (X-ray) FSDP position and intensity data are represented by filled and open squares (circles), respectively.

tional region of 35 e x + y e 55 and subsequently increases sharply with increasing metal content (Figure 10). Such small-

Figure 10. Compositional dependence of the difference in intensity of SN(Q) at Q ) 0.2 and 0.5 Å-1 used as a measure of small-angle neutron scattering.

angle scattering was not observed in SX(Q), as these data do not extend below Q ) 0.3 Å-1 due to the cutoff from a low-Q beam stop. Radial Distribution Function and Nearest-Neighbor Coordination EnWironment. The peak(s) located between 2 and 3 Å in neutron and X-ray N(r) are associated with the nearestneighbor environments of As and Ge atoms in these glasses (Figure 7). For the S-excess and stoichiometric glasses (x + y ) 35 and 39.57) the peak centered at 2.27 Å corresponds to the As-S and Ge-S nearest-neighbor distances. Since the Ge content is very small in these glasses, this peak must be primarily related to As-S correlations in the As2S3 network (Figure 7). It may be noted that this distance is in good agreement with the results of previous EXAFS and diffraction studies in GexAsyS100-x-y glasses with x:y ) 1:1 and 1:2.20-22,29,31 Similarly, for the slightly S-deficient glass with x + y ) 43, the peak at 2.26 Å is related mainly to the As-S correlations in the arsenic sulfide network. A tiny shoulder positioned at ∼2.45 Å appears in N(r) of this glass that can be readily associated with the Ge/ As-Ge/As nearest-neighbor correlations.20-22,29,31 Our previous structural studies of glasses in the Ge-As-S system have shown that Ge atoms prefer to be bonded to S atoms and only start participating in metal-metal bonding in highly S-deficient glasses once all As atoms are used up in homopolar As-As bonding. Therefore, the peak at ∼2.45 Å in the glass with x + y ) 43 can be safely assigned exclusively to As-As homopolar

6238 J. Phys. Chem. C, Vol. 113, No. 15, 2009

Soyer-Uzun et al.

TABLE 3: FSDP Parameters of GexAsyS100-x-y Glasses Used in This Studya FSDP parameters neutron position (Å-1)

chemical composition Ge1.91As33.09S65 (x + y ) 35) Ge2.16As37.41S60.43 (x + y ) 39.57) Ge2.35As40.65S57 (x + y ) 43) Ge3As52S45 (x + y ) 55) Ge3.27As56.73S40 (x + y ) 60) Ge3.41As59.09S37.5 (x + y ) 62.5) Ge3.55As61.45S35 (x + y ) 65) a

1.241( 0.002 1.237 ( 0.002 1.172 ( 0.002 1.159 ( 0.002 1.135 ( 0.002 1.125 ( 0.002 1.115 ( 0.002

X-ray

intensity

width (Å-1)

position (Å-1)

intensity

width (Å-1)

0.565 ( 0.006 0.613 ( 0.006 0.742 ( 0.007 1.273 ( 0.013 1.039 ( 0.010 0.978 ( 0.010 0.940 ( 0.009

0.316 ( 0.006 0.304 ( 0.006 0.252 ( 0.005 0.203 ( 0.004 0.207 ( 0.004 0.210 ( 0.004 0.212 ( 0.004

1.254 ( 0.003 1.251 ( 0.003 1.178 ( 0.002 1.166 ( 0.002 1.141 ( 0.002 1.131 ( 0.002 1.122 ( 0.002

0.468 ( 0.005 0.495 ( 0.005 0.601 ( 0.006 0.921 ( 0.009 0.865 ( 0.009 0.838 ( 0.008 0.776 ( 0.008

0.277 ( 0.006 0.266 ( 0.005 0.233 ( 0.005 0.195 ( 0.006 0.195 ( 0.004 0.197 ( 0.004 0.199 ( 0.004

Typical error bars for the FSDP position, intensity, and width are (0.2%, 1%, and 2%, respectively, of the reported values.

TABLE 4: Interatomic Distances and Coordination Numbers for Ge and As Atoms in GexAsyS100-x-y Glasses first coordination shell average As-S distance (Å) chemical composition

CGe(S)

CAs(S)

Ge1.91As33.09S65 (x + y ) 35) Ge2.16As37.41S60.43 (x + y ) 39.57) Ge2.35As40.65S57 (x + y ) 43) Ge3As52S45 (x + y ) 55) Ge3.27As56.73S40 (x + y ) 60) Ge3.41As59.09S37.5 (x + y ) 62.5) Ge3.55As61.45S35 (x + y ) 65)

4.0 ( 0.2a 4.0 ( 0.2a 4.0 4.0 4.0 4.0 4.0

3.0 ( 0.2a 3.0 ( 0.2a 2.7 ( 0.3b 1.7 ( 0.3b 1.4 ( 0.2b 1.0 ( 0.2b 0.9 ( 0.2b

CAs(As)

