Er-166 Mossbauer and inelastic neutron scattering measurements were pe
rformed on the tetragonal intermetallic compound ErCu2Si2(T-N = 1.65(5
) K). From the inelastic neutron scattering results a crystal field di
agram has been determined. The Er-166 Mossbauer spectra show direct re
laxation between the split levels of the magnetic ground state doublet
of the I-4(15/2) multiplet.