Abstract
Infrared spectra measurements revealed that the highest energy Eu mode (~530 cm-1) assigned to the in-plane Cu-O stretching motion in the T′ structure, becomes weak in its strength with Ce-doping up to χ=0.2 in Nd2−χCeχCuO4 while its energy shifts downwards. It is concluded that the depression of the phonon's strength is associated with a conversion to the Cu1+ configuration (Cu2+ diminution) as the doping level increases, and that the presence of Cu1+ with an ionic radius larger than Cu2+ also accounts for the downward shift of the Eu mode affecting the Cu-O bond length as the lattice parameter “a” increases with increasing of the dopant. In fact, the strength as well as the energy of the Eu correlated well with the valence state of Cu on the basis of charge neutrality for Nd2−χCeχCuO4 with different doping levels.
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