PHONON SOFTENING IN THE FAR INFRARED-SPECTRA OF YBA2(CU1-XNIX)3O7

The reflectivity of YBa2(Cu1-xNix)3O7 ceramics was measured as a function of temperature and the Ni concentration (x = 0.01, 0.03 and 0.05). The obtained spectra are presented in the range 260-340 cm-1 which contains the two out-of-plane modes of the CuO2 plane showing the highest phonon softening. The modes at 280 and 310 cm-1 soften around 5-6 and 3 cm-1, respectively, upon cooling of the samples from 300 to 10 K. The softening is almost independent of the Ni content and equivalent to the effect observed in undoped YBa2Cu3O7. Contrary to Zn doping, these results show that a partial substitution of copper with magnetic Ni ion does not affect much the phonon softening and superconductivity in the YBaCuO system.