4.2: Isomorphous Crystals
Two crystals are said to be isomorphous if ( a ) both have the same space group and unit-cell dimensions and ( b ) the types and the positions of atoms in both are the same except for a replacement of one or more atoms in one structure with different types of atoms in the other ( isomorphous replacement ), such as heavy atoms, or the presence of one or more additional atoms in one of them ( isomorphous addition ). Isomorphous crystals can form solid solutions .