0.3 ( 0.1b 1.3 ( 0.2b 1.6 ( 0.3b 1.9 ( 0.3b 2.1 ( 0.4b

c

CAs(S)

1.5 1.2 1.0 0.9

c

average As-As distance (Å)

CAs(As)

neutron

X-ray

neutron

X-ray

1.5 1.8 2.0 2.1

2.27 ( 0.01 2.27 ( 0.01 2.26 ( 0.01 2.26 ( 0.01 2.22 ( 0.03 2.22 ( 0.01 2.22 ( 0.01

2.27 ( 0.01 2.27 ( 0.01 2.26 ( 0.01 2.21 ( 0.01 2.18 ( 0.01 2.17 ( 0.01 2.17 ( 0.01

2.59 ( 0.01 2.46 ( 0.03 2.47 ( 0.01 2.46 ( 0.01 2.45 ( 0.01

2.62 ( 0.02 2.45 ( 0.02 2.43 ( 0.01 2.43 ( 0.01 2.43 ( 0.01

a These coordination numbers are obtained by solving eqs 11 and 12 together. b These coordination numbers are obtained by solving eqs 13 and 14 together taking CGe(S) ) 4. c Coordination numbers expected from stoichiometry assuming (1) CGe(S) ) 4 and (2) S atoms only participate in the formation of GeS4 tetrahedra and As4S3 molecules.

bonding. This peak grows rapidly in intensity in N(r) of the S-deficient glasses (43 e x + y e 65) with increasing metal content. It should be noted here that the peaks at ∼2.26 and ∼2.45 Å in these glasses also correspond well with the As-S and As-As nearest-neighbor distances within the As4S3 molecule.47 The exclusive assignment of the peak at ∼2.45 Å to As-As bonds is clearly valid in S-deficient glasses with an x + y value of at least 55 since the Ge K-edge EXAFS spectrum for the glass with x + y ) 55, reported in this study, indicates that all Ge atoms are indeed heteropolar-bonded to four S atoms. Raman spectra of glasses with higher S deficiencies (60 e x + y e 65) indicate the presence of some As-As bonds in Asrich regions that are not part of As4S3 molecular units. Therefore, the peak at ∼2.45 Å in glasses with 60 e x + y e 65 must correspond to both intramolecular As-As nearest-neighbor distances in As4S3 molecules and As-As bonds in As-rich regions that are not part of As4S3 molecules. In the case of S-excess and stoichiometric glasses (35 e x + y e 39.57), the total area CN of the peak positioned at 2.27 Å in the neutron N(r) corresponding to As-S and Ge-S correlations can be expressed by reducing eq 7: N N C N ) W GeS CGe(S) + W AsS CAs(S)

(11)

Similarly, eq 10 can be reduced for the X-ray case such that X X C X ) W GeS CGe(S) + W AsS CAs(S)

(12)

where CX is the total area of the peak located at 2.27 Å in the X-ray N(r). Equations 11 and 12 can be solved together to obtain CAs(S) and CGe(S) for S-excess and stoichiometric glasses. CAs(S) and CGe(S) were calculated to be 3 and 4, respectively (Table 4), without needing any additional constraints.

For S-deficient glasses (43 e x + y e65), eq 7 can be reduced for the neutron case such that N N N C N ) W GeS CGe(S) + W AsAs CAs(As) + W AsS CAs(S)

(13) Similarly, for the X-ray case, eq 10 reduces into the following form for the S-deficient glass compositions: X X X C X ) W GeS CGe(S) + W AsAs CAs(As) + W AsS CAs(S)

(14) CN and CX are the total area under the peaks positioned at ∼2.2 and ∼2.5 Å in N(r) that are obtained by integrating the first shell in the neutron and X-ray N(r). As discussed above, eqs 13 and 14 that assign all metal-metal bonding exclusively to As atoms are rigorously valid in the composition range 43 e x + y e 55. Moreover, the concentrations of Ge atoms in all these glasses are very low, and so are the X-ray and neutron weighting factors for Ge-As and Ge-Ge correlations that are nearly an order of magnitude smaller than those for As-As correlations (Table 2). Therefore, these equations are also good approximations for glasses with 60 e x + y e 65 simply because such approximations would not change the accuracy of the obtained coordination numbers significantly. These two equations (eqs 13 and 14) still contain three unknowns, i.e., CGe(S), CAs(As), and CAs(S), that require an additional constraint to solve for these coordination numbers. However, CGe(S) ) 4 as all Ge atoms are expected to be tetrahedrally coordinated, and hence, the number of unknowns is reduced to 2 in eqs 13 and 14. CAs(As) and CAs(S) values thus obtained by combining neutron and X-ray diffraction are listed in Table 4. The validity of the approximations discussed above is strongly supported by the

