What is a phase of matter? You mentioned secondary phases in some previous posts.

A phase is a state of matter. You may be familiar with solids (like ice), liquids (like water), and gases (like water vapor). You can also have different phases of solid. For example, in one phase, atoms may pack one way and in another, they may pack differently. For example,  zinc and sulfur atoms can arrange themselves in the wurtzite or zinc blende structures. These different phases look differently from different directions (have different symmetry).

Zincblende structure (Courtesy of Wikipedia)

Wurtzite structure (Courtesy of Wikipedia)

Some of these phases can be found in some of the films that we study. ZnS secondary phases likely play a significant role in limiting the amount of charge that can be collected in solar cells based on CZTS, one of the materials that we study. Secondary phases have real-world consequences. Check out this video to learn more about phases of matter.