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Journal of Alloys and Compounds xxx (2006) xxx–xxx Review Magnesium based ternary metal hydrides containing alkali and alkaline-earth elements Klaus Yvon ∗ , Bernard Bertheville Laboratoire de Cristallographie, Université de Genève, 24 Quai Ernest Ansermet, CH-1211 Genève, Switzerland Received 23 December 2005; accepted 17 January 2006 Abstract A review of structurally characterised ternary metal hydrides containing magnesium and alkali (A) or alkaline-earth (Ae) elements (including divalent Eu and Yb) is given. There exist 28 such compounds that cover 18 different crystal structures. Most were obtained by solid-state reactions under moderate hydrogen gas pressure while some required hydrostatic pressure in the kbar range. The compounds are saline and have ordered crystal structures. Their hydrogen contents can be rationalized in terms of mono and divalent metal cations, and hydrogen anions. Many have fluorine analogues, and those containing calcium and strontium tend to have ytterbium and europium analogues, respectively. Magnesium usually adopts octahedral and rarely tetrahedral or pentagonal bipyramidal hydrogen configurations. The metal configurations around hydrogen are mostly octahedral and tetrahedral, but square pyramidal, trigonal bipyramidal, trigonal and linear configurations are also found. Compared to binary hydrides, the metal–hydrogen bonds are generally shortened which is reflected by a volume contraction. The hydrogen storage efficiencies for A metal compounds reach 6.0 wt.% and 88 g l−1 (NaMgH3 ) and for Ae metal compounds 5.7 wt.% and 99 g l−1 (Ca4 Mg3 H14 ). The compounds generally desorb hydrogen above 670 K at 1 bar. The europium containing hydrides order magnetically at temperatures below 19–32 K. © 2006 Elsevier B.V. All rights reserved. Keywords: Hydrogen storage materials; Chemical synthesis; Crystal structures Contents 1. 2. 3. 4. 5. ∗ Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthesis and methods of characterisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Crystal chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Crystal structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Weight and volume efficiencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Thermal stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Electronic and magnetic properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 00 00 00 00 00 00 00 00 00 00 00 Corresponding author. Tel.: +41 22 379 6231; fax: +41 22 379 6864. E-mail address: [email protected] (K. Yvon). 0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.01.049 JALCOM-13609; No. of Pages 8 K. Yvon, B. Bertheville / Journal of Alloys and Compounds xxx (2006) xxx–xxx 2 1. Introduction Due to their light weight and low cost, magnesium based metal hydrides are of interest as constituents for advanced hydrogen storage systems. Until recently only few ternary hydrides between magnesium and alkali (A) or alkaline-earth (Ae) elements were known. The first fully characterized example was Ca4 Mg3 H14 [1] which has been reported in 1992. Since then quite a few other hydrides of this class have been discovered, most of them at the University of Geneva. No review on these compounds has appeared as yet, except for a short chapter in an article on novel solid-state metal hydrides [2] and a report on europium based saline metal hydrides [3]. Ternary hydrides between magnesium and A or Ae elements known up to 1997 are included