Ba8CuxGe46-x is a type-I clathrate material that forms as a semiconductor in a
narrow composition range corresponding to the electron-balanced Zintl
composition, with x = 5.3. We use NMR spectroscopy combined with ab initio
electronic structure calculations to probe the electronic and structural
behavior of these materials. Computational results based on a superstructure
model for the atomic configuration of the alloy provide good agreement with
the electric quadrupole-broadened NMR lineshapes. Modeling using the modified
Becke–Johnson (TB-mBJ) exchange potential is also shown to agree well with
experimental NMR Knight shifts. The results indicate that the Cu–Ge balance
is the main factor determining the carrier density, within a
narrow stability range near the ideal Zintl composition.
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