Infrared spectroscopic study of the local structural changes across the metal insulator transition in nickel-doped GdBaCo2O5.5

Phonons in GdBaCo₂O_5.5 have been identified using infrared spectroscopy and their mode assignments have been carried out using ab initio lattice dynamical calculations. Metal insulator transitions in undoped and nickel-doped GdBaCo₂O_5.5 have been probed using infrared absorption spectroscopy. The phonon modes corresponding to the bending mode of the CoO₆ octahedra/pyramids are seen to soften, broaden and develop an asymmetry across the insulator-metal transition pointing to extensive electron phonon interaction effects in these systems. Correlated changes of the phonon line shape parameters associated with the transition indicate a suppression of T_MIT with increased nickel doping of the cobalt sublattice. Temperature dependence of the octahedral stretching mode frequencies in undoped GdBaCo₂O_5.5 points to distinct structural distortions accompanying the high temperature metallic transition.