Ge-Doped Arsenic Sulfide Glasses

Figure 11. A schematic representation of the Ge2As6S7 dimer.

fact that the total coordination number of As atoms is found to be 3 in all glasses irrespective of the chemical composition (Table 4). Next-Nearest Neighbor and Longer Range Correlations. The peak positioned at ∼3.5 Å in the neutron and X-ray N(r) of the stoichiometric and S-excess glasses (Figure 7) corresponds primarily to As-As next-nearest neighbors that are connected through S atoms, i.e., the As-S-As linkages in the chalcogenide network. Ge-Ge/As distances for Ge-S-Ge/As linkages are also expected to be similar, and this correlation is indeed observed in the Fourier-transformed Ge K-edge EXAFS spectrum of the glass with x + y ) 55 (Figure 5). This metal-metal correlation at 3.5 Å has also been observed in previous diffraction and Ge and As K-edge EXAFS spectroscopic studies of Ge-As-S glasses with Ge:As ) 1:1 and 1:2.20-22,29,31 An increasing S deficiency results in a progressive shift of the center of gravity of this peak from ∼3.5 to ∼3.8 Å in the neutron and X-ray N(r) (Figure 7). For the S-excess, stoichiometric, and slightly S-deficient glasses with 35 e x + y e 43, both the neutron and X-ray radial distribution functions N(r) are characterized by broad peaks at ∼5.1 and 6.7 Å. The center of gravity of these peaks shifts to 5.6 and 7.1 Å, respectively, in the compositional range of 55 e x + y e 65 with increasing S deficiency. Moreover, for these highly As-rich glasses another broad peak is observed in X-ray N(r) at ∼9 Å, possibly signifying longer range metal-metal correlations in the structure. Discussion Short- and Intermediate-Range Order. The Ge and As atoms are 4- and 3-coordinated, respectively, in all glasses, irrespective of the chemical composition (Table 4). This observation is in accordance with the results of previous EXAFS and diffraction studies of Ge-As-S glasses with Ge:As ) 1:1 and 1:2.20-22,29,31 When taken together, the Raman spectroscopic and diffraction results indicate that the structures of slightly S-excess, stoichiometric, and slightly S-deficient glasses (35 e x + y e 43) consist predominantly of a network of cornershared AsS3 trigonal pyramids together with small concentrations of GeS4 tetrahedra and some S-S (As-As) bonds in the S-excess (S-deficient) glass. Although the S-S stretching band at ∼490 cm-1 in the Raman spectrum of the S-excess glass confirms the presence of S-S bonds in this glass, the expected

J. Phys. Chem. C, Vol. 113, No. 15, 2009 6239 S-S correlation peak at ∼2 Å is not observed in N(r) of this glass, possibly due to the weak weighting of this correlation. It should also be noted that these S-S bonds must belong to the Ge-As-S network and not to isolated S8 rings since the intense vibrational bands at 152 and 220 cm-1 characteristic of such rings are not observed in the Raman spectrum of the S-excess glass.48 The sharp Raman bands in the spectrum of the slightly S-deficient glass with x + y ) 43 imply the presence of a small concentration of As4S4 molecules that do not take part in the network formation. This result is consistent with the observation made in a previous study that similar As-rich GexAsyS100-x-y glasses show phase separation and crystallization of the As4S4 phase in the composition range of 44 e x + y e 52.32 The formation of small concentrations of As4S4 molecules has also been reported in S-deficient ternary (Ge2S3)x(As2S3)1-x glasses.28 The predominantly heteropolar-bonded Ge-As-S network, characteristic of glasses with 35 e x + y e 43, suddenly changes its dimensionality and connectivity with decreasing S content at x + y ) 55, where the structure is dominated by As4S3 molecules that are held together presumably by van der Waals forces. The relative intensities and positions of the vibrational bands in the Raman spectrum of this glass are very similar to those of the intramolecular vibrational modes of As4S3 molecular entities in the pure As4S3 molecular crystal (Figure 2).32,46 The predominantly molecular nature of this glass with a high nominal 〈r〉 value of 2.58 is to be contrasted with previous reports of small concentrations of non-cross-linked S8, P4S10, P4S9, P4S3, P4Se3, As4S4, and As4S3 molecular species in chalcogenide glasses with comparable 〈r〉 values in Ge-P-S and Ge-As-S systems.28,49-56 Large concentrations of molecular S8 rings have indeed been reported to be present in S-rich glasses in the As-S and Ge-S binary systems; however, it might be argued that such results are to be expected considering the low 〈r〉 values of such glasses that approach 2.0.48 In fact, to the best of our knowledge, the only other predominantly molecular chalcogenide glass with high nominal 〈r〉 value that has been reported in the literature belongs to the P-Se binary system and consists of isolated P4Se3 molecules that are conformationally similar to the As4S3 molecules.57,58 The Ge K-edge EXAFS results indicate that Ge atoms in this glass are still heteropolar-bonded to four S atoms. These GeS4 tetrahedra must establish some connectivity between the As4S3 molecules in the structure. The near-zero electronegativity difference between Ge and As suggests that these atoms can replace each other in the structure, at least to some extent. The apical As site in the As4S3 molecule has been shown in previous studies to be energetically favorable for replacement by atoms such as P that are chemically similar to As. Similar replacement of the apical As by Ge coordinated to four S atoms could lead to the formation of Ge2As6S7 dimers as shown in Figure 11. Formation of such dimers is completely consistent with the spectroscopic and diffraction results as well as with the similarity in electronegativity between Ge and As as discussed above. The existence of these dimers can also explain the necessity for Ge doping to increase the glass-forming tendency of this composition via a combination of steric hindrance and an increase in structural connectivity, especially since the pure As4S3 composition does not form bulk glass under normal quenching rates available in the laboratory. In the composition region of 60 e x + y e 65 the relative intensity of the 273 cm-1 band in Raman spectra gradually decreases with increasing S deficiency, clearly indicating the decrease in the concentration of As4S3 molecules. On the other hand, the relative intensity of the Raman bands at 200-250

