Evolution of the 251 cm^-1 infrared phonon mode with temperature in Ba_0.6K_0.4Fe₂As₂

The far-infrared optical reflectivity of optimally doped Ba_1-xK_xFe₂As₂ (x = 0.4) single crystal is measured from room temperature down to 4 K. We study the temperature dependence of the in-plane infrared-active phonon at 251 cm^-1. This phonon exhibits a symmetric line shape in the optical conductivity, suggesting that the coupling between the phonon and the electronic background is weak. Upon cooling down, the frequency of this phonon continuously increases, following the conventional temperature dependence expected in the absence of a structural or magnetic transition. The intensity of this phonon is temperature independent within the measurement accuracy. These observations indicate that the structural and magnetic phase transition might be completely suppressed by the chemical doping in the optimally doped Ba_0.6K_0.4Fe₂As₂ compound.