  Copper(II) silver(II) sulfate crystallizes in a monoclinic CuSO4-related
structure with P21/n symmetry. This quasi-ternary compound features
[Ag(SO4)2]2- layers, while the remaining cationic sites may be occupied either
completely or partially by Cu2+ cations, corresponding to the formula of
(CuxAg1-x)[Ag(SO4)2], x = 0.6-1.0. CuAg(SO4)2 is antiferromagnetic with large
negative Curie-Weiss temperature of -84.1 K and shows two characteristic
ordering phenomena at 19 K and 40 K. Density functional theory calculations
reveal that the strongest superexchange interaction is a two-dimensional
antiferromagnetic coupling within [Ag(SO4)2]2- layers, with the superexchange
constant J2D of -11.1 meV. This renders CuAg(SO4)2 the rare representative of
layered Ag2+-based antiferromagnets. Magnetic coupling is facilitated by the
strong mixing of Ag d(x2-y2) and O 2p states. Calculations show that M2+ sites
in MAg(SO4)2 can be occupied with other similar cations such as Zn2+, Cd2+,
Ni2+, Co2+, and Mg2+.