6240 J. Phys. Chem. C, Vol. 113, No. 15, 2009 cm-1 that are associated with the vibration of As-As bonds increases, possibly indicating the formation of As-rich regions in the structure. Specifically, the band at 235 cm-1 (Figure 2), which is related to the As-As bonds in the network, attains a maximum intensity for the highest metal-containing glass (x + y ) 65).23 The peak at ∼2.5 Å in the N(r) of these S-deficient glasses possibly corresponds to intramolecular As-As correlations both in As4S3 molecules and in the Asrich regions coexisting with As4S3 molecules since the Raman spectra contain bands corresponding to As-As bonds in both molecular (273 cm-1) and nonmolecular (235 cm-1) environments. Hence, the glass structure in the compositional region of 55 e x + y e 65 changes with increasing metal content from one consisting nearly exclusively of As4S3 molecular units into one with a partially molecular character composed of a progressively smaller fraction of As4S3 units embedded in an As-As homopolar-bonded matrix. It is important to note that very reasonable coordination numbers CAs(As) and CAs(S) are calculated from the diffraction data on the basis of the assumptions that (1) all Ge atoms are coordinated to four S atoms and (2) the remaining S atoms are associated with As4S3 units (Table 4). This result implies the formation of Ge2As6S7 dimers as hypothesized above. These structural changes at the nearest-neighbor level are also consistent with the evolution of the nearest-neighbor peak in the neutron and X-ray N(r). As mentioned before, the peak at ∼3.5 Å in N(r) of the glasses with 35 e x + y e 43 corresponds to As-As next-nearest neighbors connected through S atoms, i.e., As-S-As linkages. The position of this peak in the N(r) of the predominantly molecular glass with x + y ) 55 shifts to ∼3.6 Å. In accordance with the Raman spectroscopic results, this peak can be associated primarily with the intramolecular correlations between the apical and basal As atoms in As4S3 molecules, although partial contributions from the correlation between As (basal)-S nonbonded pairs in these molecules is also likely. This assignment is consistent with the As-As second-neighbor correlation at ∼3.6 Å in the Fourier-transformed As K-edge EXAFS spectrum of this glass (Figure 5). The distance between apical and basal As atoms and that between As (basal)-S nonbonded pairs are ∼3.53 and 3.6 Å, respectively, in the R-As4S3 crystal.47 Hence, the peak at ∼3.6 Å in the N(r) of the glass with x + y ) 55 is most likely to be representative of nonbonded intramolecular As-As and As-S correlations in As4S3 molecules. The shift in the position of the second peak in N(r) to ∼3.8 Å for glasses in the compositional range of 60 e x + y e 65 can be correlated with the presence of As-As next-nearest neighbors that are connected through As atoms, i.e., the As-As-As linkages. This distance is consistent with the nonbonded metal-metal distances in homopolar Ge/As-Ge/ As-Ge/As linkages as well as the As-As distance of ∼3.76 Å in the As-As-As linkages in crystalline As.29,31,59 Previous isotope-substituted neutron diffraction and anomalous X-ray scattering studies have indicated that FSDP in chalcogenide glasses arises primarily from metal-metal correlations.19,60-62 The monotonically decreasing trend in the position of neutron and X-ray FSDP to lower Q values with increasing metal content indicates that the length scale of intermediate-range order in the form of metal-metal correlations increases in the system (Figure 8). For instance, the neutron QFSDP position in these glasses decreases from 1.241 to 1.115 Å-1 with increasing S deficiency, indicating a corresponding increase in the length scale of the intermediate-range order from 2π/QFSDP ≈ 5.1 Å for the S-excess glass to 2π/QFSDP ≈ 5.6 Å for the most S-deficient glass. This result is consistent with the

Soyer-Uzun et al.

