Effect of temperature and doping with Cu on the reflectance spectra of PbSb$_{2}$Te$_{4}$ crystals

For the anisotropic PbSb$_{2}$Te$_{4}$ single crystal, the effect of temperature and doping with Cu on the reflectance spectrum $R(\nu)$ is studied for reflection from a cleavage plane orthogonal to the trigonal axis $C_3$. It is found that the spectrum is complex in structure and exhibits a sharp increase in the reflectance in the far-infrared region and two minima in the mid-infrared region. It is shown that the experimentally observed feature of the reflectance spectra can be quantitatively described in the context of the Drude–Lorentz theory, with the dielectric function that takes into account the contributions of hole plasma oscillations and two optical lattice vibrations at different frequencies. The static conductivity estimated from optical data is in good agreement with the results of electrical measurements of the dc conductivity.