polymorphism and allotropy
The same set of LEGO bricks can build a car or a house — identical pieces, different arrangement. In the same spirit, the same atoms can lock into more than one crystal structure, and which one forms depends on temperature and pressure. That is the idea behind polymorphism and allotropy.
The two words are the same idea at different scope. Polymorphism means one compound existing in several crystal structures; allotropy is that same phenomenon for a pure element. Iron is BCC (ferrite) below 912 degrees C, FCC (austenite) from 912 to 1394, and BCC again above that. Carbon is even more dramatic: soft grey graphite (slippery stacked sheets) versus hard clear diamond (a rigid tetrahedral network). Tin famously crumbles from shiny white to grey powder in the cold (tin pest).
This matters enormously in engineering. Iron's allotropic switch is the linchpin of steel: heating into the FCC form lets carbon dissolve, and rapid cooling then traps it, hardening the metal — essentially all heat treatment rides on this one fact. And graphite versus diamond drives home the deeper lesson that structure, not just composition, can dominate a material's properties.
Carbon shows allotropy at its most dramatic: graphite is soft, grey, and slippery (sheets that shear apart, used as a lubricant and pencil lead), while diamond is the hardest natural material (a rigid 3D tetrahedral network). Same element — only the structure differs.
Graphite versus diamond: identical atoms, opposite properties.
Same atoms, different structure, very different behaviour — composition alone does not fix a material's properties. Allotropy is simply polymorphism restricted to pure elements.