---
title: "Short note: Chalcogenide glasses"
subject: "Engineering Chemistry"
chapter: 4
key: "semester_2/engineering_chemistry:chalcogenide-glasses"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/chalcogenide-glasses
site: BCT Engineering
---

# Short note: Chalcogenide glasses

Engineering Chemistry · Chapter 4: Engineering Polymers

- Asked in: 2070 Bhadra, 2 marks; 2068 Bhadra, 2.5 marks; 2068 Magh, 2.5 marks (asked 3 times)
- Video: [Chalcogonide Glasses](https://www.youtube.com/watch?v=tW9PayLNmTk) — sab kuch
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/chalcogenide-glasses
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-4

## Question

Write a short note on chalcogenide glasses.

## Short answer

Chalcogenide glasses are amorphous solids containing S, Se, Te with As, Ge, made by melt-quenching. They are transparent in infrared, semiconducting, and photosensitive. Applications include IR optics, optical fibers, phase-change memory, and photoreceptors.

## Answer

**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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What defines a chalcogenide glass?
   A. A crystalline oxide of silicon
   B. An amorphous solid containing S, Se or Te but not oxygen, with elements like As or Ge
   C. A glass made only of oxygen and silicon
   D. A polymer of carbon and sulphur
   Answer: B. They are non-crystalline solids containing chalcogens S, Se, Te (not O) combined with As, Ge, Sb, etc. (Key point: Definition)

2. Why is the melt rapidly quenched during preparation?
   A. To remove impurities by evaporation
   B. To make the melt homogeneous
   C. So it solidifies as a glass instead of crystallising
   D. To raise the melting point
   Answer: C. Fast cooling does not give atoms time to arrange into crystals, leaving an amorphous glass. (Key point: Rapid quenching)

3. Which property makes chalcogenide glasses useful for thermal imaging lenses?
   A. Transparency to visible light only
   B. Insulation of electricity
   C. Low refractive index
   D. Transparency in the infrared
   Answer: D. They transmit infrared (about 1 to 15 micrometres), suiting IR lenses and windows. (Key point: Property)

4. Which application relies on switching reversibly between amorphous and crystalline phases?
   A. Phase-change memory and rewritable discs using GST
   B. Optical fibre cladding
   C. Window glass
   D. Insulating cable sheaths
   Answer: A. Ge-Sb-Te alloys change phase reversibly, used in rewritable discs and phase-change memory. (Key point: Phase change use)

## Related in Engineering Chemistry

- [Chalcogenide glasses and network polymers of sulphur](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Achalcogenide-glass-polymers) — Engineering Chemistry, chapter 4
- [Biodegradable polymer; carbon reinforced polymers, chalcogenide glasses](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Abiodegradable-carbon-chalcogenide) — Engineering Chemistry, chapter 4
- [Polythiazyl preparation; polyphosphazene, chalcogenide uses](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Apolythiazyl-polyphosphazene) — Engineering Chemistry, chapter 4