The infrared reflectivity spectra of a single crystal of ${\mathrm{La}}_2$${\mathrm{NiO}}_4$ and their dependence on temperature and data analyses are reported. A plasmon mode is found in the basal plane, whereas no plasmon mode is found to propogate along the tetragonal axis. The two dimensional electronic properties of this crystal, which is isostructural with (${\mathrm{Ba}}_{\mathrm{x}}$${\mathrm{La}}_{1\mathrm{-}\mathrm{x}}$${)}_2$${\mathrm{CuO}}_4$, which shows high-$T_c$ superconductivity, are discussed.

• Received 2 March 1987

DOI:https://doi.org/10.1103/PhysRevB.35.7126