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Structural Studies in the Systems ZrZn2_xAlx and HfZn2_xAlx

A. Drašner and Ž. Blažina Institute "Ruđer Boškovič", 41001 Zagreb, POB 1016, Yugoslavia

Z. Naturforsch. 36b, 1547-1550 (1981); received August 21, 1981

Aluminum, Hafnium, Zinc, Zirconium, Alloy

A great number of samples in the systems ZrZ^-arAls and HfZn2_xAlz was prepared and investigated by means of X-ray powder diffraction. In both systems hitherto unknown ternary Friauf-Laves structures of the MgCu2-type (S. G. Fd3m) were identified. Their homogeneity regions were determined to be ZrZno.eAl1.4-ZrZno.4Al1.6 and HfZn1.25Alo.75-HfZno.sAli.s, respectively. The unit cell parameter of ZrZno.sAli.s is a = 747.3 ± 0.3 pm, and for HfZno.sAli.s a = 741.4 ± 0.3 pm. For the alloys of these particular compositions the atomic positions were determined using trial and error methods. In the investigated systems, depending on the composition and/or thermal treatment, alloys having the AuCu3 structure (S. G. P m 3 m ) were also observed. Structural correlations of these ternary Friauf-Laves phases with their corresponding binary prototypes were made in terms of stacking sequences.

Introduction In our earlier papers, studies on substitution (of

both components) in cubic AB2 phases as well as in structuraly closely related AB5 phases were described [1-3]. Since no adequate theory for explaining the appearance of these phases is available, we were interested in further substitutional studies in Friauf-Laves phases.

In the systems ZrZ^-zAlx and HfZn2_iAlz the corresponding binary prototypes are isostructural with the Friauf-Laves phases of the MgCu2-type (ZrZn2, HfZn2) or MgZn2-type (ZrAl2, Hf Al2) [4-6]. Only HfZn 2 is polymorphic, i.e. crystallizes also with the hexagonal MgNi2-type of structure [5].

Some literature data of the Zr-Zn-Al and Hf-Zn-Al ternary systems, on the ZrZri3-ZrAl3 and HfZn3-HfAl3 tie lines, are also available [7, 8].

Experimental Materials and Methods

The following metals were used in this investiga-tion: hafnium filings (99.9%) and aluminum powder (99%) (obtained from Koch-Light Laboratories Ltd.), zirconium powder (Carlo Erba, reagent grade), zinc powder (Kemika, Zagreb, reagent grade).

Samples of the general formulae ZrZn2-xAlx and HfZn2_xAlx, were prepared by direct synthesis from elements in evacuated silica tubes. All samples were annealed at 800 °C for 360 and 1080 h. In order to prepare single phase alloys in some cases (especially in the system HfZn2-xAlx) the homogenization temperature was risen up to 1000 °C for 72 and 192 h.

* Reprint requests to A. DraSner. 0340-5087/81/1200-1547/$ 01.00/0

X-ray powder diffraction patterns were taken on a Philips diffractometer PW 1050, using nickel-filtered CuKa radiation and silicon as an internal standard. Relative intensities were calculated taking into account the structure factor, the Lorentz-polarization factor and multiplicities.

Results The system ZrZn^-xAlx

The results of X-ray examinations of the samples annealed at 800 °C for 1060 h indicated the existence of two new ternary phases of the composition ZrZno.sAli.s and ZrZni.sAlo.s, respectively.

The unit cell of ZrZno.sAli.s is cubic with a = 747.3 ± 0.3 pm. The powder diffraction pattern was indexed on the basis of a MgCu2-prototypes (S. G. Fd3m). Calculated and observed intensity values are in best agreement if the following atomic posi-tions are assumed:

8 Zr in 8(a) 0,0,0; 1/4,1/4,1/4; +

4Zn + 12Al in 16(d) 5/8,5/8,5/8; 5/8,7/8,7/8; (statistically) 7/8,5/8,7/8; 7/8,7/8,5/8; +

+ (0,0,0; 0,1/2,1/2; 1/2,0,1/2; 1/2,1/2,0)

The relevant diffraction data are presented in Table I. The alloy is homogeneous in the region ZrZno.eAl1.4-ZrZno.4Al1.e. The variation of the lattice parameter within the single phase region follows Vegard's law (Fig. 1, Table II).

