Six inequivalent Cu2+ EPR spectra were observed at 4.2 K in single crystals of FeSiF 6 • 6H20.
The estimated parametersgz ---2.38 and 8=40°, where 8 is the angle between the ionicz axis and
the c axis, differ from those measured in crystals of similar structure. Thes~ differences have been
explained in terms of an isotropic Cu2+ -Fe2+ exchange Hamiltonian JS1 • S2, with
J = + (0.030 ± 0.003) em - 1
' which gives a contribution gex = - 5.05 J sin2 e, where e is the
angle between the external magnetic field and the z axis. Perpendicular to the c axis, an
independent estimate of + 0.034 em -I for J was made from the low-field displacement of a
satellite spectru
