inorganic compounds Acta Crystallographica Section C
Crystal Structure Communications ISSN 0108-2701
YCu3Al2, an example of an AB5 structure type Karim Kadir,a* Tetsuo Sakai,b Itsuki Ueharab and Lars Erikssona a
Division of Structural Chemistry, Arrhenius Laboratory, Stockholm University, S-106 91 Stockholm, Sweden, and bOsaka National Research Institute, Midorigaoka 1-8-31, Ikeda-shi, Osaka 563, Japan Correspondence e-mail:
[email protected] Received 11 June 2001 Accepted 18 June 2001
Yttrium tricopper dialuminium, YCu3Al2, is isostructural with hexagonal CaCu5, in which each Cu atom at the 3g(12, 0, 12) position in space group P6/mmm (No. 191) is partially replaced by an Al atom. The hydrogen-uptake properties are usually enhanced in other AB5 structures by aluminium substitution. YCu5 does not show any hydrogen absorption, and the goal of the present work is to investigate whether aluminium substitution could expand the metal-atom lattice enough to provide better interstitial positions for hydrogen storage. However, no enthalpy change was observed up to 773 K under 3 MPa static H2 pressure by differential thermal analysis (DTA) for the title compound. The compound does not show any signi®cant hydrogen absorption/desorption in the pressure-composition isotherms (P±C±T diagrams) in the temperature range 298±673 K under 3.3 MPa H2 pressure.
Figure 1
Comment Previous investigations have shown that the light rare-earth elements, and incidentally also some heavy rare-earth elements, form hexagonal compounds of the AB5 system (CaCu5 type), as reported in the literature (Dwight, 1961; Wernick & Geller, 1959; Haszko, 1960). The stoichiometric composition of these hexagonal phases does not seem well established, as some rare-earth elements have the CaCu5 structure in compounds of composition RCu4 as well as in compounds of composition RCu5 (Gschneidner, 1961). Studies of the intermetallic AB5 system (CaCu5 type) has shown that several properties, e.g. the hydrogen-absorption properties of the phases, can be easily varied over a wide range by the partial replacement of A or B atoms by other metals (Lanker et al., 1982; Van Vucht et al., 1970). Relatively large differences in the metallic radii of the A and B metals favors the stability of the AB5 phases, making them so-called `line compounds' in their binary-phase diagrams. This stability may be essential in order to obtain a homogeneous composition of the intermetallic compound. Acta Cryst. (2001). C57, 999±1000
Pure RCu5 is not stable and crystallizes in the BaAl4-type structure. It is noted that aluminium substitution stabilizes the hexagonal CaCu5-type structure for RCu4Al compounds (R = La±Sm; Takeshita et al., 1978). The investigation of the present compound is part of a larger project aimed at ®nding alternative intermetallic AB5 compounds for hydrogen absorption, based on both elements lighter than lanthanides (for example, R = Y) and cheap elements (for example, substitution of a B atom in RB5-type structures by Cu and Al). YCu5 and YNi5 did not show any hydrogen uptake and it was therefore of interest to study the in¯uence of metal-atom replacement in YCu5 (Wernick & Geller, 1959; Buschow & Goot, 1971) by aluminium, as the atomic radius of Al Ê ) is larger than that of Cu (1.278 A Ê ; Teatum et al., (1.432 A 1960), thus expanding the metal-atom lattice and possibly also providing interstitial positions for hydrogen. The YCu5 structure (Wernick & Geller, 1959) is built from alternate layers of the CaCu5 type, viz. each Y atom is Ê ) and by surrounded by six Cu atoms in one plane (at 2.88 A Ê ). two further sets of six Cu atoms in adjacent planes (at 3.23 A
Stereoview of the unit-cell contents with ellipsoids plotted at the 90% probability level. Position M is a mixed position of Cu2 (31%) and Al2 (69%).
