Laves polyhedra in synthetic tennantite, Cu12As4S13, and its lattice dynamics

Synthetic tennantite, Cu12As4S13, is the analogue of an abundant mineral that belongs to the tennantite-tetrahedrite group with a low lattice thermal conductivity. Combined data from high-quality X-ray diffraction, electron microscopy (STEM-HAADF), Raman spectroscopy, as well as DFT calculations are used to analyze the peculiarities of its structure stability and dynamics. Atomic displacement parameters (ADP) and low-energy optical phonon modes are discussed within the context of the structure variations and pecularities of the charge distribution. The latter indicates that the tennantite structure tends to conform to the covalent polar bonds and its charge distribution is significantly affected by the atomic shifts in Laves polyhedra. According to the DFT calculations, there is a population of stable model structures with varying shifts of copper atoms in Laves polyhedra, with total energies being very close (within 0.24 eV), which can explain the observed behavior of the ADP. A specially designed technique is used for the experimental analysis of the ADP that revealed Einstein characteristic temperatures in the tennantite structure to be in the range of 50-190 K, which is attributed to low-energy optical phonon modes.