Experimental study of Ba$_{8}$Al$_{x}$Ge$_{46-x}$ clathrates by NMR and other techniques

Edward Fry; Edward Fry; Edward Fry; Edward Fry; Joseph H. Ross, Jr.

Experimental study of Ba$_{8}$Al$_{x}$Ge$_{46-x}$ clathrates by NMR and other techniques

ORAL

Abstract

Cage-structured group-IV clathrates exhibit interesting properties, for example very high thermoelectric efficiency. We have investigated Ba$_{8}$Al$_{x}$Ge$_{46-x}$, which is a ternary semiconductor for the case $x$ = 16. $^{27}$Al NMR studies for $x$ = 16 show metallic behavior, indicating doping by native defects into the metallic regime. For smaller $x$ the average relaxation rate increases, indicating an increased density of carriers. We also observe vacancies in low-$x$ samples via electron microprobe studies. For $x$ = 12 and 13, NMR exhibits a second line not seen for $x$ = 16, attributed to Al adjacent to vacancies. Magic angle spinning NMR and computer simulation of the first-order quadrupole line shape for these samples were used to analyze the observed NMR lines. In addition, we have used ab initio calculations of the electric field gradients to match the observed quadrupole broadening with the local structure. Evidence indicates that Al adjacent to vacancies are predominantly on 24$k$ sites. These sites have significantly different local electronic structure and smaller density of conduction electrons.

Oct. 20, 2007, 10:52 AM – Oct. 20, 2007, 11:04 AM

Authors

Edward Fry

Institute for Astronomy, ETH, National Optical Astronomy Observatory, Department of Physics, ETH, Texas A&M University, LANL, American Physical Society, Society of Physics Students National Office, Texas Tech University, University of Texas, Universite d'Orleans, Steward Observatory, University of Arizona, Texas A\&M University-Commerce, Texas A\&M University, Institute for Quantum Electronics, ETH Zurich, Department of Engineering and Applied Science, Harvard University, Rice University, University of Houston, Bucknell University, Department of Physics, Texas A\&M University, Air Force Research Laboratory, Department of Physics, Texas A\&M University, College Station, TX 77843, Department of Physics and MIC, Texas A\&M University, University of Texas at Arlington, Changwon National University, University of North Texas, Southwestern University, The Pennsylvania State University, University of Dallas, UT Southwestern Medical Center, Naval Air Systems Command, Institute for Quantum Studies and Department of Physics, Texas A\&M University, College Station, TX 77843, The University of Texas at Arlington, TCU, ELTE, National Institute of Chemistry, Ljubljana, Slovenia, Texas Christian University, Fort Worth, TX, Dept. of Physics, Texas A\&M University-Commerce, Univ. of North Texas, Los Alamos National Laboratory, Oswego State University of New York, Lee College, University of Texas at San Antonio, UTSA-Professor, UNT, Sam Houston State University, Sewanee: The University of the South, Director of Education for NSF, Center for Nonlinear Dynamics and Department of Physics, The University of Texas at Austin, Sackler School of Chemistry, Tel-Aviv University, Rice University, Department of Physics and Astronomy, Houston, TX 77005, Center for Nonlinear Dynamics, University of Texas at Austin
Edward Fry

Institute for Astronomy, ETH, National Optical Astronomy Observatory, Department of Physics, ETH, Texas A&M University, LANL, American Physical Society, Society of Physics Students National Office, Texas Tech University, University of Texas, Universite d'Orleans, Steward Observatory, University of Arizona, Texas A\&M University-Commerce, Texas A\&M University, Institute for Quantum Electronics, ETH Zurich, Department of Engineering and Applied Science, Harvard University, Rice University, University of Houston, Bucknell University, Department of Physics, Texas A\&M University, Air Force Research Laboratory, Department of Physics, Texas A\&M University, College Station, TX 77843, Department of Physics and MIC, Texas A\&M University, University of Texas at Arlington, Changwon National University, University of North Texas, Southwestern University, The Pennsylvania State University, University of Dallas, UT Southwestern Medical Center, Naval Air Systems Command, Institute for Quantum Studies and Department of Physics, Texas A\&M University, College Station, TX 77843, The University of Texas at Arlington, TCU, ELTE, National Institute of Chemistry, Ljubljana, Slovenia, Texas Christian University, Fort Worth, TX, Dept. of Physics, Texas A\&M University-Commerce, Univ. of North Texas, Los Alamos National Laboratory, Oswego State University of New York, Lee College, University of Texas at San Antonio, UTSA-Professor, UNT, Sam Houston State University, Sewanee: The University of the South, Director of Education for NSF, Center for Nonlinear Dynamics and Department of Physics, The University of Texas at Austin, Sackler School of Chemistry, Tel-Aviv University, Rice University, Department of Physics and Astronomy, Houston, TX 77005, Center for Nonlinear Dynamics, University of Texas at Austin
Edward Fry

