"Isomorphous" when used in the relation to crystal structures is not synonymous: in addition to the same atomic connectivity that characterises isostructural compounds, isomorphous substances crystallise in the same space group and have the same unit cell dimensions.
[1] The IUCR definition[2] used by crystallographers is: Two crystals are said to be isostructural, if they have the same structure, but not necessarily the same cell dimensions nor the same chemical composition, and with a 'comparable' variability in the atomic coordinates to that of the cell dimensions and chemical composition.
For instance, calcite CaCO3, sodium nitrate NaNO3 and iron borate FeBO3 are isostructural.
The term isotypic is synonymous with isostructural.Examples include: Many minerals are isostructural when they differ only in the nature of a cation.
For example, methane, CH4, and the ammonium ion, NH4+, are isoelectric and are isostructural as both have a tetrahedral structure.