Vibrational analysis of tetragonal La2−xMxCuO4 (M=Sr,Ba)

Abstract

A simple valence-bond-type force field has been used to fit the observed Raman and infrared-active zone-center phonons of tetragonal ${\mathrm{La}}_{2\mathrm{-}\mathrm{x}}$ ${\mathrm{M}}_{\mathrm{x}}$ ${\mathrm{CuO}}_4$ (M=Sr, Ba). The phonon frequencies and assignments used in the fitting procedure were culled from a judicious comparison of existing literature data. An excellent fit to the four Raman-active k=0 phonons and two (observed) of the seven (expected) infrared-active phonons was obtained using three adjustable parameters (one for each of the discrete near-neighbor bond types, Cu-O, La-O, and Cu-La) and a bond-scaling relationship which correlates the strength of a particular bond type, $K_{\mathrm{AB}}^i$ with the length of the bond $r_i$, i.e., $K_{\mathrm{AB}}^i$=$C_{\mathrm{AB}}$/$r_i^6$, where $C_{\mathrm{AB}}$ is a constant. Optimization of the fitting parameters in this approximate force field produced an interestingly large Cu-La interaction, which is taken as evidence for a sizeable attractive force between each lanthanum atom and its nearest copper-oxygen plane. Also, the potential energy distribution indicates that the modes above 250 ${\mathrm{cm}}^{\mathrm{-}1}$ are a mixture of Cu-O and La-O stretch, while those below 250 ${\mathrm{cm}}^{\mathrm{-}1}$ contain large contributions from the Cu-La interactions.