Crystal Structure--- Phase Diagram

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Page ID: 2987

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Chemical Concepts Demonstrated

The effect of temperature on the structure of a solid
Body-centered and face-centered cubic

Demonstration

An iron wire is enclosed in a glass vessel (variac) and is purged with N₂ or Ar to prevent a reaction with O₂.

A voltage of 6-8 volts is sent through the wire.

Observations

When the temperature of the wire exceeds 910 °C, the wire sags. When the variac is off, the wire returns to its inital dimensions.

Explanation

Iron crystallizes in a body centered cubic structure at room temperature. The density of iron in this phase (7.86 g/cm₃) implies an atomic radius of 0.124 nm. At around 910 °C, pure iron changes from body-centered cubic to face-centered cubic packing. This phase change results is accompanied by a decrease in the density of the iron, and the iron wire sags. From this decrease in density, it is assumed that the atomic radius of the iron atom at 910 °C must be significantly larger than at room temperature.

Contributors

Dr. George Bodner (Perdue University)