We report inelastic light scattering experiments on CaFe_2As_2 in the temperature range of 4 to 290 K. In in-plane polarizations two Raman-active phonon modes are observed at 189 and 211 cm-1, displaying A_1g and B_1g symmetries, respectively. Upon heating through the tetragonal-to-orthorhombic transition at about T_S=173 K, the B_1g phonon undergoes a discontinuous drop of the frequency by 4 cm-1 whereas the A_1g phonon shows a suppression of the integrated intensity. Their linewidth increases strongly with increasing temperature and saturates above T_S. This suggests (i) a first-order structural phase transition and (ii) a drastic change of charge distribution within the FeAs plane through T_S.
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