Optical phonon spectrum of the Ge$_2$Sb$_2$Te$_5$ single crystal

Raman scattering by optical phonons of the Ge$_2$Sb$_2$Te$_5$ single crystal grown by the Bridgman method is studied for the first time. Another method for obtaining this compound based on annealing of the amorphous state and used in numerous previous studies leads to the formation of various defects which significantly affect the optical phonons spectrum. This hampers the identification of observed phonon spectra. The use of the bulk Ge$_2$Sb$_2$Te$_5$ single crystal has made it possible to exclude the effect of lattice defects and to reveal new features of the phonon spectrum associated with the lattice structure. Broad features typical of the amorphous state and narrow phonon lines of the crystal state are observed in the spectra. It has been shown that the former component is due to disorder appearing because of the mixing of Ge and Sb atoms in corresponding layers of the lattice. Moreover, there are optical phonons whose frequencies are independent of this mixing in view of the specificity of their eigenvectors. These modes are manifested in spectra as intense narrow lines.