  Magnon gap excitations selectively coupled to phonon modes have been studied
in FePSe$_3$ layered antiferromagnet with magneto-Raman scattering experiments
performed at different temperatures. The bare magnon excitation in this
material has been found to be split (by $\approx~1.2$ cm$^{-1}$) into two
components each being selectively coupled to one of the two degenerated, nearby
phonon modes. Lifting the degeneracy of the fundamental magnon mode points out
toward the biaxial character of the FePS$_3$ antiferromagnet, with an
additional in-plane anisotropy complementing much stronger, out-of-plane
anisotropy. Moreover, the tunability, with temperature, of the phonon- versus
the magnon-like character of the observed coupled modes has been demonstrated.
