L Journal of Alloys and Compounds 306 (2000) 127–132 www.elsevier.com / locate / jallcom Trigonal SrAl H : the first Zintl phase hydride 2 2 a b a, * F. Gingl , T. Vogt , E. Akiba a Department of Inorganic Materials, National Institute of Materials and Chemical Research,1-1 Higashi, Tsukuba 305-8565, Japan b Physics Department, Brookhaven National Laboratory, Upton, Long Island, NY 11973-5000, USA Received 26 January 2000; accepted 1 February 2000 Abstract The title compound and its deuteride were synthesized by hydrogenating / deuterating SrAl at 50 bar H /D pressure and temperatures 2 2 2 below 2008C. As found by in situ X-ray powder diffraction, the hydrogenation of SrAl proceeds in three steps, with SrAl H as the first 2 2 2 product. SrAl H and SrAl D were characterized by X-ray and neutron powder diffraction. They crystallize with a new trigonal structure 2 2 2 2 ¯ ˚ ˚ ˚ ˚ in space group P3m1 (164), cell parameters: a54.5283(1) A, c54.7215(2) A (hydride), a54.5253(1) A, c54.7214(2) A (deuteride), 2 2 Z51. The main feature of this structure is a two-dimensional polymer Zintl anion, [AlH ] , in which one H atom is covalently bonded ` 2n to each Al atom. SrAl H is the first example of a Zintl phase hydride, i.e., the first Zintl phase in which hydrogen atoms are a part of the 2 2 Zintl anion.  2000 Elsevier Science S.A. All rights reserved. Keywords: Zintl phases; Gas–solid reaction; Crystal structure; Neutron diffraction; X-ray diffraction 1. Introduction the synthesis and the structure determination of the first of these new ternary hydrides, namely SrAl H . 2 2 Aluminum hydrides have received new attention recent- ly after the discovery [1] that doping alkali metal aluminum hydrides with Ti compounds can improve their 2. Experimental details kinetic properties significantly, making these materials possible candidates for reversible hydrogen storage appli- 2.1. Synthesis cations. Concerning metal hydride applications, aluminum is All steps of synthesis and sample preparation for X-ray used (together with Co and Mn) as a substituent for Ni to and neutron diffraction were carried out under dry argon optimize Mischmetal AB alloys for electrodes in NiMH atmosphere. SrAl was prepared by arc melting stoichio- 5 2 batteries [2,3]. Crystallographic studies on hydrides con- metric amounts of the elements. After remelting several taining Al as a major ingredient are rare. Apart from the times to ensure homogeneity, the ingots were ground to binary hydride, AlH [4], and the complex alanates LiAlH powders with particle size smaller than 100 mm. Samples 3 4 [5] and NaAlH [6], only a few crystal structures of of 1–2 g were loaded into stainless steel containers and 4 intermetallic hydrides like ZrNiAlH [7], Zr FeAl H placed in stainless steel autoclaves. Hydrogenation re- 0.57 6 2 10 [8] and AlTh H [9] are available in which the hydrogen actions were carried out at 50 bar hydrogen pressure by 2 4 atoms seem to avoid the neighborhood of Al, with Al–H slowly raising the reaction temperature from 1008C to ˚ 2008C in steps of 10–208 / day. The deuteride sample was distances typically around 3 A. prepared under comparable conditions. The products were During our hydrogenation experiments on binary al- dark gray and sensitive to moisture. kaline earth–aluminum compounds we found several new phases in the Sr–Al–H system. In this paper we report on 2.2. X-ray powder diffraction *Corresponding author. Tel.: 181-298-61-4541; fax: 181-298-61- The hydrogenation reaction of SrAl was studied by in 4541. 2 E-mail address: akiba@nimc.go.jp (E. Akiba) situ X-ray powder diffraction on a Rigaku RINT-TTR 0925-8388 / 00 / $ – see front matter  2000 Elsevier Science S.A. All rights reserved. PII: S0925-8388(00)00755-6