in the Hydride–Fluoride Crystal Structure Data Base [4], but are currently not covered by the Hydride Mate- rials Data Base [5]. The purpose of this article is to review these hydrides and to discuss their crystal chemistry and some of their properties. Only ternary compounds (i.e. no solid solutions) are covered whose structures (including hydrogen positions) have been fully characterised. Systems containing divalent europium and ytterbium are included because of their close crystal chemical relationship with the corresponding alkaline-earth systems. 2. Synthesis and methods of characterisation The compounds reported so far [1,6–27] are summarized in Table 1. They were usually synthesised by solid-state reaction between binary hydrides powder mixtures (e.g. Rb2 MgH4 ) or from binary metal alloys (e.g. EuMg2 H6 ) under moderate hydrogen pressures (up to 160 bar) and temperatures (up to Table 1 Compositions, structural properties and hydrogen storage efficiencies of magnesium based ternary metal hydrides containing alkali (A) and alkaline-earth (Ae) or lanthanide elements (Eu, Yb)a Compound Space group Vb A–Mg NaMgH3 KMgH3 d RbMgH3 CsMgH3 (hp)f,g CsMgH3 (lp)f Rb4 Mg3 H10 Cs4 Mg3 H10 g K2 MgH4 Cs2 MgH4 (hp)f,g Rb2 MgH4 Cs2 MgH4 (lp)f Rb3 MgH5 Cs3 MgH5 Pnma Pm-3m P63 /mmc R-3m Pmmn Cmca Cmca I4/mmm I4/mmm Pnma Pnma I4/mcm I4/mcm −5.1 −15.9 −15.3e −12.3 −8.7 −6.3 −10.0 −9.9 −14.3 +3.9 +3.8 <+0.1 −0.3 Ae–Mg EuMg2 H6 Sr2 Mg3 H10 Ba2 Mg3 H10 Eu2 Mg3 H10 g Sr6 Mg7 H26 g Eu6 Mg7 H26 g Ba6 Mg7 H26 SrMgH4 EuMgH4 BaMgH4 Ca4 Mg3 H14 Yb4 Mg3 H14 Sr2 MgH6 g Ba2 MgH6 Eu2 MgH6 g Ca19 Mg8 H54 g Yb19 Mg8 H54 P4/mmm C2/m C2/m C2/m I/2m I/2m Immm Cmc21 Cmc21 Cmcm P-62m P-62m P-3m1 P-3m1 P-3m1 Im-3 Im-3 +8.9 −3.8 −5.1 −3.1 −2.9 −2.6 −5.4 −0.5 −0.2 −4.6 −0.6 +0.7 −7.9 −9.9 −7.3 −4.7 −4.1 a b c d e f g CNc wt.% H2 A, Ae Mg 12 12 12 12 12 10,11 10, 11 9 9 9, 10 9, 11 8, 10 8, 10 6 6 6 6 6 6 6 6 6 4 4 4 4 6.0 4.6 2.7 1.9 1.9 2.4 1.6 3.8 1.2 2.0 1.4 1.8 1.2 12 12 12 12 10, 11 10, 11 12 9 9 13 9 9 12 12 12 8, 10, 12 8, 10, 12 6 6 6 6 6 6 6 6 6 6 7 7 6 6 6 6 6 2.9 3.9 2.8 2.6 3.6 2.4 2.6 3.5 2.2 2.4 5.7 1.8 2.9 2.0 1.8 5.4 1.5 g H2 /liter 88.3 77.1 69.7e 60.0 56.9 56.9 52.7 59.9 48.5 45.5 40.0 42.7 37.1 88.3 95.1 86.5 96.9 92.0 94.5 83.7 88.5 91.2 81.1 98.9 99.6 91.3 78.3 92.9 100.9 97.7 Reference 6 7 8 9 10 11 9 12 9 13 14 13 14 15 16 17 18 19 18 20 21 15 22 1 23 24 25 18 26 27 As of 2005, hydrides structurally characterized by neutron diffraction on deuterides only (except for KMgH3 , see d), listed in decreasing order of Mg content. Relative volume contraction compared to weighted sums of volumes of corresponding binary hydrides. Coordination number. H positions not ascertained but fixed by symmetry. Values derived from deuteride. hp, high-pressure; lp, low-pressure modification. Synthesized under high hydrostatic pressure. K. Yvon, B. Bertheville / Journal of Alloys and Compounds xxx (2006) xxx–xxx ∼700 K). Some hydrides, however, required the application of rather high quasi-hydrostatic pressure in a multi-anvil press, such as Sr2 MgH6 that was obtained at 35 kbar and T = 870 K, Sr6 Mg7 H26 at 25 kbar and 900 K, CsMgH3 (hp), Cs4 Mg3 H10 and Cs2 MgH4 (hp) at 30 kbar and 820 K, and various Eu–Mg hydrides at up to 32 kbar and 870 K. Some hydrides were obtained by hydrogenation of single-phase alloys such as the solid solution series Eu1−x Srx Mg2 (MgZn2 -type structure) that yielded Eu1−x Srx Mg2 H6 for x ≤ 0.6 and (Eu1−x Srx )2 Mg3 H10 [28] for x > 0.8, or by hydrogenation