Polyanionic Hydrides from Polar Intermetallics AeE 2 (Ae ) Ca, Sr, Ba; E ) Al, Ga, In) Thomas Bjo ¨ rling, ‡ Dag Nore ´ us, ‡ and Ulrich Ha ¨ ussermann* ,†,§ Contribution from the Inorganic Chemistry and Structural Chemistry Departments, Stockholm UniVersity, 10691 Stockholm, Sweden Received July 5, 2005; E-mail: Ulrich.Haussermann@asu.edu Abstract: The hydrogenation behavior of the polar intermetallic systems AeE2 (Ae ) Ca, Sr, Ba; E ) Al, Ga, In) has been investigated systematically and afforded the new hydrides SrGa2H2 and BaGa2H2. The structure of these hydrides was characterized by X-ray powder diffraction and neutron diffraction of the corresponding deuterides. Both compounds are isostructural to previously discovered SrAl 2H2 (space group P3 h m1, Z ) 1, SrGa2H2/D2: a ) 4.4010(4)/4.3932(8) Å, c ) 4.7109(4)/4.699(1) Å; BaGa2H2/D2: a ) 4.5334(6)/4.5286(5) Å, c ) 4.9069(9)/4.8991(9) Å). The three hydrides SrAl2H2, SrGa2H2, and BaGa2H2 decompose at around 300 °C at atmospheric pressure. First-principles electronic structure calculations reveal that H is unambiguously part of a two-dimensional polyanion [E2H2] 2- in which each E atom is tetrahedrally coordinated by three additional E atoms and H. The compounds AeE2H2 are classified as polyanionic hydrides. The peculiar feature of polyanionic hydrides is the incorporation of H in a polymeric anion where it acts as a terminating ligand. Polyanionic hydrides provide unprecedented arrangements with both E-E and E-H bonds. The hydrogenation of AeE 2 to AeE2H2 takes place at low reaction temperatures (around 200 °C), which suggests that the polyanion of the polar intermetallics ([E2] 2- ) is employed as precursor. 1. Introduction Alanates, which are aluminum hydrides of alkali metals (A) or alkaline earth metals (Ae), gained recently a tremendous amount of attention. 1-3 This was triggered by the discovery that some transition metals catalyze the reverse of the two-step decomposition NaAlH 4 f 1 / 3 (Na 3 AlH 6 ) + 2 / 3 Al + H 2 f NaH + Al + 3/ 2 H 2 , which suddenly turned long-known NaAlH 4 into a state-of-the-art media with 5.6 wt % H-storage capacity. In the past the systems AAlH 4 and A 3 AlH 6 have been intensively investigated with respect to synthesis, dehydrogenation behavior, structural characterization, and computational modeling of structural stability and physical properties. 4 Further, new alkaline earth metal alanates, such as BaAlH 5 and Ae 2 AlH 7 (Ae ) Sr, Ba), were discovered, and the structure of nanocrystalline MgAl 2 H 8 was finally solved. 5 Characteristically, alanates represent fully hydrogenated systems A m Ae n Al o H m+2n+3o . In 2000, however, Gingl et al. reported the synthesis and structural characterization of peculiar and novel SrAl 2 H 2 . 6 This aluminum hydride compound is not fully hydrogenated and was obtained by hydrogenating the alloy SrAl 2 at mild conditions (200 °C, 50 bar H 2 pressure). SrAl 2 is usually considered as a member of the large family of Zintl phases that form between active metals (alkali, alkaline earth, or rare earth metals) and a more electronegative p-block metallic or semimetallic element (the E component). According to the Zintl concept, Al is formally reduced by the electropositive Sr and features a three-dimensional four-connected (3D4C) poly- anionic network in which each Al atom is surrounded by four neighbors in a distorted tetrahedral fashion. This arrangement fits the electron count of Al - , which is isoelectronic to Si. In SrAl 2 H 2 the Al network is maintained, although its dimension is reduced to two. The Al atoms are arranged as puckered graphitic layers where they have three nearest Al neighbors, the vacant coordination is taken by hydrogen. Thus, hydrogen appears to act as a pair of scissors cutting covalent Al-Al bonds and subsequently terminating them (Figure 1). This retains a † Inorganic Chemistry Department. ‡ Structural Chemistry Department. § Present address: Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287-1604. (1) (a) Bogdanovic, B.; Schwickardi, M. J. J. Alloys Compd. 1997, 253, 1. (b) Bogdanovic, B.; Brand, R. A.; Marjanovic, A.; Schwickardi, M.; To ¨lle, J. J. Alloys Compd. 2000, 302, 36. (c) Bogdanovic, B.; Sandrock, G. MRS Bulletin 2002, September, 712. (2) Akiba, E. Curr. Opin. Solid State Mater. Sci. 1999, 4, 267. (3) Seayad, A. M.; Antonelli, D. M. AdV. Mater. 2004, 16, 765. (4) (a) Hauback, B. C.; Brinks, H. W.; Jensen, C. M.; Murphy, K.; Maeland, A. J. J. Alloys. Compd. 2003, 358, 142. (b) Hauback, B. C.; Brinks, H. 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