LATTICE-DYNAMICS OF THE INFINITE-LAYERED COMPOUNDS

We obtain for the first time the phonon-dispersion relations of ''infi nite-layered'' compounds by means of a shell model. We start from pote ntial parameters and polarizabilities available from fits of phonon da ta in several high-T-c cuprates. With modification of only a few param eters we achieve a good fit of the infrared phonon data available for the narrow-band semiconductor Ca0.86Sr0.14CuO2. A virtual crystal appr oximation is made for the (Ca0.86Sr0.14) ion. Comparing the parameters to those of other cuprates, we observe a higher covalence degree of t he Cu-O bond in this material. By substituting the Ca related paramete rs by Nd ones we also study the changes in the phonon density of state s compared to that of the superconductor Sr0.86Nd0.14CuO2 (T-c similar or equal to 40 K). We find a strong tendency to instability of an aco ustic phonon branch around the zone boundary R point, which may be rel ated to the Sr concentration dependent tetragonal-orthorhombic phase t ransition in Ca1-xSrxCuO2.