Crystal structure, magnetism and phase transformation in perovskites A(2)CrNbO(6) (A=Ca,Sr,Ba)

According to the Rietveld refinement, Ca2CrNbO6 and Sr2CrNbO6 have been found to be monoclinic with a = 5.4195 (1) Angstrom, b = 5.4907 (1) Angstrom, c = 7.7073 (2) Angstrom, and cubic with a = 7.8732 (1) Angstrom (<Fm(3)over bar m>, No. 225), respectively. Under atmospheric pressure, Ba2CrNbO6 crystallizes into an 8H hexagonal structure with a = 5.7404 (1) Angstrom and c = 18.7744 (5) Angstrom (P6(3)/mmc, No. 196), but at high pressure (80 kbar) and high temperature (930 degrees C), it undergoes a structural phase transformation to the cubic perovskite with a = 8.0550 (4) Angstrom (<Fm(3)over bar m>). 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.