The crystal structure of ZrZni.sAlo.s is also cubic, but belongs to the space group P m 3 m (AuCu3-type, a = 407.3 ± 0 . 3 pm), being stable up to 1000 °C.

Table I. X-ray diffraction data for ZrZno.sAli.s (CuKa).

h k I do (pm)

dc (pm)

Io Ic

1 1 1 430.7 431.4 11 14 2 2 0 264.3 264.2 66 57 3 1 1 225.3 225.3 100 100 2 2 2 215.7 215.7 15 15 4 0 0 186.5 186.4 1 1 3 1 3 171.3 171.4 3 3 4 2 2 152.6 152.5 22 23 3 3 3, 5 1 1 143.8 143.8 30 31 4 4 0 132.1 132.1 19 22 5 3 1 126.3 126.3 3 2 6 2 0 118.2 118.1 9 10 5 3 3 114.0 113.9 9 11 6 2 2 112.7 112.6 5 5 7 1 1, 5 5 1 104.6 104.6 2 2 6 4 2 99.8 99.8 13 14 7 3 1, 5 5 3 97.3 97.3 20 16 8 0 0 93.4 93.4 4 5

R = 8.36%.

76(H

E Q. - 750

740 o

730

Z r Z n 2 _ x A t x

7i5

7L0

735

730-

40 45 50 55 60 A l ( a t . * / . )

H f Z n 2 _ x A l x

Tab. II. The unit cell parameters in the single phase region of the systems ZrZn2_xAlx and HfZn^Alz .

0 20 30 £0 50 60 A l ( a t . * / . )

Fig. 1. The lattice parameter variation in the singel phase regions of the systems ZrZn^Al^ and HfZn2-xAlx.

Composition at. % Al a (pm) (±0 .3 )

ZrZn2 0 739.6 ZrZno.eAli.4 46.7 747.4 ZrZno.sAli.s 50.0 747.2 ZrZno.4Ali.e 53.3 748.3

HfZn2 0 732.0 HfZn1.25Alo.75 25.0 738.6 HfZnAl 33.3 738.4 HfZno.75Al1.25 41.7 739.8 HfZno.5AI1.5 50.0 741.4

The samples with the MgCu2 structure at 800 °C become multiphase when the homogenization tem-perature rises up to 1000 °C. On the other hand the latter samples are of two phase nature (MgCu2 + AuCua) if the time of homogenization is reduced.

The system HfZn^-xAlx In this system single phase alloys were obtained

only after homogenization at 1000 °C for at least 192 h. The single phase region was determined to be HfZni.25Alo.75-HfZno.5Ali.5. The diffraction pat-terns were also indexed on the basis of a cubic structure of the MgCu2-type. The comparison be-tween observed and calculated intensity values for HfZno.öAli.ö are in best agreement if the atomic arrangement within the unit cell is the same as described for ZrZno.sAli.s. The relevant diffraction data are presented in Table I I I . The plot of lattice parameter vs composition indicates a linear depend-ance according to Vegard's law (Fig. 1 and Table II).

Table III. X-ray diffraction data for HfZno 5AI1 5 (CuKa).

h k I do (pm)

dc (pm)

Io Ic

1 1 1 428.7 428.0 34 42 2 2 0 262.3 262.1 75 77 3 1 1 223.9 223.5 100 100 2 2 2 214.0 214.0 8 5 4 0 0 185.4 185.4 5 5 3 1 3 170.6 170.1 11 11 4 2 2 151.4 151.3 35 29 5 1 1, 3 3 3 142.7 142.7 31 26 4 4 0 131.0 131.1 20 17 5 3 1 125.3 125.3 6 8 6 2 0 117.1 117.2 13 13 5 3 3 113.0 113.1 11 10 6 2 2 111.8 111.8 2 2 7 1 1, 5 5 1 103.8 103.8 4 3

R = 8.73%.

It should be noted that lower temperature of homogenization (900 °C and 800 °C) and/or reduced time produced two phase samples (ternary MgCu2

and AuCu3 structure). Single phase alloys of the AuCu3-type could not be prepared.

