Also called "isotype". The phenomenon that different crystals have different chemical compositions, but have the same chemical formula and the same crystal structure (no matter whether the bond is the same or not).
For example, halite (NaCl), potash (KCl), galena (PbS), and periclase (MgO) are all AX type compounds, which belong to the Fm3m space group. Anions are cubic and most closely packed. Cations occupy all octahedral spaces, and have the same crystal structure, so they are all isomorphic.
