Mg6Ir2H11, a new metal hydride containing saddle-like [IrH4]5− and square-pyramidal [IrH5]4− hydrido complexes

Journal of Alloys and Compounds 340 (2002) 180–188

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Mg 6 Ir 2 H 11 , a new metal hydride containing saddle-like [IrH 4 ] 52 and square-pyramidal [IrH 5 ] 42 hydrido complexes ˇ ´ a , *, J.-M. Joubert a,b , H. Kohlmann a,c , K. Yvon a R. Cerny a ` , 24 Quai E. Ansermet, 1211 Geneve ` 4, Switzerland Laboratoire de Cristallographie, Universite´ de Geneve ´ Laboratoire de Chimie Metallurgique des Terres Rares, CNRS, 2 – 8 rue Henri Dunant, 94320 Thiais Cedex, France c High Pressure Science and Engineering Center, University of Nevada, 4505 South Maryland Parkway, Las Vegas, NV 89154 -4002, USA b

Received 4 January 2002; accepted 18 January 2002

Abstract Mg 6 Ir 2 H 11 has been synthesised by both hydrogenation of the intermetallic compound Mg 3 Ir at 20 bar and |300 8C, and sintering of the elements at |500 8C under 50 bar hydrogen pressure. Neutron powder diffraction on the deuteride indicates a monoclinic structure ˚ b 5 91.00(1)8, T 5 20 8C) that is closely related to (space group P21 /c, Mg 6 Ir 2 D 11 : a510.226(1), b519.234(2), c58.3345(9) A, orthorhombic Mg 6 Co 2 H 11 . It contains a square-pyramidal [IrH 5 ] 42 complex and three saddle-like [IrH 4 ] 52 complexes of which one is ordered and two are disordered. Five hydride anions H 2 are exclusively bonded to magnesium. The compound has a red colour, is presumably non-metallic and decomposes under 3 bar argon at 500 8C into Mg 3 Ir, iridium and a previously unreported intermetallic compound of composition Mg 5 Ir 2 .  2002 Elsevier Science B.V. All rights reserved. Keywords: Hydrogen absorbing materials; Gas–solid reactions; Crystal structure; X-ray diffraction; Neutron diffraction

1. Introduction Magnesium and iridium form three known ternary hydride phases of composition Mg 4 IrH |5 , Mg 2 IrH 5 and Mg 3 IrH |5 [1,2]. The two former are well characterized, do not derive from stable binary metal compounds, and belong to the class of so-called ‘complex metal hydrides’ (for reviews, see Refs. [3–6]). The latter hydride (called ‘red-phase’ [2]) is not well characterised. Although a binary compound of composition Mg 3 Ir is known to exist [7] the hydride Mg 3 IrH |5 was originally synthesised from the elements by sintering. Its possible formation by hydrogenation of Mg 3 Ir has so far not been investigated. This possibility is of some interest because the red colour of the hydride suggests the occurrence of a hydrogeninduced metal-to-non-metal transition. Transitions of this type are of technological interest and common in binary metal–hydrogen systems [8] but not very common in ternary metal–hydrogen systems. The only other welldocumented system showing a hydrogen-induced metal-to-

*Corresponding author. ˇ ´ E-mail address: [email protected] (R. Cerny).

non-metal transition is metallic Mg 2 Ni, which transforms upon hydrogenation into non-metallic brownish-red Mg 2 NiH 4 [9]. The purpose of this work was to investigate the structure and properties of Mg 3 IrH |5 and its relation to Mg 3 Ir in more detail. Mg 3 IrH |5 was originally investigated by standard X-ray powder diffraction and found to crystallise with a monoclinic structure having pseudo-hexagonal cell metric and two transition metal sites [2]. Later, the structure was refined from synchrotron powder diffraction data [10] and described in terms of layers of transition metal centred, edge-sharing magnesium cubes that were connected via common vertices such as in Mg 3 RuH 6 [11] and Mg 6 Co 2 D 11 [12]. The diffraction data displayed significant anisotropic line broadening which was modelled in terms of microtwinning parallel to the layers such as in Mg 2 NiH 4 [13]. The data confirmed the presence of at least two different iridium sites. High-resolution neutron diffraction experiments were performed on the deuteride to localize the hydrogen atoms. In the following the results of this study and of hydrogenation experiments are reported. It will be shown that Mg 3 IrH |5 can be derived from the binary metal compound Mg 3 Ir. Its composition is Mg 6 Ir 2 H 11 , and its monoclinic structure is closely related to the orthorhombic cobalt analogue Mg 6 Co 2 H 11 .

