  The infrared phonon spectrum of the spinel CdCr2O4 is measured as a function
temperature from 6 K to 300K. The triply degenerate Cr phonons soften in the
paramagnetic phase as temperature is lowered below 100 K and then split into a
singlet and doublet in the low T antiferromagnetic phase which is tetragonally
distorted to relieve the geometric frustration in the pyrochlore lattice of
Cr$^{3+}$ ions. The phonon splitting is inconsistent with the simple increase
(decrease) in the force constants due to deceasing (increasing) bond lengths in
the tetragonal phase. Rather they correspond to changes in the force constants
due to the magnetic order in the antiferromagnetic state. The phonon splitting
in this system is opposite of that observed earlier in ZnCr2O4 as predicted by
theory. The magnitude of the splitting gives a measure of the spin phonon
coupling strength which is smaller than in the case of ZnCr2O4.
