Infrared phonon spectra of 3d-metal doped YBa2(Cu1-x M x )3O7-?

Abstract

Infrared (IR) absorption spectra of doped YBa2(Cu1−xMx)3O7−δ, with M representing Co, Fe, Ni, and Zn were observed as a function of concentrationx. TheB1u IR mode at ∼ 580 cm−1 was observed for all samples which were in the orthorhombic phase, which includes all the Ni and Zn doped samples and the lower concentrations (x 2.5%) shows three new IR modes at ∼ 655 cm−1, ∼ 340 cm−1, and ∼ 175 cm−1. The frequency of the B1u mode at ∼ 580 cm−1 remains unchanged for all the Ni and Zn concentrations. In contrast, this mode shows a strong monotonic frequency softening with increase in Co substitution, while in the Fe-system it shows a comparable monotonic mode softening for 3%≧x%≧0%, and then remains unchanged for 12%≧x%≧3%. These observations indicate that Fe and Co substitute on the Cu(1) sites, whereas Ni and Zn most likely substitute on Cu(2) sites. We attribute the difference in the mode softening for Co and Fe to different Cu(1) local environments for these two elements. Indication for a chain-plane charge transfer based on our results is also discussed.