How Do Crystals and Amorphous Materials Differ?

• Crystal: atoms in even rows
• Amorphous: atoms out of position

What you will learn

• Describe what a crystal lattice and long-range order are.
• State what makes an amorphous material differ from a crystal.
• Name two properties of silicon that change when the network loses order.

Before you start

You only need to know that matter is made of atoms. No formulas are needed.

What is a crystal lattice?

A crystal lattice is the way atoms sit in one evenly repeating pattern. An article in National Science Review says crystalline materials have a long-range ordered structure, meaning a periodic arrangement of lattice points. That is why they are also called ordered materials.

Silicon is a familiar example. A Harvey Mudd College lecture says the crystal lattice of silicon has a tetrahedral pattern.

• One marked repeating unit
• Atoms in the lattice
• Order continuing far along one direction

How do amorphous materials differ?

They are disordered when seen over long distances. The National Science Review article calls amorphous materials disordered, and lists gels, amorphous carbon and silicon, defect-rich solids and melt-quenched glasses as examples.

For silicon, an MIT page states the difference clearly: amorphous silicon is made of the same silicon atoms with the same four-fold coordination as crystalline silicon. Its bond network is disordered, and that makes the electronic structure very different.

A neatly laid brick wall and a heap of fallen bricks hold the same bricks. Only the arrangement differs.

• Even bond network, neat squares
• Warped bond network, distorted cells

How does the difference change the properties?

The National Science Review article compares the two groups. It discusses useful properties of amorphous materials, including optical performance, chemical tuning and shaping; this does not establish that every amorphous material outperforms every crystal. Melt-quenched glasses in particular, however, are more brittle, mechanically weaker and softer than their crystalline counterparts. The drawbacks arise from a lower atomic packing density.

For silicon, the MIT page shows two properties reversing. In this solar-cell comparison, crystalline silicon absorbs light less strongly than amorphous silicon but transports charge carriers better. The disorder in amorphous silicon reverses these properties. It absorbs light strongly, which allows ultra-thin solar cells, but its efficiency is lower than that of crystalline silicon, so the cells must be larger to provide the same power.

• Crystalline silicon: absorbs little light
• Crystalline silicon: conducts carriers well
• Amorphous silicon: absorbs much light
• Amorphous silicon: conducts carriers less well

Real-world example

Compare an egg carton with a bag of eggs spilled on a table. Eggs in the carton sit in their cells, row after row. Eggs on the table lie anywhere. Both hold the same eggs, but you arrange and pick them up in very different ways.

Ask yourself: if one more egg were added, could you guess where it would sit? In the carton you could, in the heap you could not. That is the idea of long-range order.

Common misunderstandings

Amorphous describes an arrangement, not an element. Amorphous silicon is still made of silicon atoms; this does not mean all amorphous materials share a composition.

Amorphous does not mean faulty. Amorphous silicon absorbs light strongly in the solar-cell comparison above.

Amorphous does not mean glass. The National Science Review article notes that melt-quenched glass is an amorphous material, while an amorphous material is not necessarily glass.

Frequently asked questions

Is amorphous silicon a different element?

No. It is made of the same silicon atoms with the same four-fold coordination, and differs only in having a disordered bond network.

Why can amorphous silicon cells be thin?

Because amorphous silicon absorbs light strongly, which allows ultra-thin solar cells.

Are amorphous materials weaker than crystals?

According to the National Science Review article, melt-quenched glasses in particular are more brittle, mechanically weaker and softer than their crystalline counterparts.

Summary

• A crystal has long-range order, with lattice points repeating evenly.
• An amorphous material lacks that order even when it is made of the same kind of atoms.
• Losing order reverses silicon's light absorption and carrier conduction.