Zintl behavior and vacancy formation in type-I Ba-Al-Ge clathrates
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In a study of type-I Ba8AlxGe46−x clathrates for 4≤x≤16, several samples with different stoichiometric compositions were prepared and measured. Here we report wavelength dispersion spectroscopy, powder x-ray diffraction and resistivity measurements, coupled with first-principles calculations. The lattice constant and vacancy count showed linear behavior for low Al content, extrapolating to the values for Ba8Ge43 as x→0, with the x=4 sample exhibiting a superstructure identical to that of Ba8Ge43. In ab initio calculations we identified the most stable configurations and examined energy differences for departures from Zintl behavior. We found the energy barrier for off-Zintl compositions to be relatively low and attributed this to the presence of a pseudogap near the Fermi energy.