Depression in strength of the infrared active E_u mode with Ce-doping in Nd_2-χCe_χCuO₄

Infrared spectra measurements revealed that the highest energy E_u 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 Nd_2-χCe_χCuO₄ 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 E_u 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 E_u correlated well with the valence state of Cu on the basis of charge neutrality for Nd_2-χCe_χCuO₄ with different doping levels.