A Unique Two‐Dimensional Silver(II) Antiferromagnet Cu[Ag(SO₄)₂] and Perspectives for Its Further Modifications

Copper(II) silver(II) sulfate crystallizes in a monoclinic CuSO 4 ‐related structure with P 2 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 2 p 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+ .