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

Abstract

We describe 27 Al NMR experiments on Ba 8 Al x Ge 46−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 Pm3n symmetry in these structures.

Zintl behavior and vacancy formation in type-I Ba-Al-Ge clathrates

“…Selected results for Ba 8 19 for those with lower x, E F falls above the gap as expected. The calculated results are close to a rigidband model with little change in the band structure while the Fermi level changes with the Al variation.…”

“…The lowest-energy configurations obtained are summarized in Table III, where the first column is the total number of Al per unit cell, the next three columns indicate the Al framework occupation on the 6c, 16i, and 24k parent sites, and the fifth column is the total calculated formation energy relative to the lowest energy. The occupation of three Al on the 6c site has been found to be the preferred value from computations as reported for Ba-Al-Ge, 19 Ba-Ga-Ge, 24 and Ba-Ga-Sn ͑Ref. 25͒ clathrates.…”

“…This is somewhat smaller than previously reported for Ba 8 Al 16 Ge 30 , 26 using a different functional. The 3-1-12 configuration was shown previously 19 to have the lowest energy specifically for Al 16 configurations, and also for Ba 8 Ga 16 Ge 30 ͑Ref. 24͒ and Ba 8 Ga 16 Sn 30 .…”

NMR and Computational Studies of Ba8Ga16Sn30 Clathrates

“…We retained the cubic cell symmetry, leaving 54 independent atoms (P1 space group), except for a few cases allowing higher symmetry. Half of these configurations were taken from previous work on Ba 8 Ga 16 Ge 30 [10] and Ba 8 Al x Ge 46-x [11] where the minimum energy configurations where found. Additional configurations were modeled after the proposal by Kozina et al [12] for Ba 8 Ga 16 Sn 30 .…”

Electronic, elastic, vibrational, and thermodynamic properties of type-VIII clathrates Ba8Ga16Sn30 and Ba8Al16Sn30 by first principles

“…It has been shown for these types of semiconducting clathrates that group III elements avoid bonding to each other due to the Coulomb repulsive interactions. 70,71 Guest-guest bonds are unlikely in these compounds because the average distance between the guests is $5.5 Å , which is almost five times larger than the ionic radius of Ba 1.35 Å .…”