Phonon densities of states of La2-xSrxNiO4: Evidence for strong electron-lattice coupling

The inelastic neutron-scattering spectra were measured for several Sr conce ntrations of polycrystalline La2-xSrxNiO4. We find that the generalized pho non density-of-states is the same for x = 0 and x = 1/8. For x = 1/3 and x = 1/2, the band of phonons corresponding to the in-plane oxygen vibrations (80-90 meV) splits into two subbands centered at 75 and 85 meV. The; lower frequency hand increases in amplitude for the x = 1/2 sample, indicating th at it is directly related to the hole concentration. These changes are asso ciated with the coupling of oxygen vibrations to doped holes which reside i n the NiO2 planes and are a signature of strong electron-lattice coupling. [S0163-1829(99)02126-8].