Effect of Ca doping on the optical properties of La _1.2Sr _1.8Mn ₂O ₇

We present x-ray-diffraction, infrared reflectivity, and Raman-scattering measurements of La1.2 (Sr1.8-x Cax) Mn2 O7 as a function of temperature and doping (x=0.0, 0.4, 0.6, and 0.8). At room temperature, x-ray-diffraction data show that the replacement of Sr ions with smaller Ca ions causes a shrinkage of the unit-cell volume. The far-infrared conductivity spectra of all doped samples are typical of an insulator, showing only phonons. When doped with Ca on Sr and a corresponding systematic reduction in Curie temperature (Tc), there is a suppression of the low-frequency optical spectral weight. For the x=0.0 compound, the Raman-active phonon associated with the internal stretching of the oxygen atoms in Mn O6 octahedra shows a noticeable hardening below Tc. A polaron transport can be attributed to the observed frequency shifts. Interestingly, this hardening is reduced as the Ca content increases. These observations suggest that Ca doping disrupts the effects of the double-exchange interaction on the lattice degree of freedom, thereby accounting for the characteristic changes in the phase diagram.