0925-8388 / 02 / $ – see front matter  2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 02 )00050-6

ˇ ´ et al. / Journal of Alloys and Compounds 340 (2002) 180 – 188 R. Cerny

2. Experimental

2.1. Synthesis Five samples were synthesised by various methods from either magnesium lump (Fluka, 99.95%) or magnesium powder (Cerac, 99.6%), iridium powder (Merck, 99.95%), and hydrogen (Carbagas, 99.9999%) or deuterium gas (AGA, 99.8 isotopic purity). One method consisted of first preparing the binary compound Mg 3 Ir and then hydrogenating (deuterating) that compound, while another method consisted of synthesising the hydride directly from the elements by sintering. Intermetallic compounds were prepared from the elements by either sintering pellets of powder mixtures of the pure elements at molar ratios Mg / Ir54 under 1 bar argon at 500 8C for 7 days (sample 1), or by sealing mixtures of magnesium lumps and iridium powder at Mg / Ir53.03 under argon in steel tubes which were then enclosed in silica tubes and held at 1030 8C for four days (sample 2). The excess of magnesium was necessary to compensate for losses due to evaporation. While the hydride sample Mg 3 IrH |5 (sample 3H) was identical to that synthesised previously by sintering the elements at Mg / Ir53 under hydrogen at a pressure of 50 bar and a temperature of 500 8C in an autoclave [2], the deuteride sample (sample 2D) was obtained from sample 2 by reaction in an autoclave under deuterium pressures between 120 and 150 bar and a temperature of 410 8C for 10 days.

2.2. Hydrogenation and dehydrogenation studies The hydride formation (sample 1H) from binary Mg 3 Ir (sample 1) was studied in a conventional Sievert’s volumetric apparatus under 50 bar hydrogen pressure at 400 8C for 10 days. Dehydrogenation of Mg 3 IrH |5 (sample 3H) was studied by using a thermo-gravimetric analyser IGA001 of Hiden Analytical working under 3 bar of argon at 500 8C. Finally, hydrogenation of the already formed hydride Mg 3 IrH |5 (sample 3H) with hydrogen was studied on the same apparatus under 20 bar hydrogen pressure at 300 8C, and by in-situ X-ray powder diffraction in a reaction chamber HRK of PAAR under 1 bar hydrogen pressure between 25 and 450 8C. The purpose of these experiments was to study the possible insertion of additional hydrogen into the structure that would lead to changes of the monoclinic lattice distortion and / or anisotropic line broadening.

2.3. Structure analysis X-ray powder diffraction patterns were recorded for the intermetallic compound Mg 3 Ir (sample 1, Philips PW1820 diffractometer) and the hydride Mg 3 IrH |5 (sample 1H, Bruker D8 diffractometer) using Cu Ka 1,2 radiation. The neutron powder diffraction pattern for the deuteride (sam-

181

ple 2D, mass 2.0 g) was measured on D2B (ILL, Grenoble) ˚ in the high intensity mode (wavelength l 51.594 A, diffraction range 7.58#2u #1628, step size D2u 50.058, measuring time 22 h). In order to reduce neutron absorption (mainly by iridium) the sample ( m 51.69 cm 21 ) was enclosed in a double-walled vanadium container having inner and outer diameters of 7 and 9 mm, respectively. The powder patterns were analysed by the Rietveld refinement method using the programs FULLPROF [14] for the X-ray data and TOPAS [15] for the neutron data. For the X-ray refinements of the binary alloy the parameters of the hexagonal Cu 3 P-type structure (superstructure of the hexagonal Na 3 As structure type) as determined from single ˚ crystal data (space group P63 cm, a57.927, c58.190 A [7]) were used. For the monoclinic hydride and the deuterated alloy the atomic parameters of the metal atoms were taken from a previous synchrotron diffraction study ˚ (space group C2 /m, a58.351, b54.821, c510.247 A, b 5 90.898 [10]). For the neutron data the positions of the deuterium atoms were derived by analysing the structure of the orthorhombic cobalt analogue Mg 6 Co 2 D 11 [12] (see Section 3.2).

