On the solubility of aluminium in the intermetallic compound CoGa3

On the solubility of aluminium in the intermetallic compound CoGa3

Journal of A ~ D CO~POUHD~ ELSEVIER Journal of Alloys and Compounds 261 (1997) 250-253 On the solubility of aluminium in the intermetallic compound...

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Journal of

A ~ D CO~POUHD~ ELSEVIER

Journal of Alloys and Compounds 261 (1997) 250-253

On the solubility of aluminium in the intermetallic compound

CoGa 3

H. M e y e r , M. Ellner* Max-Planck-hlstitut fiir Metallforschung. Seestr. 75. D- 70174 Stuttgart. Germany Received 10 March 1997; received in revised form 2 April 1997; accepted 3 April 1997

Abstract The gallium atomic positions in the intermetallic comp:.,und CoGa~ (Pearson Symbol tPl6, space group P4: Imnm) can be occupied by aluminium up to xA,=0.35. Lattice parameters were measured in the whole homogeneity range of CoAI,Ga~_ ,. Both the axial ratio c/a and the unit cell volume decrease with increasing aluminium content. The extrapolated average atomic volume for the hypothetical phase 'CoAl 3 with CoGa3 structure' is significantly higher than the average atomic volume of the observed non-defect cobalt/aluminiumcontaining structure occurring near the stoichiometry CoAI~-o-Co4AI,3 (oP102, o-Co4Al, ~ type). The measured data of the pseudotemary phase CoAI,Ga3_, are compared with data for the other gallium-contzi~,ing representatives of the CoGa~ structure type. © 1997 Elsevier Science S.A. Keywords: Co-Gac~mpounds; Aluminium substitution; Intermetallic compounds; Homogeneity range

1. Introduction

2. Experimental

In contrast to the complex phase diagram of the binary system Co-AI [l ], the phase diagram of the homological system Co-Ga [21 shows only two intermetallic compounds: CoGa (cP2, CsCI type) [3] and CoGa~ (tPl6, CoGa 3 type). Whereas CoGa shows a large concentration range from XA~=0.30 (at 1483 K) to XA~=0.64 (at 1128 K), CoGa 3 is an intermetallic compound with fixed stoichiometry [2]. For the CoGa 3 structure determination, the following space groups were taken in consideration by Schubert et al. [3]: P42nm, P42/mnm and P2~n2. The resulting CoGa 3 structure proposal was made in the noncentrosymmetrical space group P?ln2 [3]. According to Parth6, this space group does not consider all symmetry elements of the CoGa 3 struc~are; concerning this fact, the standardized structure is described in the centrosymmetricai space group P421mnm [4]. No phase equilibria have been published for the ternary system C o - A I - G a [5-8]. The miscibility between CoAl (CsCI type) [9] and CoGa at 1100 K was investigated by lpser and Mikula [10]. In the following study, the influence of the decreasing d-electron concentration on the stability of the CoGa 3 structure has been investigated. This was carried out by substitution of aluminium atoms for gallium atoms in the in the CoGa 3 structure.

Mixtures consisting of cobah 99.9% (Johnson Matthey), aluminium 99.999% (Heraeus) and gallium 99.9999% (Ingal) (approximately 3 g) were put into a corundum crucible and melted in an evacuated and argon filled (Messer-Griesheim 5.0) induction furnace. Because the loss of melted alloys was smaller than 0.2%, chemical analysis was not carried out. For the heat treatment, the bulk alloys were wrapped into molybdenum foils, taken into evacuated argon filled silica ampoules and heat treated for 28 days at 773 K and then water quenched. For X-ray powder diffraction, bulk alloys were powdered in a mortar. Splat-cooling investigations were carded out in a shockwave tube with argon (Messer-Griesheim 5.0) [11]. Powder diffraction patterns were recorded in a Guinier transmission Camera (Enraf-Nonius FR552) using Co Ka~ radiation. Silicon was used as an internal calibration standard. Single coated CEA Reflex 15 Film was used for the Guinier photographs. Unit cell parameters were refined by !east square calculations. The intensities of diffraction lines observed in Guinier patterns were densitometrically analysed on the Line-Scanner LS 20 (KEJ Instruments).

Corresponding author. 0925-8388/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved Pll S0925-8388(97)00223-5

3. Results Guinier patterns of ternary alloys C062Al19Ga19 and C055A122.sGa22.5 heat treated 28 days at 773 K yielded

H. Meyer, M. Eliner I Journal of Alloys and Compounds 261 (1997) 250-253 Table I Crystal data of CoAI,Ga~_, (tPl6)

251

1.035

Alloy

a (A)

c (A)

c/a

V (A 3)

Co: sGav ~ Co: sAI ~,,Ga,, s Co,~AI:,,Ga~ Co2,.AI: sGaso Co,,sAI~2 sGa42 .s Co,.sAi~sOa4,

6.242 ( ! ) 6.238 ( i ) 6.233(!) 6.232 ( i ) 6.232 ( I ) 0.231 ( I ~

6.442 ( I ) 6.422 ( i ) 6.407(i) 6.396 ( I ) 6.386 ( ! ) 0.382 ( i )

! .0320 1.0295 1.0279 i.0263 1.0247 i.0242

251.0 249.9 248.9 248.4 248.0 247.8

!.030

=

1.025 6.244 -

I

6.242

I 1.020

\

6.240 J .<

\

6.238. ~. 6.236 •

\

Coo 2sAI0L~Ga~

Fig. 3. Composition dependence of the axial ratio cla for the pseudoternary phase Co25AI~GaT~_ ,.

