When there are two solid phases at one temperature in a phase diagram, it indicates the existence of a phase transition or equilibrium between those two solid phases. In other words, at that specific temperature, both solid phases coexist and are in equilibrium with each other.
Phase diagrams are graphical representations that show the relationships between different phases of a substance (such as solid, liquid, and gas) under various combinations of temperature and pressure. Each phase of a substance is represented by a distinct region in the phase diagram.
When two solid phases coexist at a particular temperature, it implies that there are different crystal structures or arrangements of atoms/molecules that are stable under those conditions. These solid phases may have different densities, compositions, or other structural characteristics.
The boundary line separating the regions of the two solid phases in the phase diagram is known as a phase boundary or phase transition line. It indicates the temperature and pressure conditions at which the transition between the solid phases occurs. The specific nature of the transition (e.g., whether it's a polymorphic transition or an allotropy transition) depends on the substance being considered and its properties.
Overall, the presence of two solid phases at a specific temperature in a phase diagram signifies the coexistence and equilibrium between those phases under those thermodynamic conditions.