Ab‐Initio Investigation of the Electronic Structure, Optical Properties, and Lattice Dynamics of β‐Ag 2 Te

Electronic, optical, and lattice vibrational properties of beta-Ag2Te are investigated, experimentally using spectroscopic ellipsometry and Raman spectroscopy, and theoretically using the density-functional perturbation theory (DFPT) for the first time. First-principles calculations of the electron spectra and dielectric functions are performed with the modified Becke-Johnson (mBJ) exchange potential, which solves the band-gap problem found with generalized gradient approximations (GGA). The calculated phonon spectra of beta-Ag2Te reveal that low-frequency optic modes overlap with acoustic modes and should scatter strongly with acoustic modes during heat transport. The possible Ag disorder and strong scattering of acoustic phonons by optic modes lead to an extremely low lattice thermal conductivity of beta-Ag2Te observed in the experiment. The phonon structure was also investigated experimentally using Raman spectroscopy. Three Raman-active modes were observed and identified at 92, 120.6, and 140.5 cm(-1). Symmetry assignments of all the vibrational modes were derived from considerations of point group symmetry.