Crystal structure of the Al2CuIr phase

Journal of Alloys and Compounds 496 (2010) 208–211

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Crystal structure of the Al2 CuIr phase L. Meshi a,∗ , V. Ezersky a , D. Kapush b , B. Grushko c a

Department of Materials Engineering, Ben-Gurion University of the Negev, POB 653, Beer-Sheva 84105, Israel I.N. Frantsevich Institute for Problems of Materials Science, 03680 Kyiv 142, Ukraine c Institut für Festkörperforschung, Forschungszentrum Jülich, D-52425 Jülich, Germany b

a r t i c l e

i n f o

Article history: Received 16 December 2009 Received in revised form 11 February 2010 Accepted 16 February 2010 Available online 23 February 2010 Keywords: Intermetallics Crystal structure and symmetry Electron diffraction X-ray powder diffraction

a b s t r a c t A new ternary Al2 CuIr phase was revealed in the Al–Cu–Ir system. It is formed below 1063 ◦ C from the ␤-phase (CsCl-type structure) extending at elevated temperatures from AlIr. The crystal structure of the Al2 CuIr phase was determined using a combination of precession electron diffraction and X-ray powder diffraction techniques. The phase has an orthorhombic C-centered unit cell with lattice parameters a = 8.1196(7) Å, b = 5.0646(2) Å and c = 5.18513(3) Å; its crystal symmetry can be described by the Cmme (no. 67) space group (Pearson symbol oC16). The unit cell of the new phase contains 8 Al, 4 Cu and 4 Ir atoms and exhibits a new structure type. The reliability factors characterizing the Rietveld refinement procedure are: Rp = 4.45%, Rwp = 6.45%, RB = 3.69% and Rf = 2.41%. © 2010 Elsevier B.V. All rights reserved.

1. Introduction As part of preliminary studies of the Al-rich part of the Al–Cu–Ir alloy system [1,2], the existence of four ternary phases and significant solubility of Cu in several binary Al–Ir phases was revealed (for the updated Al–Ir phase diagram see ref. [3]). For example, it was found that the congruent AlIr ␤-phase can dissolve up to ∼30 at.% Cu at practically constant Al concentration. This fact agrees with the information found in the literature regarding many other Al–Cu–TM (where TM is a transition element) ternary alloy systems [4]. More detailed investigation of the Al–Cu–Ir phase diagram in [5] showed that an extremely high-Cu concentration of the ␤-phase is only typical of elevated temperatures and below ∼1063 ◦ C a new phase, with an approximate composition of Al50 Cu25 Ir25 , is formed. Since the diffraction patterns taken from the particles of this new phase could not be indexed in terms of any structure related to this system, full solution of the structure of this phase was performed using a combination of electron diffraction and powder X-ray diffraction methods.

ing, the samples were water quenched. The materials were examined by powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The local phase compositions were determined in SEM by energy-dispersive X-ray analysis (EDX) on polished samples. Pieces of the annealed material were crushed into powder in an agate mortar, and the powder samples were analyzed by the X-ray diffraction method. The X-ray powder diffraction data were collected on a Huber Imaging Plate camera installed on an Ultrax 18-Rigaku X-ray rotating Mo anode source, with a monochromator (focal length B = 360 mm) providing pure K␣1 radiation and operating at 40 kV, 90 mA. The FULLPROF program [6] was used both for the extraction of the structure factors from the observed diffracted intensities by the Le Bail method [7] and for the final Rietveld refinement of the structure. Positions of Ir, Cu and Al atoms for the starting structural model were determined by direct methods using the SHELX-97 program [8,9] Transmission electron microscopy and precession electron diffraction experiments were carried out on a 200 kV JEOL FasTEM-2010 electron microscope equipped with an energy-dispersive X-ray spectrometer (NORAN) and Spinning Star Precessing Unit (Nanomegas). Precession Electron Diffraction patterns were taken with a nearly parallel beam and the degree of precession was in the range of 18.5–46.6 mrad. Images and diffraction patterns were recorded by a Gatan slowscan digital camera. The TEM study was performed on powdered material dispersed on Cu grids with amorphous carbon film.

3. Results 2. Materials and methods Alloys of ∼3 g were produced by levitation induction melting in a water-cooled copper crucible under a pure Ar atmosphere. The purity of Al was 99.999%, of Cu 99.95% and of Ir 99.9%. Parts of the solidified ingots were thermally annealed under an Ar atmosphere at 1100 ◦ C for 65 h to 800 ◦ C for up to 300 h. After anneal-

∗ Corresponding author. Tel.: +972 8 6472576; fax: +972 8 6472946. E-mail address: [email protected] (L. Meshi). 0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2010.02.129

The characterization of alloys with an approximate composition of Al50 Cu25 Ir25 , which were annealed at 900–1000 ◦ C, showed that the new compound is formed in a small compositional region. This fact allowed us to designate the new phase as Al2 CuIr. It was found that by heating it transforms congruently to the ␤-phase at ∼1063 ◦ C and is in equilibrium with the low-Cu branch of the ␤phase down to at least 800 ◦ C and with the high-Cu branch of the ␤-phase down to ∼1000 ◦ C [5].

