The pronounced glass-forming tendencies of alloys of S and Se with Ge and/or As are discussed topologically. An atomic model is introduced which for predominantly covalent forces constitutes the first microscopic realization of Kauzmann's description of the glass transition as an entropy (not enthalpy or volume) crisis. The model contains no adjustable parameters and predicts the glass-forming tendency as a function of composition in excellent agreement with experiment. Several related properties, including phase diagrams, radial distribution functions and crystal structures are discussed in the context of chemical bonding and short-range order in the non-crystalline covalent networks of these materials.

Topology of covalent non-crystalline solids I: Short-range order in chalcogenide alloys

Results of thermal analysis performed at different heating rates on eight glasses of the As-Sb-Se system, with compositions represented by (As, Sb) 40 Se 60 and As x Sb 15 Se 85 − x are reported and discussed. The values of the glass transition temperature ( T g ), the crystallisation temperature ( T c ) and the peak temperature of crystallisation ( T p ) are found to be independent of particle size. From the heating rate dependence of T g and T p values of the activation energy for glass transition ( E t ) and the activation energy for crystallisation ( E c ) are evaluated and their composition dependence discussed. The crystallisation data are examined in terms of recent analyses developed for non-isothermal crystallisation studies to arrive at E c and details of the crystallisation mechanism. The results indicate bulk nucleation and crystallisation with two and three dimensional growth respectively for the As x Sb 15 Se 85 − x and (As, Sb) 40 Se 60 glasses.

Calorimetric measurements on as-sb-se glasses

Over the past two decades chalcogenide glasses have been researched in order to assess their suitability as passive bulk optical component materials for 3–5μm and 8–12μm infrared applications. This research has led to a greater understanding of the physical properties of these materials, and the present paper concentrates on the optical properties and applications of these bulk chalcogenide glasses. The various factors influencing the intrinsic and extrinsic optical loss mechanisms in materials are discussed, numerical data on the basic optical properties of chalcogenide glasses is presented and applications are discussed.

Optical properties of chalcogenide glasses

We review the bonding in a wide range of non-crystalline materials, such as the elemental and compound amorphous semiconductors, liquid semiconductors and, briefly, some oxide glasses and amorphous silicon dioxide. The review concentrates on the experimental determination of electronic structure from photoemission, optical spectra and core spectra and on its theoretical description, particularly using tight-binding methods. The review also describes the bonding at defects in amorphous semiconductors and in particular that of dopants and hydrogen in hydrogenated amorphous silicon (a-Si:H).

Electronic structure of amorphous semiconductors

Two types of amorphous and glassy Sb 2 S 3 were studied: amorphous orange-red powder obtained by precipitating a SbCl 3 solution using a stream of hydrogen sulphide in a bath of boiling water and a glassy material in the form of thin platelets obtained by the method of rotating cylinders. It was found by DTA that there is a great difference in the crystallization entropy and temperature between the powder and glass. Radial electron density distribution curves were measured for both modifications and a difference analysis showed that they can be considered to be much the same. From the distribution curves it follows that the basic unit is a trigonal SbS 3 pyramid which forms in the case of Sb 2 S 3 glass, a covalent random network while for the amorphous powder a random ordering of crystalline-like and band-like (Sb 2 S 3 ) n molecules can be considered as an appropriate model. A comparison of radial electron density distribution curves for As 2 X ( X =S, Se, Te and Sb 2 S 3 shows that Sb 2 S 3 belongs to the class of glassy As 2 X 3 structures. A rule is found that in this class of glasses the mutual ratios of diameters of corresponding coordination spheres are constant. On the basis of coordination numbers and of broadening functions for individual coordination spheres it is shown that in the series As 2 S 3 :As 2 Se 3 As 2 Te 3 a gradual transition from a layer-like to a random network takes place and that glassy Sb 2 S 3 can be put beside As 2 Te 3 . This process is a result of the difference between orpiment-like configurations and the As 2 X 5 ones, which evoke chain branching and interconnecting. This leads finally for As 2 Te 3 and Sb 2 S 3 to a random continuous covalent network. This process is confirmed by increase of both occupation numbers and the widths of broadening functions corresponding to individual coordination spheres.

Structure of amorphous and glassy Sb2S3 and its connection with the structure of As2X3 arsenic chalcogenide glasses

Diffraction measurements on bulk and thin films AsTe glasses are reported, throughout the whole range of stability. FNP distances and areas, average valence angles and compactness are determined from the Fourier transform. For As-rich glasses important differences are found between short-range order in both vitreous and crystalline states. An interpretation at the molecular scale is proposed. It takes account of the particular topological properties of the AsTe molecules, which are evidenced by hand-made models. The proposed structure consists of a network of AsTe 3/2 units which are interconnected through “AsAs locks”. It is shown that these “locks” play a crucial role in the stability of the As-rich glasses in favoring strongly the covalent bonds. On the other hand, in Te-rich glasses there is no similar locking mechanism. The presence of threefold-coordinated tellurium sites enhances the metallic character, and weakens the difference between crystalline and vitreous states. The structural models which are proposed enable us to explain the anomalies and the discontinuities of most thermodynamic properties which are described in paper I.

J. Cornet

Papers

Properties and structure of AsTe glasses (II). Local order parameters and structural model