Crystal structure, electronic structure, and vibrational properties ofMAlSiH(M=Ca,Sr,Ba)<

Abstract

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− ͔.

Vibrational properties of the gallium monohydrides SrGaGeH, BaGaSiH, BaGaGeH, and BaGaSnH

“…At the same time the Zintl concept suggests that Ga atoms are considered as four-bonded. Therefore the Ga-H interaction is best described as a dative bond between H À and nominally zero-charged Ga [Ga'H 1À ] [5]. This bond is weak-due to the strong, counteracting electrostatic Ae-H interaction-but distinct, as indicated by the rigid frequency of the stretching mode with respect to the various chemical environments in AeGaTtH.…”

“…Reaction with hydrogen leads to monohydrides AeTrTtH, which are electron precise Zintl phases with a narrow band gap. In these hydrides H is attached to the Tr atom and-compared to the AlB 2 type precursors-TrTt hexagon layers are substantially puckered [5,6].…”

“…A fertile ground for precursors leading to polyanionic hydrides is provided by AlB 2 -type variants among the ternary 9-electron systems AeTrTt (Ae ¼ Ca, Sr, Ba; Tr ¼ Al, Ga, In; Tt ¼ Si, Ge, Sn) [4][5][6][7]. In the simple AlB 2 structure Tr and Tt atoms are randomly distributed on the majority site defining planar hexagon layers; variations may occur from Tr/Tt ordering and/or slight corrugations of hexagon layers [8].…”

A series of BaAl2−xSixH2−x (0.4<x<1.6) hydrides with compositions and structures in between BaSi2 and BaAl2H2

“…The structure of BaAlSiH is closely related to trigonal BaSi 2 , but the substitution modifies the electric properties. The density of states (DOS) of BaAlSiH showed a narrow band gap of 0.8 eV [10]. CaAlSiH and SrAlSiH are also semiconductors [10,11].…”

“…The density of states (DOS) of BaAlSiH showed a narrow band gap of 0.8 eV [10]. CaAlSiH and SrAlSiH are also semiconductors [10,11]. BaAlSiH can be obtained by direct hydrogenation of the precursor BaAlSi.…”