Structure of Sb_xGe_40-xSe₆₀ glasses around 2.67 average coordination number

Abstract

The evolution of the structure of Sb_xGe_40-xSe₆₀ (x = 8, 12, 15, 18 and 20) chalcogenide glasses is determined by high resolution X-ray photoelectron spectroscopy as Sb is added and the average coordination number (Z) is varied from 2.60 to 2.72. For glasses with Z < 2.67 the structure consists of deformed tetrahedra and pyramids, in which at least one Se atom is substituted by Ge or Sb atoms. Increase in the concentration of cations beyond Z ≥ 2.67 leads to a strengthening of the above regularity and formation of the structure by shared pyramids and tetrahedra with two or more Se atoms substituted by cations. At the same time Se–Se dimers are present in all the investigated compositions while they are not expected in these Se-deficient glasses. Their formation dictates that there must be metal-rich regions somewhere in the structure.

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 (n_c) becomes equal to space dimensionality (e.g., n_c = 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. Sb_xGe_40-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.