L. Meshi et al. / Journal of Alloys and Compounds 496 (2010) 208–211

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Fig. 1. Precession electron diffraction patterns taken from the particles of the Al2 CuIr phase along: (a) [0 1 0], (b) [0 0 1] and (c) [0 2¯ 1] orientations. The patterns were obtained with a precession angle of approximately 2◦ (34.9 mrad). The mirror planes are labeled by m. (d) Selected area electron diffraction patterns taken from the same particle at [1 1¯ 0] orientation. ER denotes extra reflections appearing due to double diffraction phenomenon.

The alloy selected for structural study contained essentially single ternary Al2 CuIr phase with only a small fraction of the binary AlIr phase. The chemical composition of these phases was checked by EDX in SEM and TEM. The structure of the Al2 CuIr phase was determined by applying standard structure determination methodology to the data retrieved from electron diffraction patterns and X-ray powder diffractograms. In order to determine the unit cell geometry, a series of precession electron diffraction patterns (PED) of different orientations with large angular separations was recorded. Then three diffraction zones of the highest symmetry with a characteristic rectangular net of reflections were selected so, that each pair of patterns had a common side of the rectangular basis. These patterns were ascribed to the [1 0 0], [0 1 0] and [0 0 1] directions of the orthorhombic crystal lattice. Fig. 1a–b shows PED patterns taken from the Al2 CuIr phase at [0 1 0] and [0 0 1] orientations, respectively. The values of the unit cell parameters were estimated from the PED patterns as: a = 8.15 Å, b = 5.09 Å and c = 5.2 Å within an accuracy of approximately ±0.04 Å. In the terms of this unit cell a successful indexing of all observed diffraction zones was performed thus indicating that the dimensions of the unit cell are correct. The interplanar spacings measured from the different electron diffraction patterns were in good agreement with the interplanar distances observed in the corresponding X-ray diffractogram. The latter are represented in Table 1. Using the data in Table 1 the lattice parameters were refined by the least-squares fitting procedure and more precise unit cell dimensions were obtained as: a = 8.1196(7) Å, b = 5.0646(2) Å and c = 5.18513(3) Å. A detailed analysis of the electron diffraction patterns and Xray diffraction peaks revealed the following reflection conditions: for (h k l) reflections h + k = 2n (indicating that the Bravais lattice is of the C-type); for (0 k l) reflections k = 2n, for (h 0 l) reflections h = 2n, for (h k 0) reflections h, k = 2n, for (h 0 0) reflection h = 2n and for (0 k 0) reflections k = 2n. For reflections of the (0 0 l) type no restrictions were found. However, examination of reflection

conditions only indicates an extinction symbol thus leaving an ambiguity about the precise space group. In our case, the reflection conditions found relate to the C–(ab) extinction symbol (see Table 3.2 in International Tables of Crystallography [10]). Thus the possible space group that can describe the symmetry of the structure of Al2 CuIr phase is either Cmme (no. 67) or Cm2e (no. 39). (Cmma=Cmme, Cm2a=Cm2e, according to the new notation in the Fifth edition of the International Tables of Crystallography). The

Table 1 List of 24 first most intense peaks corresponding to the Al2 CuIr phase as obtained from powder X-ray diffractogram (S.G. Cmme, a = 8.1196(7) Å, b = 5.0646(2) Å and c = 5.18513(3) Å, V = 212.97 Å3 ). No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

(h k l) 200 111 201 020 021 112 202 311 220 400 022 312 003 131 511 132 331 422 223 403 332 114 620 314

dobs (Å)

dcalc (Å)

2 (◦ )

Relative intensity (%)

4.060 3.308 3.196 2.532 2.275 2.220 2.185 2.168 2.148 2.030 1.811 1.756 1.728 1.575 1.482 1.393 1.380 1.351 1.347 1.316 1.254 1.241 1.193 1.139

4.059 3.308 3.196 2.532 2.275 2.219 2.185 2.168 2.148 2.03 1.811 1.756 1.728 1.574 1.482 1.393 1.380 1.351 1.346 1.316 1.254 1.241 1.194 1.139

