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Chem.Libretexts

Crystal Structure--- Phase Diagram

 

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.

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