of multi-phase alloys such as mixtures between binary CaMg2 and elemental Ca that yielded Ca4 Mg3 H14 . The reaction products were usually polycrystalline and contained secondary phases such as unreacted binary metal hydrides. Single crystals of sufficient size and quality for diffraction experiments (e.g. Yb19 Mg8 H54 ) were rare. The hydrides containing A and/or heavy Ae metals were particularly sensitive to air and moisture. Given the unfavourable scattering power of hydrogen for both X-ray and neutron powder diffraction the atomic arrangements had to be determined ab initio on deuterides. Difficulties during structure determination arose due to the presence of impurity phases (of which some were not detected by X-rays), the structural complexity (up to 16 atomic parameters for Ae6 Mg7 H26 ; Ae = Eu, Sr) and the limited resolution of conventional powder diffractometers. In some cases, such as for the strongly neutron absorbing europium compounds, high-resolution data had to be collected on advanced synchrotron and high-flux neutron radiation sources. The data were analysed by the Rietveld method by using joint refinement procedures, if necessary. For better convergence the number of refined parameters, in particular the atomic displacement amplitudes, were reduced by applying suitable constraints. Due to the absence of single crystals and/or single-phase powder samples and their sensitivity to moisture, measurements of physical properties were difficult and only few have been performed. Magnetic susceptibility data have been collected only on europium containing hydrides, and enthalpies of hydride formation have been determined for very few, relatively unstable hydrides by measuring pressurecomposition isotherms on a thermo-balance (Yb4 Mg3 H14 ) or by calorimetric methods (NaMgH3 [29], KMgH3 [30]). 3 3. Crystal chemistry 3.1. Composition Magnesium based ternary metal hydrides are known to occur for all A and Ae elements (including Eu and Yb) except A = Li and Ae = Be. Their compositions and some of their properties are listed in Table 1. So far, there exist 28 ternary compounds of which five occur with Eu, four with Rb, Cs, Sr and Ba each, two with K, Ca and Yb each, and one with Na. For some hydrides (CsMgH3 , Cs2 MgH4 ) there exist low-pressure (lp) and highpressure (hp) polymorphs. As expected from valence considerations the hydrogen-to-metal ratios range from H/M = 1.25 (e.g. Rb3 MgH5 ) to H/M = 2 (e.g. SrMgH4 ). For A metal compounds the Mg/A ratios cover the range 0.33–1.00 and assume simple fractions such as Mg/A = 3/4 (Rb4 Mg3 H10 ) and 1/2 (K2 MgH4 ). For Ae metal compounds the Mg/Ae ratios cover the range 0.42–2.00 and assume complex fractions such as Mg/Ae = 6/7 (Ba6 Mg7 H26 ) and 19/8 (Ca19 Mg8 H54 ). Interestingly, ternary hydrides between Ae elements other than magnesium, and quaternary hydrides between magnesium, A and Ae elements have not been reported as yet. This absence is presumably due to the fact that the binary hydrides CaH2 , SrH2 , BaH2 , EuH2 and YbH2 are isostructural (PbCl2 -type), thus favouring the formation of solid solutions, whereas MgH2 crystallizes with different structures (␣-TiO2 - or ␣-PbO2 -type [31]). 