Single chemical element, single phase (usually solid), but different 3D structures.
There are various possibilities for the axes, but some common ones:
- temperature (T) vs magnetic field strength (B)
- temperature (T) vs proportion of each chemical element of a binary alloy
- temperature (T) vs pressure
Sketch of the typical superconducting phase diagram of a Type-I superconductor
. Source. Sketch of the typical superconducting phase diagram of a Type-II superconductor
. Source.