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− ͔.

Theory Theory C 1000 Comparative Study of the High-Pressure Behavior of As, Sb, and Bi -[DFT calculations]. -(HAEUSSERMANN*, U.; SOEDERBERG, K.; NORRESTAM, R.; J. Am. Chem. Soc. 124 (2002) 51, 15359-15367; Dep. Inorg. Chem., Arrhenius Lab., Univ. Stockholm, S-106 91 Stockholm, Swed.; Eng.) -Schramke 16-001

Theory of the condensed state D 1000 Structure and Bonding of Zinc Antimonides: Complex Frameworks and Narrow Band Gaps. -Crystal structure relationships, phase stability, and chemical bonding of the thermoelectric materials ZnSb, α-Zn4Sb3, and β-Zn4Sb3 are characterized by frozen core all-electron projected augmented wave calculations. Central structural building units present in all the materials are rhomboidal Zn2Sb2 rings. The high coordination numbers in the frameworks implies the presence of multicenter bonding. A simple bonding picture where multicenter bonding is confined to Zn2Sb2 rings is given. -(MIKHAYLUSHKIN, A. S.; NYLEN, J.; HAEUSSERMANN*, U.; Chem. Eur. J. 11 (2005) 17, 4912-4920; Dep. Inorg. Chem., Arrhenius Lab., Univ. Stockholm, S-106 91 Stockholm, Swed.; Eng.) -W. Pewestorf 08-003

Structure Structure D 2000 Structural Relationships, Phase Stability and Bonding of Compounds PdSn n (n = 2, 3, 4). -Single crystals of the title compounds are prepared under self-flux condition with the stannide in equilibrium with the melt. The samples are characterized by thermal analysis, 119 Sn Moessbauer spectroscopy, and ab initio DFT calculations. As revealed by single crystal XRD PdSn2 crystallizes in the tetragonal space group I41/acd with Z = 16, PdSn 3 in the orthorhombic space group Cmca with Z = 8, and PdSn 4 in the orthorhombic space group Ccca with Z = 4. The structures of the compounds represent stackings of similar building blocks and are closely related. -(NYLEN, J.; GARCIA GARCIA, F. J.; MOSEL, B. D.; POETTGEN, R.; HAEUSSERMANN, U.; Solid

Thermodynamically stable solids M2SiH6 (M: K, Rb) are prepared from mixtures of MH and Si, or MSi using ammonia borane as hydrogen source (NaCl capsule, 4—9 GPa, 400—700 °C, 1 h).