3.2. Crystal structures As can be seen from Table 2 (A metal constituents) and Table 3 (Ae metal constituents, including Eu and Yb) the hydrides crystallize with 18 different structures types of which some are deformation variants of more symmetric ones, such as NaMgH3 that derives from cubic KMgH3 by an orthorhombic distortion, and Ae6 Mg7 H26 and AeMgH4 (Ae = Sr, Eu) that derive from orthorhombic Ba6 Mg7 H26 and BaMgH4 , respectively, by small monoclinic distortions. All hydrogen atom arrangements are ordered, in contrast to metallic metal hydrides [32] in which they are usually disordered. Many have fluorine analogues [4,12]. Mixed hydride fluorides exist such as the solid solution series NaMgH3−x Fx [6], but none have ordered anion distributions. As expected from ionic size considerations the Yb Table 2 Ternary metal hydridesa between magnesium and alkali elements Li Na K Rb Cs Structure type (space group) CsMgH3 (hp)b CsMgH3 (lp) Cs4 Mg3 H10 b Cs2 MgH4 (hp) Cs2 MgH4 (lp) Cs3 MgH5 GdFeO3 (Pnma) CaTiO3 (Pm-3m) BaTiO3 (P63 /mmc) BaRuO3 (R-3m) CsMgH3 (Pmmn) Cs4 Mg3 F10 (Cmca) K2 NiF4 (I4/mmm) -K2 SO4 (Pnma) Cs3 CoCl5 (I4/mcm) NaMgH3 b KMgH3 b RbMgH3 b Rb4 Mg3 H10 b K2 MgH4 b Rb2 MgH4 b Rb3 MgH5 a b Arranged in decreasing order of Mg content. Fluoride analogue known to exist. K. Yvon, B. Bertheville / Journal of Alloys and Compounds xxx (2006) xxx–xxx 4 Table 3 Ternary metal hydridesa between magnesium and alkaline-earths or divalent lanthanides Ca Sr Ba Eu Yb Sr2 Mg3 H10 Sr6 Mg7 H26 Ba2 Mg3 H10 b EuMg2 H6 Eu2 Mg3 H10 Eu6 Mg7 H26 Ba6 Mg7 H2 SrMgH4 b EuMgH4 BaMgH4 b Ca4 Mg3 H14 Yb4 Mg3 H14 Sr2 MgH6 Ba2 MgH6 b Ca19 Mg8 H54 a b Eu2 MgH6 Yb19 Mg8 H54 Structure type (space group) EuMg2 H6 (P4/mmm) Ba2 Ni3 F10 (C2/m) Ba6 Zn7 F26 (I2/m) Ba6 Mg7 H26 (Immm) BaZnF4 (Cmc21 ) LaNiH4 (Cmcm) Ca4 Mg3 H14 (P-62m) K2 GeF6 (P-3m1) Yb19 Mg8 H54 (Im-3) Arranged in decreasing order of Mg content (adapted from [3]). Fluoride analogue known to exist. compounds are isotypic to the Ca compounds and the Eu compounds are isotypic to the Sr compounds. The only Eu compound for which no Sr analogue has been reported yet is EuMg2 H6 . Its solid solution Eu1−x Srx Mg2 H6 has only a limited range 0 ≤ x ≤ 0.6 [28]. The structures show complicated atom arrangements (Sr6 Mg7 H26 : nine different H sites) and display a rich variety of coordination numbers (CNs) and geometries. Of major interest are the H atom environments around magnesium. They are usually octahedral (CN = 6) or distorted octahedral as in binary MgH2 . Notable exceptions are the tetrahedral H configurations (CN = 4) in -K2 SO4 -type hydrides (Rb2 MgH4 , lp-Cs2 MgH4 ) and Cs3 CoCl5 -type hydrides (Cs3 MgH5 ), and the pentagonal bipyramidal H configuration (CN = 7) in Ae4 Mg3 H14 (Ae = Ca, Yb). As expected, the MgH6 octahedra tend to be isolated at low Mg contents and connected via corners, edges and faces at higher Mg contents. However, there is no clear relationship between the connectivity of the MgH6 octahedra and the Mg content. While the MgH6 octahedra are isolated in Ae2 MgH6 (Ae = Sr, Ba, Eu), corner sharing occurs in EuMg2 H6 and AMgH3 (A = Na, K), and in AeMgH4 (Ae = Sr, Ba, Eu) and A2 MgH4 (A = K, Cs), leading to three-dimensional slabs and two-dimensional networks, respectively (see Fig. 1). On the other hand, the octahedra in Ae19 Mg8 H54 (Ae = Ca, Yb) are connected via corners to groups of 8 that are isolated from each other. Corner and edge sharing (such as in MgH2 ) leading to three-dimensional networks occurs in Ae2 Mg3 H10 and Ae6 Mg7 H26 (Ae = Sr, Ba, Eu). Corner and face sharing occurs only with A metal compounds such as A4 Mg3 H10 (A = Rb, Cs) and AMgH3 (A = K, Cs). Their Mg octahedra are condensed via common faces to dimers (RbMgH3 ) or trimers (CsMgH3 ; A4 Mg3 H10 , A = Rb, Cs) that are linked via