Analysis of infrared reflection spectra of oxides of the ${\mathrm{La}}_2$Cu${\mathrm{O}}_4$ high-$T_c$ superconductor family in polarized light

The temperature dependence of infrared reflection spectra of ${\mathrm{La}}_2$Ni${\mathrm{O}}_4$ single crystal in polarized light between 10 and 750 K is analyzed and compared to the results for an oriented sample of ${\mathrm{La}}_2$Cu${\mathrm{O}}_4$. In both compounds, the spectra are found to reflect the bidimensional character of this family of compounds with a plasmon restricted to the basal $a-b$ plane. In the nickel compound, the plasma frequency increases with temperature according to a simple scheme of semiconducting behavior with thermal filling of an empty conduction band, consistent with the $d^8$ character of Ni. Conversely, in "metallic" ${\mathrm{La}}_2$Cu${\mathrm{O}}_4$ ($d^9$ character) the plasmon is found to shift little with temperature. The analysis of a number of results obtained with ceramics samples in superconducting and normal phases are reexamined under the light of these new data. The effective charges carried by the oxygen ions are also deduced from the splittings of polar modes into transverse and longitudinal components. Results confirm the strong anisotropy of the transition-metal-oxygen bonds with strong overlap in the basal plane related to the short bond lengths and much more ionic character along the tetragonal $c$ axis.