Raman spectroscopy of alpha-FeOOH (goethite) near antiferromagnetic to paramagnetic phase transition.

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.