Hydrides of Alkaline Earth-Tetrel (AeTt) Zintl Phases: Covalent Tt-H Bonds from Silicon to Tin.

Zintl phases form hydrides either by incorporating hydride anions (interstitial hydrides) or by covalent bonding of H to the polyanion (polyanionic hydrides), which yields a variety of different compositions and bonding situations. Hydrides (deuterides) of SrGe, BaSi, and BaSn were prepared by hydrogenation (deuteration) of the CrB-type Zintl phases AeTt and characterized by laboratory X-ray, synchrotron, and neutron diffraction, NMR spectroscopy, and quantum-chemical calculations. SrGeD4/3-x and BaSnD4/3-x show condensed boatlike six-membered rings of Tt atoms, formed by joining three of the zigzag chains contained in the Zintl phase. These new polyanionic motifs are terminated by covalently bound H atoms with d(Ge-D) = 1.521(9) Å and d(Sn-D) = 1.858(8) Å. Additional hydride anions are located in Ae4 tetrahedra; thus, the features of both interstitial hydrides and polyanionic hydrides are represented. BaSiD2-x retains the zigzag Si chain as in the parent Zintl phase, but in the hydride (deuteride), it is terminated by H (D) atoms, thus forming a linear (SiD) chain with d(Si-D) = 1.641(5) Å.

In situ neutron powder diffraction of the formation of SrGa2D2, and hydrogenation behavior of YbGa2 and EuGa2.

The hydrogenation behavior of the Zintl phases MGa2 (M = Sr, Eu, Yb) was investigated. Upon being heated under 50 bar of hydrogen gas pressure, SrGa2 starts forming the polyanionic hydride SrGa2H2 at 125 °C as observed by in situ differential scanning calorimetry (DSC) and ex situ X-ray powder diffraction. The deuteration of SrGa2 was studied by in situ neutron powder diffraction at temperatures up to 300 °C in the pressure range of 50 ≤ p(D2) ≤ 61 bar with a time resolution of two minutes. Neither incorporation of deuterium into the interstitials of the crystal structure of SrGa2 nor any other intermediate of the reaction was observed. No significant variation in positional or occupational parameters as refined by the Rietveld method was observed for the deuteride SrGa2D2 under various temperature and pressure conditions (e.g., at T = 299 °C, p(D2) = 60 bar, space group P3m1, a = 4.4098(2) Å, c = 4.7429(3) Å, Sr in 1a, 0 0 0; Ga in 2d, 1/3 2/3 0.474(1); D in 2d, 1/3 2/3 0.120(2)). In situ DSC complemented by ex situ X-ray powder diffraction revealed that EuGa2 and YbGa2 do not form polyanionic hydrides in the investigated temperature-pressure ranges (20 °C ≤ T ≤ 220 °C, 5 bar ≤ p ≤ 54 bar). Instead binary hydrides and gallium-rich intermetallics are formed. The first refined crystal structural data are presented for YbGa6 in the tetragonal PuGa6-type structure (space group P4/nbm (No. 125), a = 6.0890(3) Å, c = 7.6562(4) Å, Yb in 2c, 3/4 1/4 0; Ga1 in 8m, 0.4750(4) 0.5250(4) 0.3377(4); Ga2 in 4g, 1/4 1/4 0.1691(5)).