  Ultrasonic velocity measurements on the magnetoelectric multiferroic compound
CuFeO2 reveal that the antiferromagnetic transition observed at TN1 = 14 K
might be induced by an R-3m -> C2/m pseudoproper ferroelastic transition (G.
Quirion, M. J. Tagore, M. L. Plumer, O. A. Petrenko, Phys. Rev. B 77, 094111
(2008)). In that case, the group theory states that the order parameter
associated with the structural transition must belong to a two dimensional
irreducible representation Eg (x^2 - y^2, xy). Since this type of transition
can be driven by a Raman Eg mode, we performed Raman scattering measurements on
CuFeO2 between 5 K and 290 K. Considering that the isostructural multiferroic
compound CuCrO2 might show similar structural deformations at the
antiferromagnetic transition TN1 = 24.3 K, Raman measurements have also been
performed for comparison. At ambient temperature, the Raman modes in CuFeO2 are
observed at omega_Eg = 352 cm^-1 and omega_Ag = 692 cm^-1, while these modes
are detected at omega_E_g = 457 cm^-1 and omega_Ag = 709 cm^-1 in CuCrO2. The
analysis of the temperature dependence of modes shows that the frequency of all
modes increases down to 5 K. This typical behavior can be attributed to
anharmonic phonon-phonon interactions. These results clearly indicate that none
of the Raman active modes observed in CuFeO2 and CuCrO2 drive the pseudoproper
ferroelastic transition observed at the N\'eel temperature TN1. Finally, a
broad band at about 550 cm^-1 observed in the magnetoelectric phase of CuCrO2
below TN2 could be attributed to a magnon mode.
