  Synthetic powder, ore samples and mineral single crystals of goethite
($\alpha-$FeOOH) were investigated with polarized Raman spectroscopy at
temperatures from 300 K to 473 K. The symmetry of the vibrational modes,
observed in different scattering configurations, was determined unequivocally.
The assignment of the Raman-active modes to definite atomic vibrations is
supported by two types of lattice-dynamical calculations: empirical shell-model
and $\textit{ab initio}$ DFT. The temperature dependencies of the lineshape
parameters of some Raman-active vibrations near to the
antiferromagnetic-paramagnetic phase transition infers for a significant
spin-lattice coupling in this compound. The most informative in this aspect is
the $B_{3g}$ phonon at 387 cm$^{-1}$, which overlays a broad scattering
background and displays a pronounced asymmetric Fano-lineshape. The asymmetry
increases in the paramagnetic state above the Neel temperature ($T_{\text{N}}$
= 393 K) indicating a strong interaction of this mode with the underlying
excitation continuum. The origin of the excitation background is discussed in
light of our experimental results and the existing data for the magnetic
structure and transport properties of $\alpha-$FeOOH. We rationalize that, most
probably, the background stems from magnetic excitations, and the asymmetric
shape of the $B_{3g}$ phonon is a result of a linear spin-phonon coupling of
this mode with the Fe-O1-Fe spin dimers. Another mechanism, a phonon
interaction with thermally activated charge carriers above above
$T_{\text{N}}$, is also considered.

Materials Science;Other Condensed Matter