AN INFRARED AND RAMAN-STUDY OF YBA2CU4O8 AND LA SUBSTITUTED YBA2CU4O8

The room temperature infrared and Raman spectra of ceramic YBa2Cu4O8 and La substituted YBa2Cu4O8 are presented and discussed. All the predicted B1u (infrared) and A(g) (Raman) modes are observed and assigned. When the spectra are compared with YBa2Cu3O7-delta, it is found that modes involving the CuO chain atoms are significantly altered, as may be expected given that the structure of these two compounds differs by the insertion of an extra CuO chain parallel to the b-axis. On the other hand, modes involving largely motion of the oxygens on the CuO2 planes and of Y are observed at similar frequencies in both compounds. With La3+ substitution for Ba2+, most of the B1u and A(g) modes harden with contraction of the c-axis. Modes that show softening trends in contrast to the generally observed hardening are the IR and Raman modes that involve largely motion of the CuO2 plane oxygens, which can be understood in terms of a decrease in the hole density on the planes with La3+ substitution.