Maximum entropy method analysis of thermal motion and disorder in thermoelectric clathrate Ba8Ga16Si30

Multitemperature (15, 100, 150, 200, 300, 450, 600, 900 K) single crystal neutron diffraction data on the type I clathrate Ba8Ga16Si30 are reported. For the framework atoms reciprocal space structural refinements give total occupancies in the unit cell of Ga/Si=3.8/2.2, 1.8/14.2, 10.2/13.8 for the 6c, 16i, and 24k sites respectively, thus showing that Ga avoids the tetrahedral 16i positions. The guest atom displacement parameters obtained from structure factor fitting are analyzed with semianharmonic Einstein models giving Einstein temperatures (Theta(E)) of 69(1), 98(7), and 124(2) K for Ba(2)((100)), Ba(2)([100]), and Ba(1), respectively. The analysis furthermore suggests that all guest atoms are structurally disordered, and the disorder appears to be temperature dependent with increased host-guest interaction at high temperatures. The structure factors are subsequently used in the maximum entropy method calculations to obtain direct space nuclear densities. These are modeled with anharmonic one-particle potential models to fourth order. Even at elevated temperatures anharmonicity is limited indicating that the low thermal conductivity of the clathrate has a different origin. (C) 2002 American Institute of Physics.