13.1 Crystal Structure--- Phase Diagram

Chemical Concepts Demonstrated: The effect of temperature on the structure of a solid; body-centered and face-centered cubic


    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.


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


    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.