10.02 12.31 12.74 16.10 17.94 18.39 18.68 18.83 19.00 20.13 22.58 23.31 23.69 26.03 27.70 29.49 29.77 30.43 30.54 31.27 32.87 33.22 34.58 36.28

25.69 100.00 22.15 7.14 9.71 45.90 41.85 30.45 69.50 27.41 21.87 21.54 5.48 11.26 9.23 10.77 7.53 14.07 12.92 6.36 6.94 10.88 9.10 7.35

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L. Meshi et al. / Journal of Alloys and Compounds 496 (2010) 208–211

Table 2 Crystal data and details of Rietveld refinement of the structure of the Al2 CuIr phase. Parameter

Data

Structure refined Space group Unit cell parameters (Å)

Al2 CuIr Cmme a = 8.1196(7), b = 5.0646(2), c = 5.18513(3) 4 AlIr-fraction of 4%

Number of formula units Additional phases generating diffraction peaks X-ray data range (2ϑ◦ ) Peak profile Half-width parameters

5–70 Pseudo-Voigt ( = 0.97683) U = 0.40558, V = −0.27195, W = 0.06673 P1 = 0.12123, P2 = 0.03657 309 23 Rp = 4.45, Rwp = 6.45, RB = 3.69 and Rf = 2.41

Asymmetry parameters Total number of reflections Number of refined parameters Reliability factors [in %]

difference between these two space groups is in the point groups to which they correspond: Cmme space group belongs to the mmm point group and Cm2e – to the m2m point group. Since electron diffraction patterns corresponding to the new phase were taken using the precession technique – the symmetry of these patterns is close to ideal [11,12]. Examination of the symmetry of the [0 1 0], [0 0 1], [0 v w] and [u 0 w] PED patterns can provide the correct assignment of the point group [12,13]. It can be seen from Fig. 1a–d that the ideal symmetry of all [0 1 0], [0 0 1], [0 2¯ 1] and [1 1¯ 0] Zero Order Laue Zone (ZOLZ) patterns can be unambiguously characterized as 2 mm. Therefore, it may be deduced that among the possible mmm and m2m point groups, only mmm is appropriate. Thus, the correct space group describing the symmetry of Al2 CuIr phase is Cmme (no. 67). The crystallographic data describing the AlIr and Al2 CuIr compounds were entered into the FULLPROF program [6], and a constraint was applied to the cell parameters and the space group of the Al2 CuIr phase in order to extract the “observed” structure factors according to Le Bail’s method [7]. In the extraction procedure a pseudo-Voigt function [13] was used for modeling a peak shape (with the mixing factor ), and the angular dependence of the peak width (FWHM) was presented by the Cagliotti function [14]. The extracted structure factors were obtained with the quality of profile fitting Rp = 5.38% and Rwp = 7.57%. Since the calculated unit cell volume for the Al2 CuIr structure was 212.97 Å3 , the number of atoms in the unit cell was estimated as 16 (assuming that the mean atomic volume is ∼13 Å3 , similar to that in AlIr). For the Al2 CuIr formula, the unit cell should consist of 8 Al atoms, 4 Cu atoms and 4 Ir atoms. In order to develop the starting structural model direct methods, utilized in the SHELX programs package [8,9], were applied to the above-mentioned data extracted from the X-ray powder diffractogram. The initial model was used for the final Rietveld refinement by FULLPROF leading to agreement factors of: Rp = 4.45%, Rwp = 6.45% and RB = 3.69% (for the Al2 CuIr phase). The details of the Rietveld refinement are summarized in Table 2, and the atomic positions

Fig. 2. Plot of the Rietveld refinement of the Al2 CuIr structure showing: observed Xray profile (open gray circles), calculated profile (solid line) and difference between them (on the bottom). Vertical bars refer to the calculated peak positions of the Al2 CuIr compound (upper bars) and AlIr phase (lower bars).

and thermal displacement factors are presented in Table 3. The resulting calculated and observed X-ray diffraction profiles and the difference between them as obtained from the Rietveld refinement are shown in Fig. 2. 4. Discussion Analysis of the atomic arrangements in the structure of the Al2 CuIr phase reveals that this structure can be represented by interconnected coordination polyhedra formed around heavy atoms (see Fig. 3a). Around each Ir atom a pyramid-shaped polyhedron with 5 Al atoms at the vertices can be found (an isolated Ir coordination polyhedron is shown in Fig. 3b). A polyhedron with

Table 3 Atomic coordinates and isotropic thermal parameters obtained for the Al2 CuIr structure. Atom

Site

x

y

z

B(Å2 )

Ir Cu Al(1) Al(2)

4g 4a 4b 4g

0.5 0.25 0.25 0.5

0.25 0.0000 0.0000 0.75

0.3198(4) 0.0000 0.5 0.1695(2)

0.41(7) 0.73(3) 0.24(3) 0.24(3)

Fig. 3. Crystal structure of the Al2 CuIr phase presented as the stacking of coordination polyhedra formed around heavy atoms: (a) a view along [0 1 0] axis, (b) and (c) isolated coordination polyhedron formed around Ir and Cu atom, respectively. Cu atoms are shown as black balls and Al atoms are shown as gray balls. (d) A view along [1 1 0] direction, showing the packing mode of Ir coordination polyhedra (Cu atoms omitted from the figure).