'\

6.230 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 c~ac~ ,, C~zAq ~C~,o Mole fraction xAt ofCo0.~AixGao.~5, x Fig. I. Composition dependence of the lattice parameter a for the pseudoternary phase Co.,~A! Ga;~_ ,.

diffraction lines of the CsC! type homogeneously. On the other hand, alloys CosoAlloGalo and Co4.~A1275Ga275, heat treated 28 days at 773 K, showed two-phases: the 6.45-

solid solution ct-Co(AI,Ga) (Cu type)+CoAlxGa~_ x and CoAlxGa ~_x+CoGa3, respectively. Guinier patterns of the splat-cooled alloys C062Al~gGa~9, CossA122.5Ga22.5 and C045AI27.sGa,7. s were homogeneous with CoAl~Ga~_ x (CsCi type). The substitution of aluminium for gallium in the intermetallic compound CoGa 3 was possible in the composition range C025Ga~5 to C025Ala5Ga4o (alloys heat treated 28 days at 773 K). The dependence of the unit cell parameters on mole fraction is shown in Table l as well as in Figs. 1 and 2. The mole fraction dependence of the axial ratio cla and the unit cell volume are shown in Figs. 3 and 4, respectively. 251.5 251.0

\

6.43 ~

\

0.30 0.35

Mole fraction Xal of Coo.25AIxGao.75.x

6.232

~'

.

Coo 25Gao7~

\

-~ 6.234

6.44,

I

0.00 0.05 0.10 0.15 0.20 0.25

250.5

\

i .

\\

\

~- 250.0

6.42 ~ 249.5,

~ 6.41

~ 249.o

\

~ 6.4o

248.5

\

,\ \

248.0 6.38

0.00

[

"

0.05 0.10 0.15 0.20 0.25

0.30 0.35

Mole fraction x AI°fC°o2~AIxGao 75-x Fig. 2. Composition dependence of the lattice parameter c for the pseudoternary phase Co25AI,Ga~s_ r

\

247.5 0.00 0.05 0.10 0.15 0.20 0.25 COo~G~o.75

0.30 0.35 Coo~Alo.35Gae4o

Mole fractionXAlof Coo.25AlxGao.~5.x Fig. 4. Composition dependence of the unit cell volume V for the pseudoternary phase C025AlxGa75_,.

252

H. Meyer, M. Ellner I Jounud of Alloys and Compounds 261 (1997) 250-253

4. Discussion

The continuous miscibility between CoAl and CoGa observed for the first time by Ipser and Mikula [!0] was confirmed by the following experiments: Guinier patterns of ternary alloys CossA122.sGa22.5 a n d C 0 6 z A l t g G a : 9 heat treated 28 day at 773 K yielded diffraction lines of the CsC! type homogeneously. Splat-cooled alloy Co45AI27.sGa27.5 showed also the solid solution CoAlxGa,_,, as the single phase. For alloys with constant cobalt content (CosoAIso_,,Ga,~, heat treated 2 days at 1100 K, the lattice parameter increases as expected with increasing mole fraction of gallium [101. The present investigation on the tetragonal C o G a 3 structure (tPl6) shows that the lowering of the d electron concentration made by aluminium substitution is possible up to the mole fraction XA:=0.35. Both lattice parameters a and c decrease with increasing aiuminium content (Table 1 Figs. 1 and 2). Because of the fact that the slope AclAxA~ is larger than the slope AalAxA~, the axial ratio cla decreases with mole fraction of aluminium (Fig. 3). The lattice parameters a and c as well as the axial ratios cla are shown for the gallium-containing phases with CoGa 3 structure in Fig. 5. The smallest axial ratio cla (the minimal tetragonal distortion) is shown by RhGa 3 and the largest axial ratio cla by FeGa 3. As result of the occupation of gall~..... positions by aluminium in the CoGa 3

