Lattice dynamics of La 2 NiO 4

摘要: The lattice dynamics of ${\mathrm{La}}_{2}$${\mathrm{NiO}}_{4}$, which is isostructural to ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$, was investigated by inelastic neutron scattering. All phonon branches were determined in the main symmetry directions [100], [110], and [001]. In addition, density states measured directly on a polycrystalline sample. results are analyzed basis rigid-ion model with anisotropic screening Coulomb forces account for two-dimensional character electronic properties. found lower frequency soft mode at X point. This has same eigenvector as i.e., it tilt octahedra around [110] axis. A negative planar breathing deformability Ni atom necessary reproduce pronounced anomalies involving in-plane Ni-O stretching vibrations. comparison infrared data yielded very good agreement modes polarized within basal plane, whereas serious disagreement along c ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ shows that ${\mathrm{La}}_{2}$${\mathrm{NiO}}_{4}$ not different.