Atomic displacement parameters: a useful tool in the search for new thermoelectric materials?

The atomic displacement parameters (ADPs) measure the mean-square displacement amplitude of an atom about its equilibrium position in a crystal. It is demonstrated that the ADPs can be used to identify crystalline solids with unusually low lattice thermal conductivities. A low lattice thermal conductivity is essential in the design of thermoelectric materials with improved efficiencies. The atomic displacement parameters (ADPs) have been measured using powder neutron diffraction as a function of temperature for several clathrate-like compounds (R_xCo_4-yFe_ySb₁₂, where R = La, Ce, Yb or Tl, x=0.22, 0.8, 1, y=0, 1 ;Tl₂SnTe₅ and Tl₂GeTe₅). The ADP data show that in each of the compounds one of the atoms is weakly bound and 'rattles' within its atomic cage. This atomic 'rattling' severely reduces the ability of these crystals to conduct heat and in some cases the lattice thermal conductivity approaches the theoretical minimum value. In many clathrate-like compounds, the ADP can also be used to estimate the Einstein frequency of the 'rattler', and to predict the existence of localized vibrational modes.