Figure 12. Variation in the amplitude of neutron and X-ray FSDPs (filled squares and open circles, respectively) as a function of the relative intensity of the band at 273 cm-1 in the Raman spectra of GexAsyS100-x-y glasses with x:y ) 1:17.3.

observation that the position of a broad peak covering a region between 5 and 6 Å in the neutron and X-ray N(r) of these glasses moves from ∼5.1 Å in S-excess and stoichiometric glasses to 5.6 Å in the most S-deficient glasses (Figure 7). Concomitantly, similar shifts in the short-range nearest-neighbor and nextnearest-neighbor correlations take place from ∼2.2 and 3.4 Å to ∼2.5 and 3.8 Å, respectively, with increasing metal content, which is related to the replacement of the heteropolar bonds with homopolar bonds in these glasses (Table 4). Moreover, in the highly S-deficient glasses, peaks at ∼7 and ∼9 Å become distinct especially in the X-ray N(r), indicating long-range metal-metal correlations in As-rich regions (Figure 7). Neutron and X-ray FSDP intensities increase with increasing metal content in the composition range 35 e x + y e 55 followed by a decreasing trend in the composition range 55 e x + y e 65 (Figure 8). The FSDP intensity in S-excess, stoichiometric, and slightly S-deficient glasses (35 e x + y e 43) must be related to the intermediate-range As-As correlations mainly in the As2S3 network since the Ge concentration is very small. The FSDP intensity goes through a maximum at the composition x + y ) 55, where the structure is dominated by the presence of molecular As4S3 units. Such high intensity of the FSDP in this glass can be related to well-defined intermolecular correlations in the structure over length scales of ∼5-6 Å. It is worth noting here that previous neutron diffraction and EXAFS studies of predominantly molecular glasses in the P-Se system have also indicated that the intermediate-range order in these glasses is indeed related to the packing of P4Se3 molecules.57,58,63 In the composition range of 60 e x + y e 65 the concentration of As4S3 molecules progressively decreases and that of the As-rich homopolarbonded regions increases with increasing metal content. This would result in the disruption of the topological continuity of intermolecular correlations, causing a progressive decrease in the FSDP intensity with increasing metal content as is indeed observed in Figure 8. This structural scenario is consistent with the fact that relative intensities of the Raman band at 273 cm-1 in glasses with 55 e x + y e 65, corresponding to the breathing mode of the basal As3 ring in the As4S3 molecule, are strongly correlated with the compositional variation of neutron and X-ray FSDP intensities (Figure 12). Therefore, the FSDP intensity in these glasses is likely to be primarily controlled by the presence and packing of As4S3 molecules in the structure. The compositional variation of the coherence length of intermediate-range

Ge-Doped Arsenic Sulfide Glasses order, 2π/∆QFSDP, is also consistent with this scenario of structural evolution with composition. The coherence length increases rapidly with increasing S deficiency from 20 Å at x + y ) 35 to ∼32 Å at x + y ) 55, where the structure is dominated by the presence of As4S3 molecules (Figure 9). Subsequently, as the molecular regions are interrupted by the formation of As-rich homopolar-bonded regions in the composition range 55 e x + y e 65, the coherence length decreases monotonically. A similar loss of correlation in intermediaterange order has also been shown in P-Se glasses as the structure changes from a quasi-zero-dimensional molecular P2Se glass, with a high concentration of P4Se3 cage molecules, to weakly interacting cages isolated in a P-rich matrix in P0.84Se0.16 glass.63 The coexistence of As4S3 molecules and As-rich regions in the composition range 60 e x + y e 65 is expected to give rise to strong composition and density fluctuations. This hypothesis is corroborated by the sudden rise in the intensity of small-angle neutron scattering observed in this compositional range (Figure 10). Network Dimensionality Transition. When taken together, the diffraction and spectroscopic results indicate that with increasing S deficiency and hence with increasing 〈r〉 the glass structure in the GexAsyS100-x-y system with x:y ) 1:17.3 transforms suddenly from a fully connected high-dimensional network at 2.37 e 〈r〉 e 2.45 into a nearly zero-dimensional molecular structure consisting predominantly of As4S3 molecules at 〈r〉 ) 2.58. This molecular structure then transforms into a low-dimensional structure where As4S3 molecules are embedded in an As-rich matrix in glasses with 2.63 e 〈r〉 e 2.69. Such a remarkable and abrupt structural and topological transition with composition appears to be a unique characteristic of Ge-As-S glasses with a very high As:Ge ratio. The “molecular” glass with 〈r〉 ) 2.58 is characterized by local maxima in the compositional variation of intensities of neutron/X-ray FSDPs, coherence length, relative intensity of the intramolecular Raman bands, and thermal expansion coefficient, while Tg goes through a local minimum (Figures 2, 3, 8, 9, and 12, Table 1). The unusually low Tg and high thermal expansion coefficient of this glass are consistent with its structure being made up of weakly bound As4S3 molecular entities. Ge- and As-containing chalcogenide glasses have been used as model systems for understanding the influence of the structure and topology on the compositional variation of various physical properties in amorphous covalently bonded materials.64,65 It has been argued on the basis of constraint theory that the compositional dependence of the physical properties of these glasses is solely determined by their nominal average coordination number 〈r〉. Nonlinear compositional variations of various physical properties in these glasses have been ascribed to either a rigidity percolation type of transition at 〈r〉 ) 2.4 or a topological phase transition at 〈r〉 ≈ 2.67.66,67 However, these theories are simply based on the average nearest-neighbor coordination numbers of the constituent atoms, and they do not differentiate between the characters of homo- and heteropolar bonds or between network vs molecular character of the intermediate-range structure. The latter case is particularly important for the molecular glass where the high nominal 〈r〉 ) 2.58 does not provide a correct description of the connectivity of the structure and therefore limits the utility of constraint theory in explaining the unusually high thermal expansion coefficient and low Tg of this glass. As discussed by Aitken, a more correct description of such glasses may require introduction of the concept of an effective 〈r〉 that considers only those atoms that provide connectivity to the structure.32 Thus, for the