The same planes exist in YCu3Al2, but at slightly different distances (Fig. 1). The average nearest-neighbour Cu1ÐCu2 distances are increased compared with those of YCu5 (2.49 Ê ; Wernick & Geller, 1959). The CuÐCu distances in and 2.51 A the present compound are close to those of a normal closepacked Cu atom with coordination number (CN) 12 (average Ê ; Wernick & Geller, 1959). nearest-neighbor distance = 2.56 A This compound does not show any hydrogen absorption/ desorption in the pressure-composition isotherms (P±C±T diagrams) in the temperature range 298±673 K under 3.3 MPa H2 pressure using an automated Sieverts-type apparatus. No enthalpy change was observed up to 773 K under 3 MPa static H2 pressure by differential thermal analysis (DTA) for the present compound. No disproportion of the alloy was observed by X-ray diffraction after DTA and P±C±T.
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999
inorganic compounds Experimental
Table 1
The purity of the starting materials was 99.99% for Cu and Al, and 99.9% for Y. A pressed tablet of a mixture of suitable weight% ratio of ®nely powdered starting materials was sintered at 773 K under 0.6 MPa argon-gas pressure for 1 h, followed by higher temperature annealing at 1023 K for 1 h, with subsequent rapid cooling to room temperature. The sample was crushed and reheated directly to 1023 K under the same conditions as above, followed by cooling at 20 K minÿ1 to 673 K and then rapid cooling to room temperature. Upon optical microscopic examination, the alloy exhibited regular metallic hexagons. Some of the crystallites were isolated and crushed to powder form for identi®cation by Guinier±HaÈgg X-ray powder diffraction. The atomic composition was veri®ed by EDX (energy dispersive X-ray) analysis with a Jeol 820 scanning electron microscope equipped with a LINK elemental analysis system. Crystal data YCu3Al2 Mr = 332.03 Hexagonal, P6/mmm Ê a = 5.172 (3) A Ê c = 4.141 (2) A Ê3 V = 95.93 (9) A Z=1 Dx = 5.747 Mg mÿ3
Mo K radiation Cell parameters from 50 re¯ections = 18±22 = 31.38 mmÿ1 T = 293 (2) K Metallic prism, grey 0.29 0.07 0.06 mm
Data collection Stoe AED-2 diffractometer ±2 scans Absorption correction: multi-scan (XABS2; Parkin et al., 1995) Tmin = 0.062, Tmax = 0.152 2818 measured re¯ections 196 independent re¯ections 182 re¯ections with I > 2(I)
Rint = 0.099 max = 44.9 h = ÿ10 ! 10 k = ÿ10 ! 10 l = ÿ8 ! 7 4 standard re¯ections frequency: 120 min intensity decay: 2(F 2)] = 0.021 wR(F 2) = 0.055 S = 1.38 196 re¯ections 10 parameters
w = 1/[ 2(Fo2) + (0.02P)2] where P = (Fo2 + 2Fc2)/3 (/)max < 0.001 Ê ÿ3 max = 1.86 e A Ê ÿ3 min = ÿ2.21 e A Extinction correction: SHELXL97 Extinction coef®cient: 0.167 (16)
Ê ÿ3 at (0, 0, The highest peak in the residual density map, 1.86 e A Ê 0.1241), is 0.51 A from the Y-atom position. The absorption correc-
1000
Kadir Karim et al.
YCu3Al2
Ê ). Selected bond lengths (A YÐCu1 YÐCu2
2.9861 (17) 3.3128 (13)
Cu1ÐCu2i Cu2ÐCu2ii
2.5527 (10) 2.5860 (15)
Symmetry codes: (i) ÿy; x ÿ y; z; (ii) 1 ÿ y; x ÿ y; z.
tion was carried out with XABS2 (Parkin et al., 1995) using re¯ection data from the whole sphere. Data collection: AED-2 (Stoe & Cie, 1988); cell re®nement: DIF4 (Stoe & Cie, 1988); data reduction: REDU4 (Stoe & Cie, 1988); program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to re®ne structure: SHELXL97 (Sheldrick, 1997); molecular graphics: DIAMOND (Bergerhoff, 1996).
This work has received support from the Japanese organization New Energy and Industrial Technology Development Organization (NEDO). Supplementary data for this paper are available from the IUCr electronic archives (Reference: BR1341). Services for accessing these data are described at the back of the journal.
