Structure and thermodynamic properties of liquid BiGa alloys with miscibility gap

] O U R N A L OF

Journal of Non-Crystalline Solids 156-158 (1993)153-156 North-Holland

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Structure and thermodynamic properties of liquid Bi-Ga alloys with miscibility gap M. Inui and S. T a k e d a Department of Phystcs, College of General Educatton, Kyushu Untverstty, Ropponmatsu, Fukuoka 810, Japan Neutron diffraction, ultrasonic velocity and density measurements have been carried out for liquid B1-Ga alloys along the hqmdus curve and at temperatures above the critical point where the mlsciblhty gap closes A remarkable change in the structure factor has been observed at the critical concentration around 70 at % Ga. The adiabatic compressibility of this system obtained from the sound velocity and density shows an almost linear concentration dependence The temperature dependence of the adiabatic compressibility is weaker in the concentration region corresponding to the miscibility gap than at other compositions The concentration variation of the structure factor is discussed from the point of view of the thermodynamic properties of this system

1. Introduction The liquid B i - G a alloy system has a miscibility gap in the concentration range from 40 to 90 at.% Ga [1], and the critical temperature is reported to be 262°C at 70 at.% Ga. It is well known that large concentration fluctuations just above the critical temperature are related to a number of phenomena [2-7]. It is interesting to study the correlation between the local structure and thermodynamic properties for such systems. The ultrasonic velocity [5,8] and electrical resistivity [9] of liquid B i - G a alloys have already been reported. However, there have been few studies of the relation between the structure and thermodynamic properties for systems with a miscibility gap. This prompts us to measure the structure, ultrasonic velocity and density for liquid Bi-Ga alloys.

2. Experimental procedure Neutron diffraction experiments were carried out at temperatures above the critical temperature using the two-axis diffractometer of the InCorrespondence to Dr M. Into, Department of Physics, College of General Education, Kyushu Unwerslty, Ropponmatsu, Fukuoka 810, Japan. Tel' + 81-92 771 4161, ext. 283 Telefax + 81-92 724 0790

stitute for Materials Research (KID), Tohoku University in the high-flux reactor of JRR-2. The samples were sealed in pyrex glass tubes of inner diameter 10.0 mm with 1.0 mm wall thickness under a vacuum of 10 -2 Pa. A neutron beam with wavelength of 0.995 or 1.41 A was used. The small-angle data were corrected using the spectrum of a vanadium rod with a similar size. The ultrasonic velocity of liquid B i - G a alloys was measured at 10 MHz by a pulse-transmission method using the differential path technique from the melting temperature to about 800°C under an argon atmosphere. The apparatus for the present measurement was UAC-77 by Teitsu Denshi Co. Ltd. A sample cell made of fused quartz was used. The temperature of the liquid sample was monitored by three thermocouples, one of which was put into the melt. The density of the system was measured with a "7-ray absorption technique. A quartz cell with a transmitting space about 15 mm in length was used. Detailed information about the measurements has been given elsewhere [10,11].

3. Experimental results The total structure factors, S(Q), for hquid Bi -G a alloys at temperatures above the miscibil-

0022-3093/93/$06.00 © 1993 - Elsevier Science Pubhshers B V All rights reserved

154

M Inui, S Takeda / Structure andproperttes ofhqutd Bt-Ga alloys 3

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Fig. 1 The structure factors of llqmd BI at 300°C (a), BisoGa2o at 300°C (b), Bl6oGa4o at 300°C (c), Bi4oGa6o at 320°C (d), Bl3oGa7o at 300°C (e), Bll4Gas6 at 300°C (D, BisGa95 at 300°C (g) and Ga at 200°C (h).

ity gap are shown in fig. 1. The shapes of the total structure factors are roughly divided into two patterns: Bi-based and Ga-based. As seen in the figure, the position of the first peak shifts gradually towards the high-Q region from Bi to 70 at.% of Ga. It is noteworthy that the shift becomes large around the critical concentration of Bi30Ga70. On the other hand, the peak position in Ga-rich alloys beyond the critical concentration is very close to that of pure Ga. Figure 2 shows the total pair distribution functions, g(r). Again the pattern of g(r) can be qualitatively divided into two classes: the region from Bi to Bi30GaT0 (region A) and that from Bi30GaT0 to Ga (region B). In region A, the position of the first peak shifts towards smaller r and the width becomes broad with increasing Ga concentration. The second peak around 6.5 ,~ also shifts to smaller r with increasing Ga concentration. It is noteworthy that there is no peak at around 5.6 .~ where g(r) in region B has the second peak. The positions of the first and the second peaks in region B are almost unchanged with Bi concentration. A small shoulder is found at 3.6 ,~ in g(r) in region B. Figure 3 shows the concentration dependence of the sound velocity and of the molar volume of

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liquid Bi-Ga alloys at 300°C. The sound velocity decreases with increasing temperature. Our result is in good agreement with the result reported in ref. [8]. As shown in the figure, the sound velocity decreases with increasing Bi concentration and shows a concave behaviour as a function of concentration. The slope of the concentration dependence is rather large in the Ga-rich region, and changes around 70 at.% Ga. On the other 3000

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M. Inut, S. Takeda / Structure and properttes of lututd Bt-Ga alloys

hand, the observed molar volume has a slightly positive deviation from a linear concentration dependence indicated by the broken line.

