Schwarzmüller, S.; Hoelzel, M.; Fritsch, K.; Evenson, Z.; Habicht, K.; Oeckler, O.: Lithium atom mobility in lithium germanium antimony tellurides elucidated by neutron diffraction and quasielastic neutron scattering. Journal of Alloys and Compounds 827 (2020), p. 154346/1-8
10.1016/j.jallcom.2020.154346

Abstract:
Lithium germanium antimony tellurides are known as promising thermoelectric materials. Earlier 7Li solid state NMR studies had revealed pronounced lithium atom mobility. Further insights into the diffusion mechanism are investigated by means of neutron powder diffraction in combination with quasielastic neutron scattering (QENS). In vacancy-containing LiGe3.5Sb2Te7, the isotropic displacement parameters of the cations are larger than in vacancy-free Li2Ge3Sb2Te7 and thus indicate more pronounced atom mobility in the investigated temperature range from room temperature to 600 °C. In contrast, temperature-dependent isotropic displacement parameters of the anions are similar for both solid solutions. Comparable to the situation in the important thermoelectric material PbTe, anharmonic displacement parameters for LiGe3.5Sb2Te7 e a lithium germanium antimony telluride with optimized thermoelectric properties e are significant already at room temperature. The extrapolation of displacement parameters toward low temperatures indicates that the displacements are predominantly dynamic. The temperature-dependent evaluation of lattice parameters and isotropic displacement parameters reveals three different regimes. From room temperature to 250 °C, a NaCl-type model with anharmonic displacement parameters for cation sites fits well. Between 250 °C and 400 °C, the unit cell volume increases less than in the other regimes and small lattice distortions may occur. At temperatures higher than 400 °C (investigated up to 600 °C), lithium atoms also occupy tetrahedral voids of the fcc Te substructure. Whereas QENS data probing the picosecond time scale show no significant lithium atom mobility in the Q range of 0.08 - 1.68 Å-1, a temperature-dependent change in the elastic scattering term hints at a different time window or a combination of motions with different time scales.