CoachDifferent crystal structures of the same element are its allotropes. Iron is BCC (-iron) at room temperature; it has been measured FCC (-iron) from to , and BCC again (-iron) at , up to its melting point. Carbon exists as diamond and as graphite (Chapter 6).
Examples
Example 5.21 (Steel)
Carbon, of covalent radius (half the distance in diamond), is too large for the sites of iron without distortion, but much less so in -iron, whose octahedral sites are the largest. Steel is made by dissolving carbon in hot -iron; quenched, the carbon is trapped as the iron returns to a body-centred structure, which it distorts, and the distorted crystal is hard.