title: Electronic structure, chemical bonding, and solid-state NMR spectroscopy of the digallides of Ca, Sr, and Ba.
abstract: Combined application of (69,71)Ga NMR spectroscopy and quantum mechanical calculations reveals the chemical bonding in the digallides of Ca, Sr, and Ba. An analysis of the electron localization function (ELF) shows honeycomb-like 6(3) nets of the Ga atoms as the most prominent structural features in SrGa(2) and BaGa(2). For CaGa(2) a description of a 3+1-coordinated Ga atom is revealed by the ELF and by an analysis of interatomic distances. The NMR spectroscopic signal shift is mainly due to the Knight shift and is almost equal for the investigated digallides, whereas the anisotropy of the signal shift decreases with the radius of the alkaline-earth metals. Calculated and observed values of the electric field gradient (EFG) are in good agreement for CaGa(2) and BaGa(2) but differ by about 21 % for SrGa(2) indicating structural instability. Better agreement is achieved by considering a puckering of the Ga layers. For BaGa(2) an instability of the structure is indicated by a peak in the density of states at the Fermi level, which is shifted to lower energies when taking puckering of the Ga layers into account. Both structural modifications are confirmed by crystallographic information. The Fermi velocity of the electrons is strongly anisotropic and is largest in the (001) plane of the crystal structure. This results in an alignment of the crystallites with the [001] axis perpendicular to the magnetic field as observed in (69,71)Ga NMR spectroscopy and magnetic susceptibility experiments. The electron transport is predominantly mediated by the Ga-Ga p(x)- and p(y)-like electrons in the (001) plane. The specific heat capacity of BaGa(2) was determined and indicated the absence of phase transitions between 1.8 and 320 K.
authors.affiliation: Max-Planck-Institut für Chemische Physik fester Stoffe, Nöthnitzer Strasse 40, 01187 Dresden, Germany. haarmann@cpfs.mpg.de
authors.forename: Frank
authors.initials: F
authors.lastname: Haarmann
authors.forename: Katrin
authors.initials: K
authors.lastname: Koch
authors.forename: Daniel
authors.initials: D
authors.lastname: Grüner
authors.forename: Walter
authors.initials: W
authors.lastname: Schnelle
authors.forename: Oliver
authors.initials: O
authors.lastname: Pecher
authors.forename: Raul
authors.initials: R
authors.lastname: Cardoso-Gil
authors.forename: Horst
authors.initials: H
authors.lastname: Borrmann
authors.forename: Helge
authors.initials: H
authors.lastname: Rosner
authors.forename: Yuri
authors.initials: Y
authors.lastname: Grin