A Unique Two-Dimensional Silver(II) Antiferromagnet Cu[Ag(SO 4 ) 2 ] and Perspectives for Its Further Modifications.

Copper(II) silver(II) sulfate crystallizes in a monoclinic CuSO 4 -related structure with P2 1 /n symmetry. This quasi-ternary compound features Ag(SO 4 ) 2 2- layers, while the remaining cationic sites may be occupied either completely or partially by Cu 2+ cations, corresponding to the formula of (Cu x Ag 1-x )[Ag(SO 4 ) 2 ], x=0.6-1.0. CuAg(SO 4 ) 2 is antiferromagnetic with large negative Curie-Weiss temperature of -140 K and shows characteristic ordering phenomenon at 40.4 K. Density functional theory calculations reveal that the strongest superexchange interaction is a two-dimensional antiferromagnetic coupling within Ag(SO 4 ) 2 2- layers, with the superexchange constant J 2D of -11.1 meV. This renders CuAg(SO 4 ) 2 the rare representative of layered Ag 2+ -based antiferromagnets. Magnetic coupling is facilitated by the strong mixing of Ag d(x 2 -y 2 ) and O 2p states. Calculations show that M 2+ sites in MAg(SO 4 ) 2 can be occupied with other similar cations such as Zn 2+ , Cd 2+ , Ni 2+ , Co 2+ , and Mg 2+ .