L. Meshi et al. / Journal of Alloys and Compounds 496 (2010) 208–211 Table 4 List of interatomic distances (in Å) for the Al2 CuIr compound.

5. Conclusions

Cu coordination polyhedra

Ir coordination polyhedra

2Cu (4a) 4Al (4g) 2Al (4b)

1Al (4g) 4Al (4b)

2.53(2) 2.54(9) 2.59(3)

211

2.53(7) 2.56(8)

eight vertices is formed around each Cu atom (an isolated Cu coordination polyhedron is shown in Fig. 3c). This polyhedron consists of 6 Al atoms and 2 Cu atoms. Cu and Al atoms are shown in Fig. 3b and c as black and gray spheres, respectively. Corresponding interatomic distances (see Table 4) are of the same order as the distances found in known binary compounds such as IrAl, IrAl3 , CuAl2 etc. This fact provides additional validation for the correctness of the proposed structure. In the Al2 CuIr structure, the pyramids around Ir are joined in the [1 1 0] direction through the common Al vertices in the base of the pyramid, thus the xy-plane looks like a “chess-board”, in which the bases of the pyramids serve as gray squares and the holes among them serve as white squares, see Fig. 3d in which the structure of the Al2 IrCu phase is represented by the Ir coordination polyhedra at the [0 1 1] orientation, omitting Cu atoms. It is also interesting to analyze the packing mode of the coordination polyhedra surrounding the Cu atoms. In Fig. 3a the effect of a mirror perpendicular to the x axis is evident. The coordination polyhedra around Cu atom are interconnected in a complex way, forming layers of polyhedra parallel to the (0 0 1) plane. Both types of polyhedra found in this structure interconnect in the [1 0 0] direction, forming this interesting structure which represents a new structure type.

The structure of a new ternary Al2 CuIr phase (Cmme, oC16, a = 8.1196(7)Å, b = 5.0646(2)Å and c = 5.18513(3)Å) forming in the Al–Cu–Ir system was determined using a combination of precession electron diffraction (PED) and X-ray powder diffraction techniques. The reliability factors obtained from the final Rietveld structure refinement were: Rp = 4.45%, Rwp = 6.45%, RB = 3.69% and Rf = 2.41%. Acknowledgement We thank M. Schmidt for preparing the alloys. References [1] B. Grushko, D. Pavlyuchkov, Powder Diffr. 23 (2008) 356. [2] D. Kapush, B. Grushko, D. Pavlyuchkov, T.Ya. Velikanova, Chem. Metal Alloys 2 (2009) 30. [3] D. Pavlyuchkov, B. Grushko, T.Ya. Velikanova, Intermetallics 16 (2008) 801. [4] B. Grushko, T. Velikanova, CALPHAD 31 (2007) 217. [5] D. Kapush, B. Grushko, T.Ya. Velikanova, J. Alloys Compd. 493 (2010) 99. [6] J. Rodrigues-Carvajal, Program FULLPROF-98, Version 0.2, 1998. [7] A. Le Bail, H. Duroy, J.L. Fourquet, Mater. Res. Bull. 23 (1988) 447. [8] G.M. Sheldrick, SHELXS-97, Program for Automatic Solution of Crystal Structure, University of Goettingen, Germany, 1997, release 97-2. [9] G.M. Sheldrick, SHELXL-97, Program for Crystal Structure Refinement, University of Goettingen, Germany, 1997, release 97-2. [10] T. Hahn (Ed.), International Tables for Crystallography, vol. A, Kluwer Acad. Publ., Dordrecht, Boston, London, 1992. [11] R. Vincent, P.A. Midgley, Ultramicroscopy 53 (1994) 271. [12] J.P. Morniroli, A. Redjaïmia, S. Nicolopoulos, Ultramicroscopy 107 (2007) 514. [13] R.A. Young, D.B. Wiles, J. Appl. Cryst. 10 (1982) 262. [14] G. Cagliotti, A. Paoletti, F.P. Ricci, Nucl. Instrum. 3 (1958) 223.