Crystal Structure--- Phase Diagram

<h2>Chemical Concepts Demonstrated</h2> <ul> <li>The effect of temperature on the structure of a solid</li> <li>Body-centered and face-centered cubic</li> </ul> <h2>Demonstration</h2> <p><img alt="" class="left" src="http://chemed.chem.purdue.edu/demos/moviesheets/graphics/113-iron.gif" style="width: 391px; height: 168px;" /><br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> An iron wire is enclosed in a glass vessel (variac) and is purged with N₂ or Ar to prevent a reaction with O₂.</p> <p>A voltage of 6-8 volts is sent through the wire.</p> <h2>Observations</h2> <p>When the temperature of the wire exceeds 910 °C, the wire sags. When the variac is off, the wire returns to its inital dimensions.</p> <h2>Explanation</h2> <p>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.</p> <p style="margin-top: 1.5em; margin-bottom: 3pt; line-height: normal; margin-right: 1.5em; "><strong><span class="mt-font-tahoma" style="font-size:14pt;color:black;">Contributors</span></strong></p> <ul style="margin-top:1.5em;margin-bottom:0px;list-style-type:disc;"> <li><span class="mt-font-verdana" style="font-size:10.5pt;">Dr. George Bodner (<a href="http://chemed.chem.purdue.edu/" title="http://chemed.chem.purdue.edu/"><span style="color:rgb(0, 84, 166);">Perdue University</span></a>)</span></li> </ul>