Chemical trends of the rattling phonon modes in alloyed germanium clathrates

Alloys based on Ge clathrates are promising thermoelectric materials becaus e of their expected "rattling" properties. We have incorporated the element s of columns I and II into the cages of (Ge, Ga)(46) type-I clathrates as c ation guests and have theoretically examined their "rattling" behavior usin g density functional theory. The potential energy curves of guest atoms in the cages are evaluated to understand the nature of the weak guest-framewor k interaction. Some atoms are unstable at the center of the cages, while ot hers appear to be bonded by weak restoring forces. We calculate the phonon modes and the Raman spectra and find that heavy alkali-earth elements, such as Sr and Ba, induce low-frequency "rattling" phonon modes as predicted by Slack's model, while heavy alkali metal atoms (K, Rb, and Cs) are less "ra ttler-like" since they interact less with the acoustic modes of the Ge-base d clathrate framework. (C) 2000 American Institute of Physics. [S0021-8979( 00)06311-8].