References Bergerhoff, G. (1996). DIAMOND. Gerhard-Domagkstraûe 1, 53121 Bonn, Germany. Buschow, K. H. J. & van der Goot, A. S. (1971). Acta Cryst. B27, 1085±1088. Dwight, A. E. (1961). Trans. Am. Soc. Met. 53, 479±500. Gschneidner, K. A. (1961). Rare Earth Alloys. New York: Van Notsrand. Haszko, S. E. (1960). Trans. Metall. Soc. AIME, 218, 763. Lanker, J. F., Uribe, F. S. & Steward, S. A. (1982). J. Less Common Met. 72, 87± 105. Parkin, S., Moezzi, B. & Hope, H. (1995). J. Appl. Cryst. 28, 53±56. Sheldrick, G. M. (1990). Acta Cryst. A46, 467±473. Sheldrick, G. M. (1997). SHELXL97. University of GoÈttingen, Germany. Stoe & Cie (1988). AED-2 (Version of July 1988), DIF4 (Version 7.04) and REDU4 (Version 6.2). Stoe & Cie GmbH, Darmstadt, Germany. Takeshita, T., Malik, S. K. & Wallace, W. E. (1978). J. Solid State Chem. 23, 225±229. Teatum, E., Gschneidner, K. A. Jr & Waaber, J. (1960). Los Alamos Technical Report No. LA-2345. Los Alamos National Laboratory, New Mexico, USA; http://lib-www.lanl.gov/la-pubs/00320829.pdf. Van Vucht, J. H. N., Kuijpers, F. A. & Bruning, H. C. A. M. (1970). Philips Res. Rep. 25, 133±140. Wernick, J. H. & Geller, S. (1959). Acta Cryst. 12, 662±665.
Acta Cryst. (2001). C57, 999±1000
supporting information
supporting information Acta Cryst. (2001). C57, 999-1000
[doi:10.1107/S0108270101010009]
YCu3Al2, an example of an AB5 structure type Karim Kadir, Tetsuo Sakai, Itsuki Uehara and Lars Eriksson Computing details Data collection: DIF4 (Stoe & Cie, 1988); cell refinement: DIF4; data reduction: REDU4 (Stoe & Cie, 1988); program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: DIAMOND (Bergerhoff, 1996). Yttrium tricopper dialuminium Crystal data YCu3Al2 Mr = 332.03 Hexagonal, P6/mmm a = 5.172 (3) Å c = 4.141 (2) Å V = 95.93 (9) Å3 Z=1 F(000) = 151
Dx = 5.747 Mg m−3 Mo Kα radiation, λ = 0.71073 Å Cell parameters from 50 reflections θ = 18–22° µ = 31.38 mm−1 T = 293 K Metallic prism, grey 0.29 × 0.07 × 0.06 mm
Data collection Stoe AED2 diffractometer Radiation source: fine-focus sealed tube Graphite monochromator θ–2θ scans Absorption correction: multi-scan (XABS2; Parkin et al., 1995) Tmin = 0.062, Tmax = 0.152 2818 measured reflections
196 independent reflections 182 reflections with I > 2σ(I) Rint = 0.099 θmax = 44.9°, θmin = 4.6° h = −10→10 k = −10→10 l = −8→7 4 standard reflections every 120 min intensity decay: 2σ(F2)] = 0.021 wR(F2) = 0.055 S = 1.38 196 reflections 10 parameters 0 restraints
Acta Cryst. (2001). C57, 999-1000
w = 1/[σ2(Fo2) + (0.020P)2] where P = (Fo2 + 2Fc2)/3 (Δ/σ)max < 0.001 Δρmax = 1.86 e Å−3 Δρmin = −2.21 e Å−3 Extinction correction: SHELXL97, Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4 Extinction coefficient: 0.167 (16)
sup-1
supporting information Special details Experimental. This residual peak is most probably due to errors in the low-angle reflections. Two refinements using reflection data with 2θ > 40° and one with 2θ > 60° did not give any high residual peaks. The high internal R value is most probably an effect of the slightly irregular crystal shape together with a very high absorption coefficient. Heavily absorbing crystals give contributions to the reflection intensities proportional to the respective surface areas exposed to the X-ray beam. The internal R values calculated from the data with 2θ > 40° and the data with 2θ > 60° were not significantly improved. At present, our absorption-correction programs work on the principle of correcting the intensities of the reflections transmitted through the crystal. Attempts to use the observed shape of the crystal and analytical corrections with the use of this shape failed. Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger. Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2)
Y Cu1 Cu2 Al2
x
y
z
Uiso*/Ueq
Occ. (
