  Silver(II) compounds exhibit powerful oxidizing properties and strong
magnetic superexchange. AgSO4 is a rare fluorine-free salt of Ag(II) which
found some application in organic chemistry. Here, we report a discovery of a
new AgSO4 polymorph (\b{eta}). The distinct nature of the two polytypes of
AgSO4 is established using powder x-ray diffraction, vibrational spectroscopy
and theoretical calculations. The \b{eta} polymorph crystallizes in the
monoclinic system (P21/n) and shows structural similarities with CuSO4. DFT
calculations indicate very small differences in the energy of the two
polymorphs AgSO4, but the relative stability of the \b{eta} polymorph should
increase with temperature. The monoclinic distortion of the orthorhombic CuSO4
prototype originates from an unprecedented strong antiferromagnetic interaction
between Ag sites along the unit cell diagonal.