3. Results

3.1. Hydrogenation and dehydrogenation The results of the hydrogenation and dehydrogenation studies are summarised in Table 1. The hydride Mg 3 IrH |5 forms both by hydrogenation of the intermetallic compound Mg 3 Ir at 50 bar and |400 8C (sample 1H), and sintering of the elements at |500 8C under 50 bar hydrogen pressure (sample 3H). While both methods lead to hydrides of reddish colour, the sample prepared by the hydrogenation of the alloy had a somewhat darker colour than the sintered sample. Furthermore, long storage times tend to transform the reddish colour into black, although no significant structural changes were apparent in the X-ray powder diffraction patterns. The amount of absorbed hydrogen in the alloy sample was estimated from the decrease of hydrogen pressure in the Sievert’s apparatus and found to correspond to 4(1) hydrogen atoms per formula unit (H / f.u.). This value is consistent with the previously estimated hydrogen content of 5 H / f.u. [2] but lower than the composition Mg 3 IrH 5.5 refined from neutron diffraction data (see Section 3.2). The relatively large error is due to the experimental conditions (high pressure and temperature, relatively long experiment time), which favoured leakage of hydrogen and its diffusion through the stainless steel container. The two hydrogenation experiments on the already formed monoclinic hydride phase Mg 3 IrH 5.5 did not lead to any significant changes in cell parameters and / or line broadening, but yielded some cubic ˚ [2]). On the other hydride phase Mg 2 IrH 5 (a56.658 A

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Table 1 Summary of hydrogenation and dehydrogenation treatments performed on Mg 6 Ir 2 H 11 and Mg 3 Ir Sample Starting product

3H Mg 6 Ir 2 H 11

3H Mg 6 Ir 2 H 11

3H Mg 6 Ir 2 H 11

1 Mg 3 Ir

Treatment

Hydrogenation, in-situ X-ray diffraction, PAAR HRK, 1 bar H 2 , 25→450 8C

Hydrogenation, thermo-gravimetry, Hiden IGA-001, 20 bar H 2 , 300 8C, 2 h

Dehydrogenation, thermo-gravimetry, Hiden IGA-001, 3 bar Ar, 500 8C, 15 h

Hydrogenation, volumetry, Sievert’s apparatus, 50 bar H 2 , 400 8C, 10 days

Products

230–380 8C: appearance of Mg 2 IrH x 400–450 8C: decomposition to Ir and MgO

Mg 6 Ir 2 H 11 precipitation of Mg 2 IrH x

Mg 3 Ir, Mg 5 Ir 2 , Ir and MgO

Mg 6 Ir 2 H 11

hand, dehydrogenation of the hydride sample (sample 3H) on the thermobalance at 500 8C and 3 bar argon pressure yielded binary Mg 3 Ir, elemental Ir, new phase Mg 2 Ir 5 (see Section 3.2) and some MgO impurity phase. Due to sample oxidation the quantification of the removed hydrogen by weight loss was difficult. Dehydrogenation appeared to occur gradually and the reaction was not finished at 500 8C. In view of these observations the desorption temperature at 1 bar hydrogen equilibrium pressure is probably much lower than 500 8C.

3.2. Metal atom structure of Mg3 Ir and its hydride Rietveld refinement results on the X-ray data of sample 1 confirmed the hexagonal Cu 3 P structure type. A comparison between the observed and refined diffraction

patterns is given in Fig. 1. The refined cell parameters ˚ showed a slightly higher (a57.882(1), c58.228(1) A) axial ratio (c /a 5 1.044) compared to that reported (1.033). The sample also contained a small amount of MgO impurity phase, and a secondary phase of composition Mg 5 Ir 2 that was found to crystallize with the hexagonal Al 5 Co 2 -type structure (P63 /mmm [16]). The ˚ was formation of this phase (a58.601(1), c58.145(1) A) confirmed by its synthesis from the elements, and appears to be a newly reported equilibrium phase in the Mg–Ir phase diagram. As expected, hydrogenation of hexagonal Mg 3 Ir (sample 1H) leads to a strong lattice expansion and a monoclinic lattice distortion. Furthermore, the diffraction data showed strong anisotropic line broadening which was modelled according to the results of a previous X-ray synchrotron diffraction study [10]. The results confirmed

Fig. 1. Rietveld plot of Mg 3 Ir (sample 1, R wp 53.7%, x 2 57.8). Observed (dots) and calculated (solid line) diffraction patterns (Cu Ka 1,2 ) are shown with the difference curve below. Ticks indicate the line positions of the main phase Mg 3 Ir (Cu 3 P-type structure, R B 55.35%), the secondary phase Mg 5 Ir 2 and the impurity phase MgO. Inset shows two of the superstructure lines (202 and 211) differentiating the Cu 3 P-type structure from the Na 3 As-type structure.

