Raman scattering study of (La_1-xSr_x)₂NiO₄

Phonon Raman spectra of polycrystalline (La_1-xSr_x)₂NiO₄ crystals have been measured at room temperature and at liquid nitrogen temperature for x=0, 0.1, 0.2, 0.25, 0.3, 0.35, 0.4, 0.5. We have observed two Raman active phonons at about 450cm^-1 and at about 240cm^-1. We assign the mode at 450cm^-1 to NiO stretching along c-axis (A_lg) and that at 240cm^-1 to optical oxygen along a- or b-axis (E_g). The frequencies of the A_lg mode does not show a distinct Sr dependence. While the energy of the E_g mode strongly depends on x; with increasing x, the energy of the E_g mode decreases for x<0.3 and slightly increases for x>0.3. We have found that the x value of the minimum energy corresponds to the characteristic concentration, where the electronic conduction changes from semiconductive to metallic. We suggest that the E_g mode couples with the electronic state of (La_l-xSr_x)₂NiO₄.