The concentration-concentration fluctuation structure factor in the long-wavelength limit, See(0), which can be deduced from the thermodynamic data [12], shows a broad maximum around 70 at.% Ga where the position of the first peak of S(Q) changes, as seen in fig. 1. This change can be seen also in g(r) of Bi30Ga70, in which the second peak around the position of 5.2 ,~ in region B is almost smeared out. This can be explained as follows. The second peak of g(r) of liquid Ga is observed around 5.2 A, while the g(r) of liquid Bi takes a minimum value at the same position. Consequently the total g(r) of the alloy must be averaged out to the mean value in that region if there is a segregation tendency. It is therefore inferred that there is a large segregation tendency around the concentration of 70 at.% Ga. In order to investigate how this concentration fluctuation varies with temperature at the critical concentration, the temperature dependence of the total structure factor of Bi30Ga70 is examined at 300°C and 400°C. Figure 4 shows the structure factor of Bi30Ga70 at 300°C (broken line) and 400°C (solid line), together with that of Bin0Ga60 at 320°C (dots). Several characteristic features can be seen in the figure: (a) the structure factor, S(Q), at 300°C has a larger value in the small-angle region around 1.2 ,~-1 than that at 400°C. (b) the value of the second peak decreases at around 5 ,~-t with increasing temperature. The profile of the second peak of Bi30Ga70 at 400°C is similar to that of Bia0Ga60 at 320°C. If this trend does not change at higher temperature, then the structure factor becomes more Bi-like. The specific heat of this system is reported to have a large value around the critical concentration [7]. This endothermic behaviour is related to the structural change. In this respect, it may be

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useful to study the compressibility of this system. Figure 5(a) shows the concentration dependence of the adiabatic compressibility of liquid Bi-Ga alloys at 300°C, deduced from the observed sound velocity and density. As seen in the figure, there appears to be a small dip around 40 at.% Ga 4

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Fig 5. (a) Concentration dependence of the adiabatic compressibdlty of hquid Bl1_xGa x at 300°C. The sohd hne denotes the ideal cornpressibihty gwen by eq. (1). (b) Concentration dependence of the temperature coefficient of the adiabatic compressibdlty of liqmd Bi] _xGax at 300°C

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M 1nut, S Takeda / Structure and propertws of hqutd Bt-Ga alloys

which corresponds to the eutectic region, but no anomalous behaviour can be found around the critical concentration. The compressibility of a liquid system is, in general, expected to have a large value when it has large density and concentration fluctuations. The observed compressibility obeys very closely the relation

)(;deal= [(1-X)VBIXsB1+XVGaXsGa]//V,

(1)

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where is the adiabatic compressibility of element t (i = Bi, Ga) and V is the observed molar volume of Bil_xGa x. This result is by contrast with results from compound-forming liquid alloys, where the compressibility around the stoichiometric composition is reported to have a large positive deviation from the ideal value [13-15]. Figure 5(b) shows the temperature coefficient of the adiabatic compressibility at 300°C. The coefficient can be expressed by the equation 1 dxs X, dT

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where and v~ are the volume expansion coefficient and sound velocity of the liquid alloys, respectively. The temperature coefficient of the adiabatic compressibility, therefore, can be obtained from the volume expansion coefficient and the temperature coefficient of the sound velocity. It is noteworthy that the temperature coefficient shows a large negative deviation from the linear concentration dependence in the region of the miscibility gap. This behaviour means that the density fluctuations due to the thermal agitation do not affect the compressibility so much in this region. In other words, this suggests that the

atoms can easily be rearranged to have a structure with smaller density fluctuations. Further investigation such as partial structure factors is necessary to solve this problem. The authors are grateful to Professor S. Tamaki for stimulative and fruitful discussions. They express their thanks to the Institute for Materials Research, Tohoku University, for providing use of the diffractometer KID in JRR-2.

References [1] R.P. Elllott, Constituhon of Binary Alloys, 1st Suppl. (McGraw-Hall, New York, 1965) p. 182. [2] G.D, W~gnall and P A. Egelstaff, J. Phys. C1 (1968) 1088. [3] P.D. Adams, Phys. Rev. Lett. 25 (1970) 1012 [4] H.K. Schtirmann and R D. Parks, Phys. Rev. Lett 26 (1971) 835 [5] M. Puls and J.S Kirkaldy, J. Chem. Phys. 54 (1971) 4468 [6] M.G gam and S V. Lether, J Chem. Phys. 55 (1971) 1164. [7] J Mikler, F. Gehringer and R. Komarek, Z. Metallkd. 79 (1988) 755. [8] M. Rosen and Z. Salton, Mater So. Eng. 58 (1983) 189. [9] G Grater, J G Gasser and R. Klmm, Phllos Mag B54 (1986) 543. [10] M Inul and S. Takeda, J. Phys. Soc. Jpn 61 (1992) 1585. [11] M. Into, S. Takeda and T. Uechi, J. Phys Soc Jpn. 61 (1992) 3203 [12] B. Predel, M Frebel and W Gust, J. Less-Common Met. 17 (1969) 391. [13] S. Takeda, S Harada and S. Tamaki, J. Phys. Soc. Jpn. 58 (1989) 544 [14] S.P McAlister, E.D. Crozier and J.F. Cochran, J. Phys C6 (1973) 2269 [15] Y. Tsuchlya and F. Kakmuma, J. Phys.: Condens, Matter 4 (1992) 2117.