Abstract
Lattice dynamical rigid-ion model calculations of phonon dispersion relation and density of states in La 2CuO 4 and its isostructural compound La 2NiO 4 are reported. The interatomic potential used in these calculations are obtained from structural information alone without fitting to dynamical data, unlike other empirical calculations reported for these systems. The symmetry properties of the tetragonal and the orthorhombic phases have been studied and the equivalence of wave vectors in the two phases is given. The computed phonon dispersion relation and density of states in these systems are in good agreement with experimental data. Our calculations reproduce the soft mode behavior observed experimentally in La 2CuO 4 for the ∑ 4 TO phonon at the zone boundary X point in the tetragonal phase; this mode is hardened in the orthorhombic phase. The partial densities of states have helped in understanding the anisotropy in the dynamical contributions to frequency distribution from various atoms.
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