J. Phys. Chem. C, Vol. 113, No. 15, 2009 6241 Ge3As52S45 glass Aitken has derived an effective coordination number of ∼0.24 under the assumption that all As atoms take part in the formation of As4S3 molecules and the remaining S and Ge atoms take part in establishing connectivity to the structure. Such a low coordination number can then rationalize the unusual thermophysical properties of this glass. It is important to note that our recent high-pressure diffraction and spectroscopic studies have shown that the molecular glass undergoes an abrupt pressure-induced structural/topological transition from the zero-dimensional molecular structure to a three-dimensionally connected network at ambient temperature at pressures g10 GPa.68,69 On the other hand, high-temperature nuclear magnetic resonance spectroscopic studies have shown that the constituent molecules in this glass undergo rapid isotropic tumbling motion even at temperatures well below Tg, where the extrapolated structural relaxation time would nearly diverge!70 The translationally frozen As4S3 molecules were observed to perform rotational dynamics in the glassy state, much like what is observed in plastic crystals. These novel structural and dynamical properties of the molecular glass may prove to be important for future technological applications. Summary The short- and intermediate-range structural characteristics of As-rich GexAsyS100-x-y glasses with x:y ) 1:17.3 are determined using combined neutron/X-ray diffraction, Ge and As K-edge EXAFS, and Raman spectroscopy. Three compositional regions can be identified on the basis of the structural characteristics of these glasses. The glasses with compositions close to stoichiometry (35 e x + y e 43) consist primarily of a heteropolar-bonded As-S network of corner-shared AsS3 pyramids. An increasing metal content results in an abrupt structural and topological transition at x + y ) 55, where the structure is found to be quasi-zero-dimensional, predominantly consisting of isolated As4S3 molecules and possibly occasional Ge2As6S7 dimers, held together presumably via weak van der Waals bonding. A further increase in the metal content (60 e x + y e 65) results in a sparsely connected structure consisting of As4S3 molecules and As-As homopolar-bonded regions. Such unique and nonmonotonic evolution of structure and topology is intimately related to the compositional variation of thermophysical properties and density fluctuation in these glasses. Acknowledgment. This work was supported by National Science Foundation Grant DMR 0603933 to S.S. The XSD and IPNS Divisions at Argonne National Laboratory are supported by the U.S. DOE under Contract Number DE-AC02-06CH11357. We sincerely thank Dr. Chris Benmore and Ms. Joan Siewenie for their valuable technical assistance with X-ray and neutron diffraction experiments. References and Notes (1) Nalwa, H. S., Ed. Handbook of AdVanced Electronic and Photonic Materials and DeVices, Vol. 5: Chalcogenide Glasses and Sol-Gel Materials; Academic Press: San Diego, 2001. (2) Harbold, J. M.; Ilday, F. O.; Wise, F. W.; Sanghera, J. S.; Nguyen, V. Q.; Shaw, L. B.; Aggarwal, I. D. Opt. Lett. 2002, 27, 119. (3) Kasap, S. O.; Koughia, K.; Munzar, M.; Tonchev, D.; Saitou, D.; Aoki, T. J. Non-Cryst. Solids 2007, 353, 1364. (4) Bureau, B.; Zhang, X. H.; Smektala, F.; Adam, J.-L.; Troles, J.; Ma, H.-li.; Boussard-Ple`del, C.; Lucas, J.; Lucas, P.; Le Coq, D.; Riley, M. R.; Simmons, J. H. J. Non-Cryst. Solids 2004, 345-346, 276. (5) Zakery, A.; Elliott, S. R. J. Non-Cryst. Solids 2003, 330, 1. (6) Meinders, E. R.; Mijritskii, A. V.; van Pieterson L.; Wuttig, M. Optical Data Storage: Phase Change Media and Recording; Springer: Berlin, 2006.

