Energy Conversion Materials Group

Electronic structure (WIEN2k, VASP, Crystal14) and transport properties (BoltzTraP) calculations

As part of computer modeling, we perform calculations for pure and doped periodic systems. In the latter case, calculations are made using the n x n x n supercell approximation to simulate the appropriate dopant concentration in the system. These calculations are done using the programs WIEN2k, VASP or Crystal14 (head of the group has the respective licenses for these programs, and the programs are installed on group's computational servers as well as on Prometheus supercomputer available within PL-Grid infrastructure).

In the first step of the calculation, full structure relaxation (unit cell size and atomic positions) is performed to bring the system to the potential energy minimum (a necessary requirement for harmonic approximation, used to model many properties). Then, for a relaxed system, the densities of states, total (TDOS) and projected (PDOS) onto individual atoms and atomic orbitals are calculated, and a band structure is generated for the selected path in the reciprocal lattice, connecting special high symmetry points (k vectors). Examples of the results of such calculations are presented below.

The obtained energy dispersion relations are next used to assess transport properties of a given compound using BoltzTraP2 program.

Fig. 1 Band structures of Fd3m AgSbSe2 (a) and AgSbTe2 (b) structures with projected characters of silver (green color), antimony (blue color) and Se/Te (band thickness)

Fig. 2. (a) Seebeck coefficient S and (b) power factor S2σ scaled by relaxation time τ in relation to chemical potential μ calculated for selected structures (pristine and containing various defects) at 600 K.