According to the Rietveld refinement, Ca₂CrNbO₆ and Sr₂CrNbO₆ have been found to be monoclinic with a= 5.4195 (1)Å, b= 5.4907 (1)Å, c= 7.7073 (2)Å, and cubic with a= 7.8732 (1)Å(Fm3m, No. 225), respectively. Under atmospheric pressure, Ba₂CrNbO₆ crystallizes into an 8H hexagonal structure with a= 5.7404 (1)Å and c= 18.7744 (5)Å(P6₃/mmc, No. 196), but at high pressure (80 kbar) and high temperature (930 °C), it undergoes a structural phase transformation to the cubic perovskite with a= 8.0550 (4)Å(Fm3m). Magnetic susceptibility measurements show that Ca₂CrNbO₆, Sr₂CrNbO₆ and Ba₂CrNbO₆ with cubic symmetry follow the Curie–Weiss law, while the magnetic behaviour of Ba₂CrNbO₆ with hexagonal symmetry is significantly different from the others. Magnetic calculations based upon several models indicate the presence of strong (Cr—Cr) covalent bonding through face sharing of the octahedra in the hexagonal phase. The variation of crystal structures and magnetic properties in these compounds can be rationalized in terms of cationic size and the nature of the covalent bonding between metal and oxygen ions.