Year: 2009
Type: article
Abstract: We describe $^{27}\text{A}\text{l}$ NMR experiments on ${\text{Ba}}_{8}{\text{Al}}_{x}{\text{Ge}}_{46\ensuremath{-}x}$ type-I clathrates coupled with ab initio computational studies. For $x=16$, calculated spectra determined by the ab initio results gave good agreement with the measurements, with best-fitting configurations also corresponding to the computed lowest-energy atomic arrangements. Analysis of the NMR results showed that a distribution of Knight shifts dominates the central portion of the line. Computational results demonstrate that this stems from the large variation of carrier density on different sites. Al-deficient samples with $x=12$ and 13 exhibited a split central NMR peak, signaling two main local environments for Al ions, which we connected to the presence of vacancies. Modeling of the wide-line spectrum for $x=12$ indicates a configuration with more Al on the $24k$ site than for $x=16$. The results indicate the importance of nonbonding hybrids adjacent to the vacancies in the electronic structure near ${E}_{F}$. We also address the static distortions from $Pm\overline{3}n$ symmetry in these structures.
Source: Physical Review B
Institutions: Texas A&M University, University of Puerto Rico-Mayaguez
Language: English
Subfield: Materials Chemistry
Field: Materials Science
Domain: Physical Sciences
Open Access status: bronze
OpenAlex ID: W1972377219