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Fig. 2. Rietveld plot of hydrogenated Mg 3 Ir (sample 1H, R wp 521.5%, x 2 526.3). Observed (dots) and calculated (solid line) diffraction patterns (Cu Ka 1,2 ) are shown with the difference curve below. Ticks indicate the line positions of the main phase Mg 6 Ir 2 H 11 and the impurity phase MgO. In view of the anisotropic diffraction line broadening of Mg 6 Ir 2 H 11 , the contributions of hkl (h|k), hkl (h.k), hkl (h,k), hk0 and 00l have been separated (from top to bottom).

space group C2 /m for the average metal atom substructure and the existence of two transition metal sites similar to those in the cobalt-based metal hydride Mg 6 Co 2 H 11 [12]. The diffraction patterns are presented in Fig. 2 and show the contributions of the various families of diffraction planes and of the impurity phase MgO to the total intensity.

3.3. Structure of the deuteride Mg3 IrD|5 Based on the neutron powder diffraction data (sample 2D) and the metal atom positions reported in Ref. [10] the structure of the deuterated phase was initially refined in space group C2 /m (a58.339(1), b54.8094(7), c5 10.229(2), b 5 91.03(1)8). Six deuterium sites were identified (two of type 8j and four of type 4i) of which four were octahedrally arranged around two iridium sites and two were surrounded by magnesium sites only. While the occupancies of the former were around and slightly above 2 / 3, those of the latter were 1 and 1 / 4, thus suggesting an overall deuterium content of Mg 3 IrD |5.5 . This composition is close to, but higher than, those measured by absorption (4 H / f.u., see Section 3.1) and originally estimated (5 H / f.u. [2]). Furthermore, some relatively strong diffraction ˚ remained unexplained by the peaks (d52.70, 1.99 A) structure model. Given the similarities in composition, cell parameters and metal atom substructure between monoclinic Mg 3 IrD |5.5 and orthorhombic Mg 6 Co 2 D 11 (Pnma, ˚ [12]; a Ir phase | a58.112, b510.080, c518.603 A

a Co phase , b Ir phase |1 / 4c Co phase , c Ir phase |b Co phase ) the baxis was quadrupled and the structure parameters of the iridium compound were transformed to the monoclinic space group P21 /b11 (maximal subgroup of the space group Pnma). The new structure model required a splitting of the following transition metal sites (Ir1–Ir1a, Ir2–Ir2a) and their corresponding deuterium neighbour sites (Ir1, D11–D15; Ir1a, D11a–D15a; Ir2, D21–D26; Ir2a, D21a– D26a), while half of the magnesium sites (Mg1–Mg4) and the deuterium sites exclusively surrounded by magnesium (D31–D35) required no splitting. Preliminary structure refinements indicated that some of the deuterium sites that were disordered in the cobalt compound were ordered in the iridium compound, and inversely. In particular, the iridium compound showed no occupancy of the D22, D24 and D24a sites, while the cobalt compound showed no occupancy of the D15a, D16a and D26a sites. In the final refinement the occupancies of the six deuterium sites surrounding Ir1 and Ir1a, respectively, were constrained to be equal, while those surrounding Ir2 and Ir2a were fixed to unity. Apart from nine background and one zero correction parameters the following structural parameters were refined: 126 positional, four isotropic displacement, two occupancy, four lattice parameters and nine profile parameters. Furthermore, for better convergence the following anti-bump restraints (minimal expected interatomic distance) were used: [Ir–D]51.65, [Ir–Mg]52.70, ˚ [D–D]52.10, [D–Mg]51.80 and [Mg–Mg]52.75 A. Refined structure parameters are summarised in Table 2, and the diffraction pattern is shown in Fig. 3.

