Superconducting AlB 2 -type silicides CaAlSi, SrAlSi, and BaAlSi ͑MAlSi͒ absorb hydrogen and form semiconducting monohydrides where hydrogen is exclusively attached to Al. This induces a metal-nonmetal transition which is accompanied with only a minor rearrangement of the metal atoms. We report the synthesis and structure determination of CaAlSiH and BaAlSiH as well as a first-principles study of the electronic structure and vibrational property changes associated with the metal-nonmetal transition. We find that incorporation of H in MAlSi removes the partly occupied antibonding ‫ء‬ band responsible for metallic behavior and turns it into an energetically low-lying Al-H bonding band. The fully occupied bonding band in MAlSi changes to a weakly dispersed band with Si p z ͑lone-pair͒ character in the hydrides, which becomes located below the Fermi level. The soft phonon mode in MAlSi pivotal for the superconducting properties stiffens considerably in the hydride. This mode is associated with the out-of-plane Al-Si vibration and is most affected by the formation of the Al-H bond. The mode of the Al-Si in-plane vibration, however, is unaffected, indicating that the Al-Si bond is equally strong in the metallic precursor and the semiconducting hydride. Al-H modes for MAlSiH are weakly dispersed. The frequencies of the stretching mode are around 1200 cm −1 and virtually invariant to the M environment, indicating a covalent but weak Al-H interaction, which is interpreted as a dative bond from hydridic hydrogen to Al ͓Al← H 1− ͔.

Strongly correlated electron systems, generally recognized as d- and f-electron systems, have attracted attention as a platform for the emergence of exotic properties such as high-Tc superconductivity. However, correlated electron behaviors have been recently observed in a group of novel materials, electrides, in which s-electrons are confined in subnanometer-sized spaces. Here, we present a trend of electronic correlation of electrides by evaluating the electronic correlation strength obtained from model parameters characterizing effective Hamiltonians of 19 electrides from first principles. The calculated strengths vary in the order 0D ≫ 1D > 2D ∼ 3D electrides, which corresponds to experimental trends, and exceed 10 (a measure for the emergence of exotic properties) in all of the 0D and some of the 1D electrides. We also found the electronic correlation depends on the cation species surrounding the s-electrons. The results indicate that low-dimensional electrides will be new research targets for studies of strongly correlated electron systems.

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and BaAl2H2. -The title hydrides are prepared by direct reaction of BaAl2-xSix (obtained by melting stoichiometric mixtures of the elements) with hydrogen (autoclave, 70 bar H2, 300-700°C, 3-48 h). The samples are characterized by powder XRD and diffuse reflectance spectroscopy. DFT calculations show that BaSi 2 and BaAl 2 H 2 are conductors whereas BaAlSiH is a semiconductor. The cell parameters for the trigonal cell (hexagonal space group P6/mmm for BaAl2-xSix) of the hydrides vary linearly as a function of the Si -/(Al-H)substitution, opening up the possibility of continuously tunable electric properties. -(MOSER, D.; HAEUSSERMANN, U.; UTSUMI, T.; BJOERLING, T.; NOREUS*, D.; J. Alloys Compd. 505 (2010) 1, 1-5, DOI:10.1016/j.jallcom.2010.05.065 ; Dep. Mater. Environ. Chem., Stockholm Univ., S-106 91 Stockholm, Swed.; Eng.) -W. Pewestorf 42-011