corners to two-dimensional slabs or threedimensional networks. Interestingly, the trimers in CsMgH3 are quasi linear in the high-pressure and cyclic in the low-pressure modification. The H atom environments around A atoms and Ae atoms other than magnesium show a great variety of CNs and configurations that differ substantially from those in the corresponding binary hydrides. As expected from their larger size, heavy A and Ae atoms have higher CNs than Mg atoms. They range from CN = 8 (A3 MgH5 ) to CN = 13 (BaMgH4 ), in contrast to the 12fold configuration in binary NaCl-type hydrides AH (A = K, Rb, Cs) and the 9-fold configuration in binary PbCl2 -type hydrides AeH2 (Ae = Ca, Sr, Ba, Eu, Yb). The influence of cation size on CNs and H atom configurations is particularly apparent in barium and strontium based analogues. As shown in Fig. 2 the bigger Ba atoms in orthorhombic Ba6 Mg7 H26 occupy two sites with CN = 12, while the smaller Sr atoms in monoclinic Sr6 Mg7 H26 occupy three sites with CN = 10 and 11 that have quite different H atom configurations. Similar differences in CNs and H atom configurations also occur between centrosymmetric BaMgH4 (CN = 13) and its non-centrosymmetric strontium congener SrMgH4 (CN = 9). Another example for the influence of cation size is the perovskite-related series AMgH3 . As the atomic size of A decreases the structures change from rhombohedral BaRuO3 -type (A = Cs) to hexagonal BaTiO3 -type (A = Rb), cubic CaTiO3 -type (A = K) and orthorhombic GdFeO3 -type (A = Na). A similar size effect is also observed in the A2 MgH4 series (A = K, Rb, Cs) that changes from the orthorhombic K2 SO4 -type (tetrahedral Mg coordination) for the relatively big Rb and Cs atoms (CN = 9, 10) to the tetragonal K2 NiF4 -type (octahedral Mg coordination) for the smaller K atom (CN = 9), a transition that can also be induced by external pressure as shown for A = Cs. Clearly, pressure stabilizes the octahedral MgH6 groups in CsMgH3 , Cs4 Mg3 H10 and Cs2 MgH4 , and these have smaller volume requirements and thus better volume efficiencies for hydrogen storage than the tetrahedral MgH4 groups formed under lower hydrogen gas pressure as in Cs2 MgH4 and Cs3 MgH5 . The metal configurations around the H atom sites display a large inventory of coordination numbers and site symmetries. Typical examples are shown in Fig. 3. Many are specific to ternary hydrides as they do not occur in the corresponding binary hydrides. They range from linear (CN = 2) to trigonal planar or nearly planar as in ␣- and ␥-MgH2 , respectively (CN = 3), distorted tetrahedral (CN = 4), square pyramidal or trigonal-bipyramidal (CN = 5), and octahedral (CN = 6). The linear Mg–H–Mg co-ordination in EuMg2 H6 and its strontium containing solid solution Eu1−x Srx Mg2 H6 is remarkable because such a geometry is atypical for saline compounds. The H site symmetries range from relatively high symmetric 4/mmm in EuMg2 H6 to low symmetric 1 in Ba6 Mg7 H26 . The metal–deuterium distances are consistent with those in the corresponding binary deuterides, but tend to be shorter. K. Yvon, B. Bertheville / Journal of Alloys and Compounds xxx (2006) xxx–xxx 5 Fig. 1. Connectivity of MgH6 octahedra in magnesium based ternary metal hydrides containing alkali (A) and alkaline-earth (Ae) or lanthanide (Eu, Yb) elements; lp = low-pressure modification, hp = high-pressure modification. A striking example is the Mg–D bond of 1.77 Å in the linear Mg–D–Mg co-ordination of EuMg2 D6 which is significantly