NMR experiments and electronic structure calculations in type-I BaAlGe clathrates

TL;DR

This study combines NMR experiments with ab initio calculations to analyze the local electronic and atomic structures of BaAlGe type-I clathrates, revealing the effects of composition and vacancies on their properties.

Contribution

It provides a detailed correlation between NMR spectra and atomic configurations, highlighting the role of vacancies and local environments in electronic structure.

Findings

Good agreement between experimental and calculated spectra for x=16.

Presence of vacancies causes split NMR peaks for x=12 and 13.

Variation in carrier density explains Knight shift distribution.

Abstract

We describe 27Al NMR experiments on Ba8AlxGe46-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 EF. We also address the static distortions from Pm-3n symmetry in these structures.