  We studied the optical properties of antiferromagnetic ZnCr2Se4 by infrared
spectroscopy up to 28,000 cm-1 and for temperatures from 5 to 295 K. At the
magnetic phase transition at 21 K, one of the four phonon modes reveals a clear
splitting of 3 cm-1 as a result of spin-phonon coupling, the other three
optical eigenmodes only show shifts of the eigenfrequencies. The
antiferromagnetic ordering and the concomitant splitting of the phonon mode can
be suppressed in a magnetic field of 7 T. At higher energies we observed a
broad excitation band which is dominated by a two-peak-structure at about
18,000 cm-1 and 22,000 cm-1, respectively. These energies are in good agreement
with the expected spin-allowed crystal-field transition of the Cr3+ ions. The
unexpected strength of these forbidden onsite d-d transitions is attributed to
a considerable hybridization of the selenium p with the chromium d orbitals.