6242 J. Phys. Chem. C, Vol. 113, No. 15, 2009 (7) Shimakawa, K.; Kolobov, A. V.; Elliott, S. R. AdV. Phys. 1995, 44, 475. (8) Lowe, A. J.; Elliott, S. R.; Greaves, G. N. Philos. Mag. B 1986, 54, 483. (9) Zhou, W.; Paesler, M. A.; Sayers, D. E. Phys. ReV. B 1991, 43, 2315. (10) Zhou, W.; Paesler, M. A.; Sayers, D. E. Phys. ReV. B 1992, 46, 3817. (11) Armand, P.; Ibanez, A.; Dexpert, H.; Philippot, E. J. Non-Cryst. Solids 1992, 139, 137. (12) Eckert, H. NMR 1994, 33, 125. (13) Greaves, G. N.; Sen, S. AdV. Phys. 2007, 56, 1. (14) King, W. A.; Clare, A. G.; LaCourse, W. C.; Volin, K.; Wright, A. C.; Wanless, A. J. Phys. Chem. Glasses 1997, 38, 269. (15) Jackson, K.; Briley, A.; Grossman, S.; Porezag, D. V.; Pederson, M. R. Phys. ReV. B 1999, 60, R14985. (16) Petri, I.; Salmon, P. S.; Fischer, H. E. Phys. ReV. Lett. 2000, 84, 2413. (17) Bureau, B.; Troles, J.; Le Floch, M.; Guenot, P.; Smektala, F.; Lucas, J. J. Non-Cryst. Solids 2003, 319, 145. (18) Bureau, B.; Troles, J.; Le Floch, M.; Smektala, F.; Lucas, J. J. Non-Cryst. Solids 2003, 326-327, 58. (19) Bychkov, E.; Benmore, C. J.; Price, D. L. Phys. ReV. B 2005, 72, 172107. (20) Sen, S.; Ponader, C. W.; Aitken, B. G. J. Non-Cryst. Solids 2001, 293-295, 204. (21) Sen, S.; Ponader, C. W.; Aitken, B. G. Phys. ReV. B 2001, 64, 104202. (22) Sen, S.; Aitken, B. G. Phys. ReV. B 2002, 66, 134204. (23) Aitken, B. G.; Ponader, C. W. J. Non-Cryst. Solids 2000, 274, 124. (24) Lippens, P. E.; Jumas, J. C.; Olivier-Fourcade, J.; Aldon, L. J. NonCryst. Solids 2000, 271, 119. (25) Nemanich, R. J. Phys. ReV. B 1977, 16, 1655. (26) Mitsa, V.; Babinets, Y.; Gvardionov, Y.; Yermolovich, I. J. NonCryst. Solids 1991, 137-138, 959. (27) Boulmetis, Y. C.; Perakis, A.; Raptis, C.; Arsova, D.; Vateva, E.; Nesheva, D.; Skordeva, E. J. Non-Cryst. Solids 2004, 347, 187. (28) Mamedov, S.; Georgiev, D. G.; Qu, T.; Boolchand, P. J. Phys.: Condens. Matter 2003, 15 (31), S2397. (29) Soyer Uzun, S.; Sen, S.; Benmore, C. J.; Aitken, B. G. J. Phys. Chem. C 2008, 112, 7263. (30) Neov, S.; Gerasimova, I.; Skordeva, E.; Arsova, D.; Pamukchieva, V.; Mikula, P.; Lukas, P.; Sonntag, R. J. Mater. Sci. 1999, 34 (15), 3669. (31) Soyer Uzun, S.; Sen, S.; Benmore, C. J.; Aitken, B. G. J. Phys.: Condens. Matter 2008, 20, 335105. (32) Aitken, B. G. J. Non-Cryst. Solids 2004, 345, 346, 1. (33) Savage, J. A.; Nielsen, S. Infrared Phys. 1965, 5, 195. (34) Binsted, N. EXCURV98, CLRC Daresbury Laboratory Computer Program; CLRC Daresbury Laboratory: Warrington, U.K., 1998. (35) Gurman, S. J.; Binsted, N.; Ross, I. J. Phys. (Paris) 1984, C17, 143. (36) Ellison, A. J. G.; Crawford, R. K.; Montague, D. G.; Volin, K. J.; Price, D. L. J. Neutron Res. 1993, 4, 61. (37) Soper, A. K.; Howells, W. S.; Hannon, A. C. ATLAS-Analysis of Time of Flight Diffraction Data from Liquid and Amorphous Samples; Technical Report RAL 89-046; Rutherford Appleton Laboratory: Chilton, Oxon, U.K., 1989. (38) Sears, V. F. In Neutron Scattering (Methods of Experimental Physics); Skold, K., Price, D. L., Eds.; Academic: Orlando, FL, 1986; Vol. 23, Part A, p 521.

