Neutron Vibrational Spectroscopy and First-Principles Calculations of the Ternary Hydrides Li4Si2H(D) and Li4Ge2H(D): Electronic Structure and Lattice Dynamics

Using combined neutron spectroscopy and first-principles calculations, we investigated the electronic structure and vibrational dynamics of the recently discovered class of ternary hydrides Li4Tt2H (Tt=Si and Ge). In these compounds, all hydrogen atoms are located in a single type of Li6-defined octahedral site. The Tt atoms form long-range Tt-Tt chains sandwiched between each Li6-octahedra layer. The Li-H interactions are strongly ionic, with bond lengths comparable to those in LiH. Our density functional theory calculations indicate that Li atoms transfer their electrons to both H and Tt atoms. Tt atoms within the Tt-Tt chain are bonded covalently. The electronic density of states reveals that both hydrides exhibit metallic behavior. The observed vibrational spectra of these hydrides are in good overall agreement with the calculated phonon modes. There is evidence of dispersion induced splitting in the optical phonon peaks that can be ascribed to the coupling of H vibrations within the Li6-octahedra layers. Disciplines Physical Sciences and Mathematics | Physics Comments Suggested Citation: H. Wu, et al. (2007). "Neutron Vibrational Spectroscopy and First-Principles Calculations of the Ternary Hydrides Li4Si2H(D) and Li4Ge2H(D): Electronic Structure and Lattice Dynamics." Physical Review B. 76, 224301. © 2007 The American Physical Society http://dx.doi.org/10.1103/PhysRevB.76.224301 Author(s) Hui Wu, Wei Zhou, Terrence J. Udovic, John J. Rush, Tanner Yildirim, Michael R. Hartman, Robert C. Bowman Jr., and John J. Vajo This journal article is available at ScholarlyCommons: http://repository.upenn.edu/physics_papers/120 Neutron vibrational spectroscopy and first-principles calculations of the ternary hydrides Li4Si2H„D... and Li4Ge2H„D...: Electronic structure and lattice dynamics Hui Wu,1,2,* Wei Zhou,1,3 Terrence J. Udovic,1 John J. Rush,1,2 Taner Yildirim,1,3 Michael R. Hartman,4 Robert C. Bowman, Jr.,5 and John J. Vajo6 1NIST Center for Neutron Research, National Institute of Standards and Technology, 100 Bureau Drive, MS 6102, Gaithersburg, Maryland 20899-6102, USA 2Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742-2115, USA 3Department of Materials Science and Engineering, University of Pennsylvania, 3231 Walnut Street, Philadelphia, Pennsylvania 19104-6272, USA 4Department of Nuclear Engineering and Radiation Health Physics, Oregon State University, 116 Radiation Center, Corvallis, Oregon 97331-5903, USA 5Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109-8099, USA 6HRL Laboratories, LLC, Malibu, California 90265, USA Received 21 May 2007; revised manuscript received 7 August 2007; published 17 December 2007 Using combined neutron spectroscopy and first-principles calculations, we investigated the electronic structure and vibrational dynamics of the recently discovered class of ternary hydrides Li4Tt2H Tt=Si and Ge . In these compounds, all hydrogen atoms are located in a single type of Li6-defined octahedral site. The Tt atoms form long-range Tt-Tt chains sandwiched between each Li6-octahedra layer. The Li-H interactions are strongly ionic, with bond lengths comparable to those in LiH. Our density functional theory calculations indicate that Li atoms transfer their electrons to both H and Tt atoms. Tt atoms within the Tt-Tt chain are bonded covalently. The electronic density of states reveals that both hydrides exhibit metallic behavior. The observed vibrational spectra of these hydrides are in good overall agreement with the calculated phonon modes. There is evidence of dispersion induced splitting in the optical phonon peaks that can be ascribed to the coupling of H vibrations within the Li6-octahedra layers. DOI: 10.1103/PhysRevB.76.224301 PACS number s : 63.20.Dj, 71.20. b, 71.15.Mb, 78.70.Nx