What material primarily demonstrates allotropy based on temperature changes?

Iron is the material that primarily demonstrates allotropy based on temperature changes. Allotropy refers to the existence of two or more different physical forms of the same element in the same physical state. In the case of iron, it can exist in several allotropes, most notably alpha iron (ferrite) and gamma iron (austenite).

Alpha iron is stable at room temperature and has a body-centered cubic (BCC) structure, which allows it to accommodate carbon atoms, making it softer and more ductile. As the temperature increases, specifically around 912 degrees Celsius, iron transforms into gamma iron, which has a face-centered cubic (FCC) structure. This allotrope can dissolve more carbon and has different magnetic properties and hardness compared to alpha iron.

At even higher temperatures, above about 1394 degrees Celsius, gamma iron reverts back to a different structure, known as delta iron, which again has a body-centered cubic configuration. These transformations illustrate the concept of allotropy clearly as they are reversible with changes in temperature, showcasing how iron's physical and chemical properties can significantly change with temperature variations. This characteristic of iron is crucial in metallurgy and material science as it affects processing and application.