shorter than the Mg–D bonds in MgD2 (1.95 Å). It is the shortest known among magnesium containing solid-state metal deuterides and reflects the partially covalent character of magnesium–deuterium (hydrogen) bonds. A shortening of metal–deuterium bonds also occurs for other constituents, in particular A metals such as Na (2.27 Å in NaMgD3 versus 2.44 Å in NaD), K (2.78 Å in K2 MgD4 versus 2.85 Å in KD), Rb (2.77 Å in Rb2 MgD4 versus 3.01 Å in RbD) and Cs (2.94 Å in Cs4 Mg3 D10 versus 3.19 Å in CsD). The shortest metal–deuterium distances for Ae metals other than Mg and for divalent lanthanides do generally not much differ from those in the corresponding binary deuterides, such as for Ca (2.29 Å in Ca4 Mg3 D14 versus 2.24 Å in CaD2 ), Sr (2.43 Å in SrMgD4 versus 2.43 Å in SrD2 ), Ba (2.55 Å in Ba2 Mg3 D10 versus 2.57 Å in BaD2 ), Eu (2.37 Å in EuMgD4 versus 2.38 Å in EuD2 ) and Yb (2.27 Å in Yb4 Mg3 D14 versus 2.24 Å in YbD2 ). However, a significant shortening occurs in those deuterides that were obtained by application of high quasi-hydrostatic pressure, such as Ca19 Mg8 D54 (Ca–D = 2.20 Å) and Eu6 Mg7 D26 (Eu–D = 2.20 Å). As expected, the metal–deuterium distances also depend on coordination numbers and matrix effects. The average Eu–D distances, for example, generally increase with the CN of europium and range from 2.53 Å in EuMgD4 (CN = 9) to 2.56–2.57 Å in Eu6 Mg7 D26 (CN = 10), 2.64 Å in Eu6 Mg7 D26 (CN = 11) and 2.65–2.71 Å in Eu2 Mg3 D10 (CN = 12) [3]. The anomalously small value in EuMg2 D6 (2.58 Å for CN = 13) is presumably a result of its unusual crystal structure. The importance of matrix effects on metal–deuterium bond lengths 6 K. Yvon, B. Bertheville / Journal of Alloys and Compounds xxx (2006) xxx–xxx Fig. 2. Deuterium coordination polyhedra around the Ba sites (top) in Ba6 Mg7 D26 (left and centre) and BaMgD4 (right), and around the corresponding Sr sites (bottom) in the less symmetric Sr analogues Sr6 Mg7 D26 and SrMgD4 ; same shading (colour) of H atoms indicate symmetry equivalent sites. and bond angles can be seen in K2 NiF4 -type deuterides such as A2 + MgD4 (A+ = K, Cs) in which the bigger Cs+ lengthens the Mg–D bonds and flattens the MgD6 octahedron (axial Mg–D = 2.00 (K), 2.01 Å (Cs), equatorial Mg–D = 2.02 (K), 2.16 Å (Cs)). As in the corresponding binary deuterides, the Eu–D distances in the ternary deuterides are similar to, but slightly shorter than, the corresponding Sr–D distances in the strontium analogues, while the Yb–D distances in the ternary deuterides are similar to, but slightly longer than, the correspond- ing Ca–D distances in the calcium analogues, thus underlining the closely related hydride chemistry of Ae metals and divalent lanthanides. The observed differences (∼0.04 Å, on the average, between the Sr and Eu analogues, and ∼0.02 Å, on the average, between the Ca and Yb analogues) are somewhat larger than expected from tabulated effective ionic metal radii for oxides and fluorides (Sr2+ : 1.36 Å versus Eu2+ : 1.35 Å for CN = 10; Ca2+ : 1.14 Å versus Yb2+ : 1.12 Å for CN = 8 [33]) which suggests that different sets of metal radii should be used for rationalizing Fig. 3. Examples for deuterium coordinations in magnesium based ternary metal hydrides containing alkali and alkaline-earths or divalent lanthanides; small spheres: D atoms, medium spheres: Mg atoms, large spheres: A or Ae atoms (perspective views). K. Yvon, B. Bertheville / Journal of Alloys and Compounds xxx (2006) xxx–xxx metal hydride structures. In