Infrared spectra and normal vibrations of (La,Nd)BaCaCu₃O₇

Abstract

The Fourier transform infrared spectra have been recorded for the series of (La,Nd)BaCaCu₃O_7-y at 300 and 85 K. These compounds are isostructural to YBa₂Cu₃O_7-y and belong to the space group Pmmm (D¹_2h) of orthorhombic crystal system with 39 lattice vibrations distributed among the various species of the group. The spectra of the entire series have been discussed and utilized to conclude the final vibrational analysis. A normal coordinate analysis has been done for LaBaCaCu₃O₇ by fitting the observed IR values at zero wavevector. The strong bands are assigned to A_g and B_1u which are vibrations along the c direction. The bands which are assigned to B_2g, B_3g, B_2u and B_3u vibrations in the ab plane are weak because of the screening effect from free charge carriers in the plane. The computation results in the values of force constants and eigenvectors giving complete description of the lattice vibrations of (La,Nd)BaCaCu₃O₇.