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Table 2 Results of the crystal structure refinement of the Mg 6 Ir 2 D 11 phase (P21 /c, ˚ b519.234(2) A, ˚ c58.3345(9) A, ˚ b 591.00(1)8) a510.226(1) A, Atom

Site

x

y

z

˚ 2) B (A

N

Ir1 Ir1a Ir2 Ir2a Mg1 Mg2 Mg3 Mg4 Mg5 Mg5a Mg6 Mg6a Mg7 Mg7a Mg8 Mg8a D11 D11a D12 D12a D13 D13a D14 D14a D15 D15a D16 D16a D21 D21a D22a D23 D23a D25 D25a D26 D26a D31 D32 D33 D34 D35

4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e

0.999(2) 0.502(2) 0.003(2) 0.497(2) 0.252(5) 0.250(3) 0.253(4) 0.250(4) 0.069(5) 0.571(4) 0.928(4) 0.422(5) 0.071(5) 0.539(4) 0.952(4) 0.439(5) 0.118(5) 0.629(5) 0.127(5) 0.602(4) 0.090(5) 0.653(4) 0.079(5) 0.596(4) 0.076(5) 0.560(4) 0.912(5) 0.416(5) 0.113(3) 0.596(3) 0.570(3) 0.113(3) 0.611(3) 0.136(3) 0.618(3) 0.902(3) 0.392(3) 0.265(4) 0.240(3) 0.253(4) 0.252(4) 0.249(3)

0.938(1) 0.071(1) 0.188(1) 0.811(1) 0.967(2) 0.065(2) 0.224(1) 0.738(2) 0.059(2) 0.936(2) 0.065(2) 0.950(2) 0.195(2) 0.809(2) 0.185(2) 0.815(2) 0.627(3) 0.378(2) 0.003(2) 0.985(2) 0.124(2) 0.873(2) 0.577(3) 0.451(2) 0.055(2) 0.936(3) 0.994(2) 0.000(2) 0.832(1) 0.128(1) 0.757(2) 0.306(1) 0.684(2) 0.825(1) 0.185(2) 0.879(1) 0.120(2) 0.581(2) 0.315(2) 0.676(2) 0.423(2) 0.438(2)

0.750(2) 0.250(3) 0.768(3) 0.246(3) 0.793(5) 0.161(4) 0.830(4) 0.791(5) 0.598(5) 0.438(5) 0.930(5) 0.110(5) 0.103(6) 0.906(4) 0.401(6) 0.573(5) 0.314(6) 0.672(6) 0.391(7) 0.634(5) 0.345(6) 0.690(7) 0.053(6) 0.926(5) 0.074(6) 0.954(4) 0.183(8) 0.804(5) 0.380(4) 0.678(4) 0.647(4) 0.125(4) 0.906(4) 0.124(4) 0.936(4) 0.177(4) 0.807(4) 0.570(6) 0.617(6) 0.075(5) 0.059(5) 0.438(4)

0.28(9) BIr1 BIr1 BIr1 0.40(8) BMg1 BMg1 BMg1 BMg1 BMg1 BMg1 BMg1 BMg1 BMg1 BMg1 BMg1 0.23(9) BD11 BD11 BD11 BD11 BD11 BD11 BD11 BD11 BD11 BD11 BD11 BD11 BD11 BD11 BD11 BD11 BD11 BD11 BD11 BD11 1.5(1) BD31 BD31 BD31 BD31

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0.69(1) 0.65(1) ND11 ND11a ND11 ND11a ND11 ND11a ND11 ND11a ND11 ND11a 1 1 1 1 1 1 1 1 1 1 1 1 1 1

of which are shown in Fig. 4. Two (Ir1, Ir1a) are disordered and consist of four hydrogen (deuterium) ligands, on average, that occupy six symmetry independent sites in a distorted octahedral configuration (average site occupancy 2 / 3). The structural analogy with the cobalt compound suggests that their local configurations are saddle-like. The two other complexes are ordered. One (Ir2) has four hydrogen (deuterium) ligands in a saddlelike configuration and the other (Ir2a) five ligands in a square-pyramidal configuration. The saddle-like configuration is the first of that type reported for homoleptic iridium hydrido complexes, while a partially disordered squarepyramidal configuration has been reported for the tetragonal low-temperature structure of Sr 2 IrD 5 [17]. The other known (ordered) homoleptic hydrido complexes of iridium are octahedral [IrH 6 ] in Na 3 IrH 6 and Li 3 IrH 6 [18], and in Ba 3 Ir 2 H 12 [19]. Interestingly, the nature of, and the ratio between, the ordered and disordered hydrido complexes differs between the iridium and cobalt compound. While Ir1 and Ir1a centre disordered saddle-like [IrH 4 ] complexes, and Ir2 and Ir2a ordered saddle-like [IrH 4 ] and square-pyramidal [IrH 5 ] complexes, respectively, Co1 centres an ordered saddlelike [CoD 4 ] complex and Co2 an average between (disordered) saddle-like [CoH 4 ] and square-pyramidal [CoH 5 ] complexes. These differences are presumably related to the symmetry lowering of the iridium compound compared to the cobalt compound, and the different formation mechanisms of the two metal hydrides. On the other hand, the molar ratio between the saddle-like and square-pyramidal configurations (3:1) and the number of anionic deuterium sites (see D31–D35 in Fig. 5) are equal in both compounds. Thus Mg 6 Ir 2 H 11 can be rationalised by the limiting ionic formula 4Mg 6 Ir 2 H 11 5 5MgH 2 ? 19Mg 21 ? 2[IrH 5 ] 42 ? 6[IrH 4 ] 52