this context it is useful to recall that metal–hydrogen distances in such compounds cannot be described in terms of a fixed hydrogen radius. This is due to the strong polarizability of the hydride anion which leads to a wide spread of molar hydrogen volumes [34]. The D–D contact distances in all deuterides studied so far all exceed 2.0 Å, which can be taken as evidence for repulsive D− –D− interactions. Repulsive interactions also occur between the metal cations as shown, for example, by the displacements of the Mg2+ cations from the centres of the face-sharing H octahedra in the [3MgH3 ]3− trimers of the CsMgH3 polymorphs (Mg–D = 1.94–1.96 Å at the periphery, 2.00–2.08 Å at the centre of the trimers). Finally, the metal–hydrogen distances tend to be slightly longer than the corresponding distances in the deuterides, thus leading to hydride volumes that are typically 0.5% bigger than those of the corresponding deuterides which is consistent with the usually observed isotope effect. 4. Properties 4.1. Weight and volume efficiencies As shown in Table 1 the gravimetric hydrogen storage efficiencies of Mg based ternary metal hydrides reach 6.0 wt.% for A metal constituents (NaMgH3 ) and 5.7 wt.% for Ae metal constituents (Ca4 Mg3 H14 ). The volume efficiencies reach 101 g H2 l−1 (Ca19 Mg8 H54 ) and thus come close to that of binary MgH2 (109 g H2 l−1 ) while exceeding by far that of liquid hydrogen (71 g H2 l−1 ). This is partly due to the fact that saline ternary metal hydrides tend to be denser than the weighted mixtures of the corresponding binary hydrides (see V values in Table 1). This contraction presumably originates from packing effects. It is reflected by a general shortening of the metal–hydrogen bonds [12] and can be discussed in terms of atomic volume increments [34]. A particularly large volume contraction occurs in the high-pressure phase of Cs2 MgH4 (V = −14%), while volume expansions occur in the (low-pressure) -K2 SO4 -type phases A2 MgH4 (A = Rb: V = +3.8, A = Cs: V = +3.9%) which are presumably due to the relatively large space requirement of the tetrahedral [MgH4 ]2− units. The exceptionally big volume expansion of EuMg2 H6 (V = +9%) is presumably a consequence of its relatively open structure displaying linear coordinated H atoms. 4.2. Thermal stability The hydrogen dissociation temperatures at atmospheric pressure usually exceed 670 K, i.e. Mg based ternary hydrides are thermally more stable than binary MgH2 (∼550 K). The least stable ternary hydrides known so far are Rb2 MgH4 and Rb3 MgH5 that decompose at 630–640 K. Note that the dehydrogenation reactions usually proceed in several steps. Sr6 Mg7 H26 , for example, decomposes at 700 K and 30 bars hydrogen pressure into SrMgH4 and SrH2 , while Yb4 Mg3 H14 decomposes according to the reaction Yb4 Mg3 H14 → 4YbH2 + 3Mg + 3H2 7 Fig. 4. Pressure-composition isotherms during desorption of Yb4 Mg3 H14 (from [23]). as can be seen from the pressure-composition isotherms shown in Fig. 4. The desorption enthalpy associated with the latter reaction (H = −103 kJ/mol H2 ) is intermediate between those of binary MgH2 (−74 kJ/mol H2 ) and YbH2 (−182 kJ/mol H2 ) [27]. These finding are in line with similar results obtained for complex transition metal hydrides such as CaMgNiH4 , Yb4 Mg4 Fe3 H22 and Ca4 Mg4 Fe3 H22 [35]. The desorption enthalpies of these quaternary hydrides are significantly higher than those of the corresponding ternary hydrides Mg2 NiH4 and Mg2 FeH6 , respectively, thus reflecting a greater strength of Ca–H versus Mg–H bonds. On the other hand, the formation enthalpies of NaMgH3 (H = −154 kJ/mol H2 [29]) and KMgH3 (H = −185 kJ/mol H2 [30]) are considerably larger than those of the corresponding binary A metal hydrides NaH (H = −114 kJ/mol H2 ) and KH (H = −116 kJ/mol H2 ). 