Institute for Astronomy, ETH, National Optical Astronomy Observatory, Department of Physics, ETH, Texas A&M University, LANL, American Physical Society, Society of Physics Students National Office, Texas Tech University, University of Texas, Universite d'Orleans, Steward Observatory, University of Arizona, Texas A\&M University-Commerce, Texas A\&M University, Institute for Quantum Electronics, ETH Zurich, Department of Engineering and Applied Science, Harvard University, Rice University, University of Houston, Bucknell University, Department of Physics, Texas A\&M University, Air Force Research Laboratory, Department of Physics, Texas A\&M University, College Station, TX 77843, Department of Physics and MIC, Texas A\&M University, University of Texas at Arlington, Changwon National University, University of North Texas, Southwestern University, The Pennsylvania State University, University of Dallas, UT Southwestern Medical Center, Naval Air Systems Command, Institute for Quantum Studies and Department of Physics, Texas A\&M University, College Station, TX 77843, The University of Texas at Arlington, TCU, ELTE, National Institute of Chemistry, Ljubljana, Slovenia, Texas Christian University, Fort Worth, TX, Dept. of Physics, Texas A\&M University-Commerce, Univ. of North Texas, Los Alamos National Laboratory, Oswego State University of New York, Lee College, University of Texas at San Antonio, UTSA-Professor, UNT, Sam Houston State University, Sewanee: The University of the South, Director of Education for NSF, Center for Nonlinear Dynamics and Department of Physics, The University of Texas at Austin, Sackler School of Chemistry, Tel-Aviv University, Rice University, Department of Physics and Astronomy, Houston, TX 77005, Center for Nonlinear Dynamics, University of Texas at Austin
Edward Fry

Institute for Astronomy, ETH, National Optical Astronomy Observatory, Department of Physics, ETH, Texas A&M University, LANL, American Physical Society, Society of Physics Students National Office, Texas Tech University, University of Texas, Universite d'Orleans, Steward Observatory, University of Arizona, Texas A\&M University-Commerce, Texas A\&M University, Institute for Quantum Electronics, ETH Zurich, Department of Engineering and Applied Science, Harvard University, Rice University, University of Houston, Bucknell University, Department of Physics, Texas A\&M University, Air Force Research Laboratory, Department of Physics, Texas A\&M University, College Station, TX 77843, Department of Physics and MIC, Texas A\&M University, University of Texas at Arlington, Changwon National University, University of North Texas, Southwestern University, The Pennsylvania State University, University of Dallas, UT Southwestern Medical Center, Naval Air Systems Command, Institute for Quantum Studies and Department of Physics, Texas A\&M University, College Station, TX 77843, The University of Texas at Arlington, TCU, ELTE, National Institute of Chemistry, Ljubljana, Slovenia, Texas Christian University, Fort Worth, TX, Dept. of Physics, Texas A\&M University-Commerce, Univ. of North Texas, Los Alamos National Laboratory, Oswego State University of New York, Lee College, University of Texas at San Antonio, UTSA-Professor, UNT, Sam Houston State University, Sewanee: The University of the South, Director of Education for NSF, Center for Nonlinear Dynamics and Department of Physics, The University of Texas at Austin, Sackler School of Chemistry, Tel-Aviv University, Rice University, Department of Physics and Astronomy, Houston, TX 77005, Center for Nonlinear Dynamics, University of Texas at Austin
Joseph H. Ross, Jr.

Department of Physics, Texas A\&M University, Texas A\&M University