Discussion An interesting feature of both systems is the

occurrence of the ternary AuCu3 structure. We believe that the presence of this structure on Zr(Hf)Zn2-Zr(Hf)Al2 tie lines has to be attributed to the widely extended homogeneity region of the AuCu3 structure observed on the Zr(Hf)Zn3-Zr(Hf)Al3 lines in the regions ZrZn2Al-ZrZno.8Al2.2 and HfZn2.4Alo.6-HfZnAl2, respectively [7]. Thus the occurrence of AuCu3 struc-ture: 1) after the reaction of the components, 2) at lower temperature of homogenization and 3) at reduced time of homogenization, speaks for less energy required for formation, of the simple close packed arrangement of three kinds of atoms. Additional energy (and time) is necessary in order to facilitate further ordering (MgCu2 structure). This could also explain the higher temperature required for samples containing hafnium (heavier atom).

The atomic arrangement in the AuCu3 unit cell was calculated for the single phase sample of the composition ZrZn1.5Alo.5- The best agreement be-tween observed and calculated intensity values is obtained if 1 Zr atom is at the position 1(a) and (0.333 Zr + 2 Zn + 0.666 Al) statistically in 3(c). Thus the formula for this compound can conve-niently be written as Zr(Zro.333Zn2Alo.666) or gen-erally, for all samples with the AuCu3 structure Zr(Zr,Zn,Al)3. The presence of Zr atoms in posi-tions 1 (a) as well as in 3 (c) is not unusual, because in the binary system Zr-Al, zirconium atoms were found to occupy alternatively the very same posi-tions in Zr3Al and ZrAl3 [9].

I t seems to be of interest to consider the Friauf-Laves phases described here and their binary proto-types in terms of stacking sequences. All these phases have stacking sequence motives based on kagome nets (smaller atoms), interleaved with three triangular nets (two of larger and one of smaller atoms) [10]. Fig. 2 shows a characteristic kagom6

Z r Z n 2 I H f Z n 2 ) (MgCu 2 - type)

O-ZflHf» . . ®Q25Zn*(J7SAI , Q (statistically)

ZrZnQ 5Al,j5 (HfZnggAl^J (MgCu2 -type >

OiO,

Z r A l 2 l H f A l 2 ) (MgZn2 - type)

Fig. 2. Kagom6 nets and projections of triangular nets in ZrZn2(HfZn2), ZrZr0 5AI1 5(HfZn0 5AI1 5) and ZrAl2(HfAl2).

net and a projection of triangular nets for these structures, along [111] for the cubic and [001] for the hexagonal cell. I t is apparent that the substitu-tion of zinc atoms with aluminium atoms and/or vice versa always takes place both in kagome and in the triangular net. This is in agreement with our earlier findings in related systems [2, 3]. Namely, it was pointed out that the substitution takes place within kagome net only if the crystal structure changes, and/or within triangular net only, if the prototype structure is preserved. In the systems described here, depending on the adopted view-point, the crystal structure changes as in ZrAl2...ZrZn0.5Ali.5 and HfAl2...HfZno.5Ali.5, or is preserved (ZrZn2...ZrZno.sAli.s and HfZn2(cub.)...HfZno.5Ali.5). In this way the sub-stitution within kagome and triangular nets could be consistently explained.

We wish to express our gratitude to Prof. Z. Ban, for his continued interest, sugestions and support.

[1] 2. Blazina, R. Trojko, and Z. Ban. J. Less-Common Met., to be published.

[2] 1. Blazina and Z. Ban, Z. Naturforsch. 35 b, 1162 (1980).

[3] Z. Blazina, A. Drasner, and Z. Ban, J. Nucl. Mater. 96, 141 (1981).

[4] A. E. Dwight, Trans. Am. Soc. Met. 53, 479 (1961). [5] W. Rossteutscher and K. Schubert, Z. Metallkde.

56, 730 (1965). [6] C. G. Wilson, Acta Crystallogr. 12, 660 (1959).

[7] K. Schubert, H. G. Meissner, A. Raman, and W. Rossteutscher, Naturwissenschaften 51, 287 (1964).

[8] A. Raman and K. Schubert, Z. Metallkde. 56, 40 (1965).

[9] W. B. Pearson, A. Handbook of Lattice Spacings and Structure of Metals and Alloys, Pergamon Press, New York 1958, pp. 391.

[10] F. C. Frank and J. S. Kasper, Acta Crystallogr. 12, 483 (1959).