Phonon spectra of La-containing Bi₂Sr₂CaCu₂O₈-based solid solutions measured by inelastic neutron scattering

Abstract

Phonon spectra of Bi₂Sr_2−y Ca_1−x La _{x + y} Cu₂O_{8 + δ} solid solutions (x=y=0; x=0, y=0.25; x=0.25, y=0; x=0, y=0.5) were measured by inelastic neutron scattering on a DIN-2PI direct-geometry spectrometer. A full-profile analysis of the x-ray diffraction data yielded an estimate of the distribution of alkaline-earth (AE) and La atoms over the Ca and Sr positions in the Bi₂Sr₂CaCu₂O₈ structure. An assignment of the main features in the phonon spectrum to dominant vibrations of certain atoms was made. In particular, the correlation of the x-ray structural data with spectroscopic data implies that the feature due to cation vibrations in the Sr crystallographic positions of the basic 2212 structure (near 11 meV) is single-mode in behavior. Substitution of La for an AE atom modifies the high-frequency part (>40 meV) of the phonon spectrum (which corresponds to vibrations of oxygen in the SrO and CuO₂ layers of the Bi₂Sr₂CaCu₂O₈ structure) and affects the cut-off frequencies in the spectra of solid solutions of various compositions. A comparison of the results obtained in this study with earlier spectra of the 2212 compound, in which Ca was substituted for by Nd, indicates that the shape and intensity of the high-frequency vibration spectra of the samples studied are related to the average cation charge in the SrO layer of the Bi₂Sr₂CaCu₂O₈ structure.