4.3. Electronic and magnetic properties Most compounds are colourless or white. Those containing europium are red (EuMg2 H6 ), red-orange (Eu1−x Srx Mg2 H6 ; x ≤ 0.6), yellow-grey (Eu1−x Srx )2 Mg3 H10 ; x > 0.8 or brown (EuMgH4 ), which are typical colours for divalent Eu2+ . Ytterbium containing samples are often grey due to the presence of some unreacted YbH2+x . All hydrides are presumably insulating or semi-conducting and can be considered as saline compounds, in agreement with quantum mechanical calculations [36]. Magnetic properties are known for Eu containing compounds such as Eu2 MgH6 , EuMgH4 and EuMg2 H6 . They show Curie–Weiss paramagnetism and ferromagnetic ordering similar to EuH2 which is a ferromagnetic semi-conductor [37]. Interestingly, as the minimum Eu–Eu distance in these hydrides increases (from 3.72 to 3.93 Å) the Curie temperature decreases (from 31.6 to 18.7 K). The extrapolated magnetic moments (7.5–8.1 B ) are consistent with divalent europium (Eu2+ free ion value: 7.95 B ). The solid solution series Eu1−x Srx Mg2 H6 and (Eu1−x Srx )2 Mg3 H10 show paramagnetic behaviour with magnetic moments close to the free ion value of Eu2+ , and they order ferromagnetically below TC = 21 and 15 K for x = 0.2 and 0.4, respectively [28]. 8 K. Yvon, B. Bertheville / Journal of Alloys and Compounds xxx (2006) xxx–xxx 5. Conclusions and outlook Ternary metal hydrides based on magnesium and A or Ae elements crystallize with a great variety of crystal structures in which the valence and size of the constituents play dominant roles. The compounds have saline character and display numerous structural analogies with ternary fluorides. The calcium and strontium members tend to have ytterbium and europium analogues, respectively. As expected, the hydrogen configurations depend on the size and nature of the metal cations. Relatively small cations such as Mg2+ show low CNs and usually adopt octahedral (CN = 6) and rarely tetrahedral (CN = 4) or pentagonal bipyramidal (CN = 7) H configurations, while large cations such as K+ , Rb+ and Cs+ , or Ca2+ , Sr2+ and Ba2+ (CN = 8–13) show high CNs and usually adopt irregular H configurations. Magnesium clearly occupies a special position within the Ae series. The frequent occurrence and relative stiffness of the MgH6 octahedra is consistent with covalent (directional) contributions to the metal–hydrogen interactions as in binary MgH2 , in contrast to the relatively flexible H configurations around the other metal cations that suggest predominantly ionic (nondirectional) metal–hydrogen interactions. Compared to binary hydrides, the metal–hydrogen bonds in ternary hydrides are generally shortened which is reflected by a volume contraction. Due to the strong polarizability of the hydride anion, these bonds cannot be rationalized in terms of a fixed hydrogen radius. The hydrogen storage efficiencies for A metal compounds reach 6.0 wt.% and 88 g l−1 (NaMgH3 ) and for Ae metal compounds 5.7 wt.% and 99 g l−1 (Ca4 Mg3 H14 ). They generally desorb hydrogen only above 670 K and thus are not suitable for reversible hydrogen storage applications at room temperature. 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