Locating hydrogen in the Mg₅Bi₃H_x Zintl phase

The preparation of Zintl phases with pronounced spin-orbit coupling has received substantial scientific interest because of their distinctive electronic properties. In the context of superconductivity and topological phenomena related to band inversion, intermetallic compounds of bismuth have come into focus recently. While bismuth forms a rich variety of Zintl phases with the heavier alkaline-earth metals, there are significantly fewer magnesium compounds. Here we show that high-temperature high-pressure synthesis opens a convenient route for the preparation of Mg₅Bi₃H_x already at moderate conditions. The compound (space group Pnma, a = 11.5399(3) Å, b = 8.9503(2) Å and c = 7.8770(2) Å) adopts a Ca₅Sb₃F crystal structure. The minute amounts of hydrogen could only be detected by thermal decomposition of the compound in combination with mass spectroscopy of the gas phase. Direct space analysis of the chemical bonding allowed for allocating the hydrogen position at a partially occupied interstitial site and reveals strongly polar Mg-Bi and Mg-H bonds in accordance with the Zintl concept. Calculated band structures exhibit substantial electronic reorganization upon hydrogen insertion. The combination of advanced analytical tools in concert with modern quantum chemical techniques provides an efficient approach to allocate trace amounts of interstitial atoms stabilizing intermetallic phases.