Electronic Structure, Chemical Bonding, and Solid-State NMR Spectroscopy of the Digallides of Ca, Sr, and Ba

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Abstract

Delving into digallides: The characteristics of the chemical bonding of the digallides of the alkaline-earth metals (see figure) have been studied by application of experimental methods, such as single-crystal X-ray diffraction and solid-state NMR spectroscopy, in combination with quantum mechanical calculations.

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Combined application of 69,71Ga 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 63 nets of the Ga atoms as the most prominent structural features in SrGa2 and BaGa2. For CaGa2 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 CaGa2 and BaGa2 but differ by about 21 % for SrGa2 indicating structural instability. Better agreement is achieved by considering a puckering of the Ga layers. For BaGa2 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 69,71Ga NMR spectroscopy and magnetic susceptibility experiments. The electron transport is predominantly mediated by the Ga–Ga px- and py-like electrons in the (001) plane. The specific heat capacity of BaGa2 was determined and indicated the absence of phase transitions between 1.8 and 320 K.

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