Chalcogenide glasses and network polymers of sulphur

Chalcogenide glasses

Chalcogenide glasses are amorphous (non-crystalline) solids that contain one or more chalcogen elements (S, Se, Te; but not oxygen) combined with elements such as As, Ge, Sb, Ga or P. Examples: As₂S₃, As₂Se₃, GeSe₂, Ge–Sb–Te (GST), As–Se–Te.

Preparation: the high-purity elements are weighed in the required ratio, sealed in an evacuated silica ampoule and melted at 600–1000 °C with rocking for a homogeneous melt. The melt is rapidly quenched (cooled quickly in air or water) so that it solidifies as a glass instead of crystallising.

Properties: transparent in the infrared (about 1 to 15 µm), high refractive index, semiconducting, photosensitive, and can switch reversibly between amorphous and crystalline phases.

Engineering uses

• Infrared optics: lenses, windows, thermal-imaging and night-vision cameras.
• IR optical fibres for sensing and laser power delivery.
• Phase-change memory and rewritable optical discs (CD-RW, DVD-RAM) using GST alloys.
• Photoreceptor drums of photocopiers (amorphous Se, As₂Se₃); photoresists and solar cells.
• Switching devices, chemical sensors and waveguides.

Network polymers of sulphur: preparation and uses

Plastic (polymeric) sulphur. Ordinary rhombic sulphur has S₈ rings. When sulphur is heated above about 160 °C, the rings open and join into long chains (S)ₙ, and the liquid becomes very viscous. If this hot liquid (heated to near its boiling point) is poured into cold water, the chains are frozen as a brown, rubber-like, elastic solid called plastic sulphur. It slowly reverts to the S₈ form.

S₈ → (heat above 160 °C) → ·S–S–S...S–S· (chain) → quench in water → plastic sulphur

Cross-linked sulphur polymers. In vulcanisation, a small amount of sulphur forms –S– and –S–S– cross-links between rubber chains, making the rubber harder and stronger (used for tyres). Polysulphide rubber (Thiokol) is obtained from ethylene dichloride and sodium polysulphide: n ClCH₂CH₂Cl + n Na₂S₄ → [–CH₂CH₂–S₄–]ₙ + 2n NaCl.

Cross-linking of sulphur chains into a network (for example with As or Ge, as in As₂S₃) gives chalcogenide glass networks. Uses of sulphur polymers: vulcanisation of rubber for tyres, Thiokol polysulphide rubber for sealants, fuel hoses and rocket binders, sulphur concrete and sulphur-based roofing and paving materials, and (SN)ₓ as a conducting polymer.