Gray Tin (α-Tin) is a non-metallic allotrope of tin that is stable only at low temperatures. Unlike metallic white tin (β-Sn), α-tin is brittle, semiconducting, and powder-forming.
The transformation of β-tin into α-tin is historically known as “Tin Pest”, a destructive phase change that causes metallic tin to crack, expand, and disintegrate.
Basic Identification
| Property | Value |
|---|---|
| Chemical Symbol | Sn |
| Allotrope | α-Tin |
| Crystal Structure | Diamond cubic |
| Stability Range | Below 13.2 °C |
| Volume Change | ~26% expansion from β-Sn |
Chemical Composition
| Element | Content |
|---|---|
| Tin (Sn) | ~100% |
✔ Forms only in very high-purity tin
✔ Trace alloying elements suppress α-tin formation
Mechanical Properties
| Property | Value |
|---|---|
| Tensile Strength | Extremely low |
| Hardness | Very brittle (ceramic-like) |
| Elastic Modulus | ~50–60 GPa |
| Ductility | 0% (brittle) |
| Fracture Mode | Cleavage fracture |
Physical Properties
| Property | Value |
|---|---|
| Density | ~5.77 g/cm³ |
| Electrical Behavior | Semiconductor |
| Band Gap | ~0.08 eV |
| Thermal Conductivity | Low |
| Magnetic Behavior | Diamagnetic |
| Appearance | Dull gray, powdery |
✔ Much lower density than white tin (β-Sn: 7.31 g/cm³)
✔ Structurally similar to silicon and germanium
Tin Pest – Phase Transformation
Below 13.2 °C, metallic white tin transforms into gray tin:
β-Sn → α-Sn
This causes:
✔ ~26% volume expansion
✔ Cracking and pulverization
✔ Loss of electrical continuity
✔ Catastrophic component failure
Transformation is accelerated by:
✔ High purity tin
✔ Prolonged cold exposure
✔ Mechanical stress
✔ Presence of α-tin nucleation sites
Refining & Processing Behavior
α-Tin is not intentionally produced in industry.
It appears only as a failure product in:
✔ Ultra-pure tin
✔ Cold-climate exposure
✔ Cryogenic conditions
It cannot be rolled, cast, welded, or machined.
Available Forms
✔ Powder (disintegrated tin)
✔ Granular fragments
✔ Laboratory-grown crystals (research only)
⚠ No commercial structural forms exist.
Applications of Gray Tin
🧪 Materials science & semiconductor research
🧊 Cryogenic and low-temperature failure analysis
📡 Reliability testing of solder joints
🔬 Phase transformation and allotrope studies
⚠ Gray tin has no commercial engineering applications.
Gray Tin vs White Tin
| Property | α-Tin (Gray) | β-Tin (White) |
|---|---|---|
| Stability | < 13.2 °C | > 13.2 °C |
| Structure | Diamond cubic | Tetragonal |
| Behavior | Brittle | Ductile |
| Conductivity | Semiconductor | Metallic |
| Density | 5.77 g/cm³ | 7.31 g/cm³ |
| Engineering Use | ❌ None | ✅ Extensive |
Why Gray Tin Matters
You do not choose gray tin for engineering — you avoid it.
However, understanding α-tin is essential when:
✔ Designing cold-climate electronics
✔ Preventing tin pest in soldered joints
✔ Selecting tin alloys with stabilizing elements
✔ Studying low-temperature phase failures
Gray Tin (α-Sn) is a brittle, semiconducting allotrope of tin with no structural use, but immense importance in materials science, failure analysis, and low-temperature reliability engineering.