Thermal lattice expansion of key thermoelectrics as input for the calculation of phonon anharmonic effects using the stochastic temperature-

Ab-initio phonon calculations rely on the harmonic approximation to estimate atomic forces and phonon dispersion relations at low temperatures, but this approximation fails in the case of thermoelectric materials. These discrepancies are due to insufficient incorporation of temperature effects, such as lattice expansion, anharmonicity. These parameters are temperature-dependent and it is necessary to benchmark anharmonic ab-initio calculations that include lattice expansion to crystallographically simple systems, as we are currently pursuing with SrTiO3 (STO), Mg2Si and GeBi2Te4 (GBT). Our initial results from neutron scatterings show that the calculations using sTDEP method are not too far from the experimental results, but that fine tuning is necessary. This can be achieved by including in the calculations the experimental thermal lattice expansion parameters extracted from single crystal XRD for a large temperature range, namely 100 K to 600 K.