Numbering of atoms is conserved as it is in the paper on the Mg 6 Co 2 D 11 phase [12]. The letter a indicates a split position from a position in Mg 6 Co 2 D 11 .

i.e. all hydrido complexes conform to the 18-electron rule [4]. The absence of noticeable changes in cell parameters and / or anisotropic diffraction line broadening during reaction with additional hydrogen supports the view that the hydride is stoichiometric.

4. Discussion

4.2. Bond distances

4.1. Composition and structure

The refined iridium–deuterium bond distances (see Table 3) are consistent with those generally expected for transition metal–hydrido complexes [4]. In the ordered 52 saddle-like [IrH 4 ] configuration around Ir2 they are in ˚ (average 1.69 A) ˚ and in the the range Ir–D51.68–1.72 A 42 ordered square-pyramidal [IrH 5 ] configuration around ˚ (average 1.76 A). ˚ This Ir2a in the range 1.67–1.92 A compares favourably with the average bond length of 1.72 ˚ in the disordered square-pyramidal [IrH 5 ] 42 complex in A Sr 2 IrD 5 at 4.2 K [17]. In the disordered saddle-like

The Mg 3 IrH |5 phase has a refined composition of Mg 6 Ir 2 H 11 , and its structure is a monoclinic derivative of orthorhombic Mg 6 Co 2 H 11 [12]. Both hydrides contain mixtures of transition metal hydrido complexes and hydride anions and can be classified as complex metal hydrides [3–6]. In contrast to the cobalt compound that contains only two symmetry independent hydrido complexes, the iridium compound contains four, the geometries

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Fig. 3. Rietveld plot of Mg 6 Ir 2 D 11 (sample 2D, R wp 59.2%, R B 52.0%, x 2 51.65). Observed (dots) and calculated (solid line) diffraction patterns ˚ are shown with difference curve below. Ticks indicate line positions. (neutrons, l 5 1.594 A)

configurations around Ir1 and Ir1a they are longer (1.67– ˚ average 1.80 A). ˚ 2.07 A; By comparison, the Ir–D distances in octahedral [IrH 6 ] 32 complexes are 1.60 and ˚ (average 1.68 A ˚ [18]). The anionic deuterium 1.76 A

Fig. 4. Atomic environments of iridium in monoclinic Mg 6 Ir 2 D 11 viewed approximately along the c-axis. Ir1 and Ir1a center saddle-like complexes [IrH 4 ], which are disordered, and Ir2 and Ir2a center saddle-like [IrH 4 ] and square-pyramidal [IrH 5 ] complexes, respectively, which are ordered. (+) Ir, (s) Mg, (d) D. Atom numbering according to Table 2. The figure corresponds to Fig. 2 in Ref. [12].

Fig. 5. One layer of magnesium cubes and anionic hydrogen positions D31–D35 in Mg 6 Ir 2 D 11 viewed along the a-axis. The cubes are centred at x50. The cell outlines are also drawn. The figure corresponds to Fig. 4 in Ref. [12].

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Table 3 Selected bond distances in Mg 6 Ir 2 D 11 (estimated standard deviations in parentheses) Ir1–

D15 D13 D16 D11 D14 D12 Mg1 Mg2 Mg8 Mg6 Mg5 Mg7 Mg6 Mg5

1.68(1) 1.68(1) 1.69(2) 1.81(1) 1.87(1) 2.07(2) 2.64(1) 2.67(1) 2.72(2) 2.75(1) 2.76(1) 2.92(2) 2.97(1) 2.97(1)

Ir1a–

D16a D11a D12a D15a D14a D13a Mg2 Mg7a Mg1 Mg5a Mg8a Mg6a Mg5a Mg6a

1.68(1) 1.78(1) 1.79(1) 1.80(1) 1.80(1) 1.97(2) 2.65(1) 2.68(1) 2.69(1) 2.71(2) 2.72(1) 2.72(1) 3.12(2) 3.15(2)