6.60 6.55 6.50

structure, both the tetragonal distortion and the unit cell volume decrease. The gallides show the largest unit cell o~ volume: RuGa 3, V=281.72 A ; OsGa 3, V=284.05 ,~,~ and the high-pressure phase ReGa3(p), V=287.84 ,~3. This unit cell volume sequence corresponds to the increasing metallic atomic radii of the AT-A 8 elements: Ru, r = 1.339 ,~; Os, r = 1.353 ,A and Re, r = 1.375 ,~, [131. The composition dependence of the CoAI,,Ga3_ x unit cell volume is shown separately in Fig. 5. The extrapolated average atomic volume (vA=vINC; V=volume of the unit ceil, NC=number of atoms in the unit ceil) for xA~=0.75 amounts 15.25 ,~3. This is larger than the average atomic volume of the binary alumil~ium-containing structure occurring near the stoichiometry CoAl 3, namely o-Co4AI,3 (oP102) with the average atomic volume VA=14.27 ,~3 [14]. The homeotypic structures o-C04All3 and m C°4A! ~3 [ 15] are characterized by formation of pentagonal channels [16] and show significant short interatomic distances Co-AI (e.g. c~-Co4A1=3, dco_Ai =2.24 ,~, [141). In contrast to this, the "shortest' C o - A I / G a interatomic distance in the ternary alloy Co25A135Ga4o (CoGa 3 type) is clearly larger, dCo_A~/G.~=2.38 ,~,. Concerning the large affinity between cobalt and aluminium on one hand and both the estimated large average atomic volume of the h y p o t h e t i c a l ' C o A l 3 with CoGa 3 structure' as well as the larger interatomic C o - A I / G a distance in Co:.~Al~sGa4o on the other hand, the formation of the CoGa 3 structure in the binary system Co-AI is obviously unsuitable. In fact, the representatives of the CoGa 3 structure are known only with the metals gallium and indium. None representative of the CoGa.~ structure type has been found in the transition metal-containing alloys with the homologous elements aluminium and thallium 117].

B P-hG=l

References

,< ~ ' 6.40

IrGa3

6.35

~' 6.30 °,~

CoGa~

~ 6.25, ..1

B FeOa 3

El CoAIxGa3..

6.20. 6.15 6.10 I , 6.3

,"

, 614

'

/

I 6.5

'

I 6.6

,

I 6.7

'

I 6.8

'

I 6.9

Lattice parameter c [A] Fig. 5. Lattice parameters a and c and the axial ratios cla for the gallium-containing representatives of the CoGa~ structure (tPI6) (FeGa. RuGa. OsGa. RhGa3 and lrGa3 [31, the high-pressure phase ReGa~ (p)

[~21.

[!i T. Gfdecke, M. EIIner, Z. Metallkd. 87 (1996) 854. [2] T. Massalski, H. Okamoto, P.R. Subramanian, L. Kacprzak, Binary Alloys Phase Diagrams, Second Edition, ASM International, The Materials Information Society, 1990. [31 K. Schubert, H.L. Lukas, H.-G. Meitlner, S. Bhan, Z. Metallkd. 50 (1959) 534. [4l E. Parthg, L. Gelato, B. Chabot, M. Penzo, K. Cenzual, R. Gladyschewkii. TYPIX, Standardized Data and Crystal Chemical Characterization of Inorganic Structure Types, Springer Verlag. Berlin, 1994. [5] G. Petzow, G. Effenberger (Eds.) Ternary Alloys, A Comprehensive Compendium of Evaluated Constitutional Data and Phase Diagrams, Vol. 4, VCH Verlagsgesellschaf: mbH, Weinheim, 1991. [6] Red Book, Phase Diagrams of Metallic Systems, Voi. 36, MSI, Materials Science International Services, GmbH, Stuttgart, 1994. 17] E Villars, A. Prince, H. Okamoto, Handbook of Ternary Alloy Phase Diagrams, ASM International, 1995. [8l Cumulative Index of the Journal of Phase Equilibria, 17 (1996) 556. [9] A.J. Bradley, G.C. Seager, J. Inst. Metals 64 (1939) 81. [10] H. Ipser, A. Mikula, Mh. Chemic 123 (1992) 509. [! !1 G. Bucher, M. Ellner, F. Sommer, B. Predel, Mh. Chemie 117 (1986) 1367.

H. Meyer, M. Ellner I Joun~al of Alloys and Compounds 261 (1997) 250-253 [12] S.V. Popova, L.N. Fomicheva, Inorganic Materials 18 (1982) 205. [13] W.B. Pearson, The Crystal Chemistry and Physics of Metals and Alloys, Wiley-lnterscience, New York, 1972, p. 151. [14] Yu. Grin, U. Burkhardt, M. Eliner, K. Peters, J. Alloys Comp. 206 {1994) 243. [15] R.C. Hudd, W.H. Taylor, Acta Crystalogr. 15 (1962) 441.

253

[16] J. Grin, U. Burkhardt, M. Ellner, K. Peters, Z. Kristallographie 209 (1994) 479. [171 P. Villars, L.D. Calvert, Pearsons's Handbook of Crystallographic Data for lntermetallic Phases, ASM International, The Materials Information Society, Materials Park, OH, 1991.