Soyer-Uzun et al. (39) Lorch, E. A. J. Phys. C: Solid State Phys. 1969, 2, 229. (40) Computer code FIT2D: Hammersley, A. P. Internal Report No. ESRF98HA01T; European Synchrotron Radiation Facility: Grenoble, France, 1998 (unpublished); see the Web site for details at http://www.esrf.fr/ computing/scientific/FIT2D/. (41) Qiu, X.; Thompson, J. W.; Billinge, S. J. L. J. Appl. Crystallogr. 2004, 37, 678. (42) Chantler, C. T. J. Phys. Chem. Ref. Data 1995, 24, 71. (43) Susman, S.; Volin, K. J.; Montague, D. G.; Price, D. L. J. NonCryst. Solids 1990, 125, 168. (44) Leadbetter, A. J.; Wright, A. C. J. Non-Cryst. Solids 1972, 7, 23. (45) Zallen, R.; Slade, M. L. Phys. ReV. B 1978, 18, 5775. (46) Christian, B. H.; Gillespie, R. J.; Sawyer, J. F. Inorg. Chem. 1981, 20, 3410. (47) Whitfield, H. J. J. Chem. Soc. A 1970, 1800; J. Chem. Soc., Dalton Trans. 1973, 1737. (48) Bychkov, E.; Miloshova, M.; Price, D. L.; Benmore, C. J.; Lorriaux, A. J. Non-Cryst. Solids 2006, 352, 63. (49) Aitken, B. G.; Ponader, C. W. J. Non-Cryst. Solids 1999, 256&257, 143. (50) Lucovsky, G.; Galeener, F. L.; Keezer, R. C.; Geils, R. H.; Six, H. A. Phys. ReV. B 1974, 10, 5134. (51) Cherry, B.; Zwanziger, J. W.; Aitken, B. G. J. Phys. Chem. B 2002, 106, 11093. (52) Aitken, B. G.; Youngman, R. E.; Ponader, C. W. J. Non-Cryst. Solids 2001, 284, 34. (53) Cherry, B. R.; Zwanziger, J. W.; Aitken, B. G. J. Non-Cryst. Solids 2004, 333, 28. (54) Georgiev, D. G.; Mitkova, M.; Boolchand, P.; Brunklaus, G.; Eckert, H.; Micoulaut, M. Phys. ReV. B 2001, 64, 134204. (55) Ewen, P. J. S.; Sik, M. J.; Owen, A. E. Solid State Commun. 1980, 33, 1067. (56) Tsuchihashi, S.; Kawamoto, Y. J. Non-Cryst. Solids 1971, 5, 286. (57) Verrall, D. J.; Elliott, S. R. J. Non-Cryst. Solids 1989, 114, 34. (58) Verrall, D. J.; Elliott, S. R. Phys. ReV. Lett. 1988, 61, 974. (59) Bryden, J. H. Acta Crystallogr. 1962, 15, 167. (60) Arai, M.; Johnson, R. W.; Price, D. L.; Susman, S.; Gay, M.; Enderby, J. E. J. Non-Cryst. Solids 1986, 83, 80. (61) Petri, I.; Salmon, P. S.; Fischer, H. E. Phys. ReV. Lett. 2000, 84, 2413. (62) Fuoss, P. H.; Eisenbergen, P.; Warburton, W. K.; Bienenstock, A. Phys. ReV. Lett. 1981, 46, 1537. Barnes, A. C. J. Non-Cryst. Solids 1999, 250-252, 393. (63) Phillips, R. T.; Wolverson, D.; Burdis, M. S.; Fang, Y. Phys. ReV. Lett. 1989, 63, 2574. (64) Phillips, J. C. J. Non-Cryst. Solids 1979, 34, 153. (65) Thorpe, M. F. J. Non-Cryst. Solids 1983, 57, 355. (66) Tatsumisago, M.; Halfpap, B. L.; Green, J. L.; Lindsay, S. M.; Angell, C. A. Phys. ReV. Lett. 1990, 64, 1549. (67) Tanaka, K. Phys. ReV. B 1989, 39, 1270. (68) Soyer-Uzun, S.; Gaudio, S. J.; Sen, S.; Mei, Q.; Benmore, C. J.; Tulk, C. A.; Xu, J.; Aitken, B. G. J. Phys. Chem. Solids 2008, 69, 2336. (69) Sen, S.; Gaudio, S.; Aitken, B. G.; Lesher, C. E. Phys. ReV. Lett. 2006, 97, 025504. (70) Gjersing, E. L.; Sen, S.; Yu, P.; Aitken, B. G. Phys. ReV. B 2007, 76, 214202.

JP810446G