Structure of SbxGe40-xSe60 glasses around 2.67 average coordination number
Highlights
► Validation of the concept of nano-scale segregation in chalcogenide glass, ChG. ► First clue to the chemical identity of nano-scale inhomogeneity in ChG. ► Powerful demonstration of XPS as tool for identifying structural moieties in glass. ► Basic structure of commercially important ternary ChG.
Introduction
The overall connectivity of network is shown to have significant impact on the physical properties of glasses such as molar volume, glass transition temperature, energy band gap, chemical solubility, etc. [1], [2], [3], [4], [5], [6]. Accepting nearest neighbor bonds as Lagrangian constraints (bending and stretching), a theoretical description of the topology of the glass network was developed by Phillips and Thorpe [1], [2]. Such mean-field theory of elasticity predicts solitary floppy to rigid transition of structure when the number of Lagrangian constraints per atom (nc) becomes equal to space dimensionality (e.g., nc = 3 for a three-dimensional network). In random covalent-bonded networks this topological threshold occurs at the average number of covalent bonds per atom (average coordination number) Z ≈ 2.4. Numerous data on compositional dependences of physical and chemical properties of different glass-forming systems show extrema or peculiarities around this composition, thus supporting the theory [3], [4], [5].
Further increase in connectivity of glass backbone beyond Z = 2.4 (adding chemical elements with higher coordination, like Ge or Si, than two-fold coordinated chalcogen atoms) leads to the formation of rigid or stressed rigid phases and observation of a second set of extrema or peculiarities around Z ≈ 2.67 in the compositional dependences of physical and chemical properties of glass-forming systems [4], [5]. Subsequently, it was suggested that ‘nanoscale phase separation’ effects are responsible for the extrema on compositional dependences [6]. However, systematic and comprehensive studies of this proposal have not been performed and structure–property correlation in this region remains to be demonstrated. For verifying structural origin of these experimental data, ternary chalcogenide systems are especially attractive because of their wide glass forming regions across the Z = 2.67 composition.
In the present paper we examine a typical Sb–Ge–Se ternary chalcogenide glass system viz. SbxGe40-xSe60, specifically the nonstoichiometric Ge2Se3–Sb2Se3 cut across the glass-forming region. The variation of composition is chosen to cover Z = 2.67 transition point (2.6 ≤ Z ≤ 2.72) incorporating a wide variation of antimony concentration (8 ≤ x ≤ 20). High-resolution X-ray photoelectron spectroscopy (HR-XPS) technique is used to determine the chemical environments and electronic states of elements in these glasses. As the suitability of HR-XPS for determining the structure of network chalcogenide glasses and thin films is well established [7], [8], [9], it should help establish the structure of so called nanophases, if present. This kind of knowledge would have a significant impact on the practical applications of Sb–Ge–Se glasses as IR optical materials [10], multilayered waveguides for evanescent waveguide sensing [11], phase-change memory devices [12], etc.
Section snippets
Experimental
The samples of ternary SbxGe40-xSe60 (x = 8, 12, 15, 18 and 20) glasses were prepared by conventional melt-quench method from a mixture of high purity (99.9999%) Sb, Ge and Se elemental powders in evacuated quartz ampoules (at 10− 6 Torr). The sealed ampoules were heated until the mixture melted, and kept inside a rocking furnace at 900 °C for 24 h. The ampoules were quenched in ice-water to form bulk glass.
HR-XPS spectra were recorded with a Scienta ESCA-300 spectrometer using monochromatic Al Kα
Results
The volumetric and glass transition temperature (Tg) measurements, performed by Mahadevan et.al. for SbxGeySe100-x-y glass system [5], show a maximum with increasing y or Z (at constant x). The initial position of this extremum occurs at the average coordination number Z*, which has the value of 2.67 for Sb-free glass (x = 0). It shifts systematically to lower values as the Sb content increases, following the empirical expression [6]:
Indeed, we can apply this relation to the
Discussion
More precise information about the structure can be obtained by analyzing the core-level XPS spectra. The fit parameters, such as BE, FWHM and partial area (A) for various components are given in Table 2. Since all of the investigated materials are Se-poor compositions and belong to Sb2Se3–Ge2Se3 (so-called non-stoichiometric) cut-line of the glass-forming region, only Ge–Se–Ge, Ge–Se–Sb and Sb–Se–Sb fragments are expected for Se nearest neighbor environment. Owing to close electronegativity
Conclusions
From the analysis of Sb, Ge and Se core level XPS spectra, we conclude that the structure of Se-poor glasses within the ‘nanoscale phase separated region’ of SbxGe40-xSe60 family exhibits distinct regularities. Formation of the deformed Ge-based tetrahedra and Sb-based pyramids where one of the chalcogen atoms is substituted by Ge or Sb cation, prevails for the glasses with Z < 2.67. Approaching the transition point at Z ~ 2.67 and especially for Z > 2.67 domain, more than one Se atoms in pyramids or
Acknowledgments
The authors thank U.S. National Science Foundation, through International Materials Institute for New Functionality in Glass (IMI-NFG), Lehigh University for initiating the international collaboration and providing partial financial support for this work (NSF Grant Nos. , ).
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Present address: Dept. of Physics & Astronomy, Austin Peay State University, Clarksville, TN 37044.