Ir2–

D23 D26 D25 D21 Mg3 Mg8 Mg7 Mg4 Mg6 Mg7 Mg5 Mg8 Mg6a

1.68(1) 1.68(1) 1.69(1) 1.72(1) 2.69(2) 2.74(1) 2.75(1) 2.77(1) 2.83(1) 2.87(1) 2.93(2) 3.09(2) 3.01(2)

Ir2a–

D21a D22a D23a D26a D25a Mg7a Mg3 Mg4 Mg8a Mg7a Mg8a Mg5a

1.67(1) 1.70(1) 1.74(1) 1.79(2) 1.92(2) 2.69(1) 2.72(1) 2.75(1) 2.81(1) 2.87(1) 2.87(1) 2.98(2)

D31–

Mg6 Mg6a Mg1 Mg2

1.85(1) 1.88(1) 2.49(2) 2.92(2)

D32–

Mg7 Mg7a Mg2 Mg3

1.85(1) 2.18(1) 2.33(2) 2.50(2)

D33–

Mg8a Mg8 Mg4 Mg4

1.95(1) 2.09(2) 2.46(2) 2.65(1)

D34–

Mg5a Mg5 Mg1 Mg3

1.85(1) 1.92(2) 2.88(2) 3.33(2)

D35–

Mg2 Mg1 Mg3

1.87(1) 2.20(1) 3.24(2)

atoms (i.e. those not bonded to the transition element) have magnesium coordinations similar to those in Mg 6 Co 2 D 11 , i.e. D31–D34 are coordinated by four atoms in a saddlelike, and D35 by three atoms in a triangular, coordination (for a structural drawing, see Fig. 5 in Ref. [12]). As expected from atomic size considerations (matrix effect) the Mg–D distances in the iridium compound are somewhat longer than those in the cobalt compound, but are in ˚ Note that the precision of the usual range (2.79–3.18 A). the distances in the Mg 6 Ir 2 D 11 structure is relatively low ˚ This is presumably due to the (typically 0.01–0.02 A). complexity of the structure, the strong diffraction line overlap due to pseudo-symmetry, and the anisotropic diffraction line broadening due to microtwinning.

4.3. Related metal atom frameworks and symmetry considerations The transition metal complexes in Mg 6 Ir 2 D 11 are sur-

rounded by deformed magnesium cubes that are connected by common corners and edges to a three-dimensional framework. This framework is also found in other transition metal (T) hydrides of composition Mg 3 TH x (T5Re, Ru, Co; see Fig. 26 in Ref. [4]). It can be derived from the hexagonal aristotype Mg 3 ReD 7 [20] (or Na 3 As) according to crystallographic group–subgroup relationships, as shown in Fig. 6. While orthorhombic Mg 3 RuD 6 is derived 52 by substituting the (ordered) octahedral [ReD 6 ] complex 52 by a partially disordered [RuD 6*5 / 6 ] complex, orthorhombic Mg 6 Co 2 D 11 and monoclinic Mg 6 Ir 2 D 11 derive from Mg 3 RuD 6 by ordered (or partially ordered) substitutions of the [RuD 6*5 / 6 ] 52 complex by hydrido complexes of various compositions and geometries and by adding an additional D 2 anion (D35). These structural relationships show that the Na 3 As-type metal framework is quite flexible with respect to hydrogen-to-metal ratio, lattice symmetry and valence electron count and thus may allow other complex metal hydrides to be stabilized. As to the temperature dependence of the lattice symmetry and hydrogen ordering monoclinic Mg 6 Ir 2 H 11 is likely to transform into a more disordered structure of higher symmetry at high temperature. The existence of a monoclinic-to-cubic order–disorder transition associated with microtwinning in the complex metal hydride Mg 2 NiH 4 [21] supports this view.

