Sodium fluoride (NaF) and magnesium oxide (MgO) are not isomorphous compounds. They have different crystal structures and are not able to form solid solutions with each other.

Isomorphism is the ability of two or more compounds to crystallize in the same crystal structure. For this to happen, the compounds must have similar chemical formulas and ionic radii. NaF has a cubic crystal structure, while MgO has a cubic close-packed crystal structure. The ionic radii of Na+ and F- ions are 0.95 Å and 1.36 Å, respectively, while the ionic radii of Mg2+ and O2- ions are 0.66 Å and 1.40 Å, respectively. The difference in ionic radii between Na+ and F- ions is smaller than the difference in ionic radii between Mg2+ and O2- ions. This means that NaF is more likely to form a solid solution with a compound that has a cubic crystal structure and similar ionic radii, such as potassium chloride (KCl).

In contrast, MgO is more likely to form a solid solution with a compound that has a cubic close-packed crystal structure and similar ionic radii, such as calcium oxide (CaO).