4.4. Reversibility of hydrogenation Mg 6 Ir 2 H 11 can be obtained by hydrogenation of the intermetallic compound Mg 3 Ir (400 8C, 50 bar) and transformed by dehydrogenation into the latter. Thus the hydride can be considered as ‘reversible’ in the usual sense, in contrast to its cobalt analogue for which no intermetallic compound of composition Mg 3 Co exists. Although the metal atom substructure of the hydride appears to differ significantly from the binary metal structure, both are in fact closely related. They derive from the hexagonal Na 3 As type according to the cell parameter relations a m 5 c Na 3 As , b m 5 4a Na 3 As , c m 5 œ3a Na 3 As , b 5 908 (monoclinic Mg 6 Ir 2 H 11 ; pseudo-hexagonal cell) and a h 5 œ3a Na 3 As , c h 5 c Na 3 As (hexagonal Mg 3 Ir) (see Fig. 6), and can be transformed into each other by atomic ˚ as shown in Fig. 7. Hydrogendisplacements of |2 A induced atom displacements of that magnitude are rare in ‘reversible’ metal hydrides, i.e. those that derive from stable binary (or ternary) metal compounds. The only other well-documented example appears to be Mg 2 NiH 4 . The hydride derives from intermetallic Mg 2 Ni by similar large atomic displacements and contains metal–hydrido complexes (tetrahedral [NiH 4 ] 42 [22]) that obey the 18-electron rule. The reconstructive transition in that system leads from a metallic, optically opaque state to a non-metallic coloured state [23]. It proceeds at lower temperature (200 8C, 120 bar of H 2 ) and shows better reversibility and faster kinetics than in the iridium system, for which reproducible

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Fig. 6. Crystallographic group–subgroup relationships between the crystal structures of Mg 3 ReD 7 , Mg 3 RuD 6 , Mg 6 Co 2 D 11 , and Mg 3 Ir and Mg 6 Ir 2 D 11 . The symbols t, k and i stand for the translationengleiche, klassengleiche and isomorphic subgroups. The number after the symbol indicates the subgroup index.

reactions are more difficult to achieve due to the narrower temperature and pressure ranges for hydrogenation and the appearance of the competing cubic Mg 2 IrH 5 phase. Interestingly, the volume changes during hydrogenation in both systems are relatively high (140% for Mg 6 Ir 2 H 11 , 132% for monoclinic Mg 2 NiH 4 ) compared to conventional intermetallic hydrides (125% for LaNi 5 D 6.7 [24]). This is presumably due to the important space requirement of the relatively rigid metal–hydrido complexes in complex metal hydrides.

5. Conclusions Mg 6 Ir 2 H 11 is the third known ternary metal hydride in the Mg–Ir–H system. Its monoclinic structure is closely related to the orthorhombic cobalt analogue Mg 6 Co 2 H 11 and has 126 free positional parameters. It is the most complex metal hydride structure solved so far by powder diffraction methods. Mg 6 Ir 2 H 11 is a complex metal hydride, and contains four symmetry independent iridium hydrido complexes and five hydride anions (the only other

known cases being K 2 ReH 9 and Mg 6 Co 2 D 11 containing two complexes each). In contrast to Mg 4 IrH |5 and Mg 2 IrH 5 , it derives from a stable binary metal compound (Mg 3 Ir) and shows non-metallic properties such as colour. The other well-known ternary metal hydride system showing changes in optical properties typical for a hydrogeninduced metal-to-non-metal transition is Mg 2 Ni–H (metallic Mg 2 Ni versus non-metallic, rusty Mg 2 NiH 4 ). Both hydride systems contain well-characterised metal–hydrido complexes, the formation of which triggers a relatively important but reversible rearrangement of the metal atom host structure. In addition, Mg 2 NiH 4 shows a temperatureinduced transition to a disordered high-temperature phase. Whether a similar transition also occurs in Mg 6 Ir 2 H 11 remains to be investigated.

Acknowledgements The authors thank Dr. F. Bonhomme for supplying the hydride sample. This work was supported by the Swiss National Science Foundation and the Swiss Federal Office

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Fig. 7. Atomic plane (010) of the hexagonal structure of Mg 3 Ir (elementary cell in full line). (d) Mg ( 2 1 / 3 , y , 1 / 3), (s) Ir ( y 5 0). The shifts of atoms marked by dotted lines lead to the (010) plane of the monoclinic structure of Mg 6 Ir 2 H 11 (elementary cell in dotted-dashed line) containing the magnesium cubes (dashed) centred by Ir1 or Ir1a ( y | 0.071). Note that only six magnesium atoms from the eight contained in each cube are drawn, the two others being shared between cubes in the b-axis direction. The same relations can be drawn for atomic planes centred at y 5 1 / 3 and y 5 2 / 3 in the Mg 3 Ir cell leading to Ir2 or Ir2a centred cubes and to all the others symmetrically equivalent.

of Energy. The help of Emmanuelle Suard as a local contact for the D2B diffractometer at ILL (Grenoble) is highly appreciated.

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