Phase equilibria in the Sm-In system

Phase equilibria in the Sm-In system

Journal of the Less-Common PHASE EQUILIBRIA A. SACCONE, S. DELFINO Metals, 84 (1982) 281- 289 IN THE Sm-In SYSTEM and R. FERRO Istituto di Chi...

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Journal of the Less-Common

PHASE EQUILIBRIA

A. SACCONE,

S. DELFINO

Metals, 84 (1982) 281- 289

IN THE Sm-In

SYSTEM

and R. FERRO

Istituto di Chimica Generale dell’Universit& (Received October

281

di Genova, Genoa (Italy)

13, 1981)

Summary The Sm-In system was studied using differential thermal, metallographic and X-ray analyses. The following intermediate phases were observed: Sm,In (melting point, 1090flO “C), Sm, +Jn (melting point, 1210&20 “C), Sm,In, (melting point, lllO+ 10 “C) and SmIn, (melting point, 1130 + 10 “C). Five eutectics occur: l3-Sm,In (905 + 5 “C, 16.5 f 1 ato/, In), SmJn-Sm ,+,In (108O)lO “C, 35kO.5 at.% In), Sm,+xIn-Sm,In, (1090+10 “C, 59.5 f 1 at.% In), Sm,In,-SmIn, (1075 k 10 “C, 67 f 1 at.% In) and SmIn,-In (154 “C, more than 99.5 at.% In). l3-Sm,In decomposes eutectoidally at 800 + 10 “C and 12 f 1 at.% In. The crystal structures of Sm,In (hP6, Ni,In type), SmJn, (oC32, Pu;Pd,-like type) and SmIn, (cP4, AuCu, type) were confirmed. A structure of the cP2 CsCl type or the cI2 tungsten type is suggested for the Sm ,+,In compound. The general characteristics of the phase diagram are briefly discussed and compared with those of the La-In, CeIn and Pr-In systems.

1. Introduction Some rare earth-indium (R-In) systems have recently been studied, and the La-In [l], Ce-In [Z] and Pr-In [3, 41 systems have now been characterized. The Yb-In system [5] was also studied and, as is often the case for ytterbium systems, anomalous behaviour was observed in the sequence and structure of the intermediate phases. The phase diagrams of other R-In systems are not known, although some of the alloy characteristics have been determined. References to data pertaining to the crystallographic characteristics of intermetallic phases in R-In systems can be found in ref. 6. A number of thermodynamic measurements have been made on R-In alloys; these indicated high AH of formation for solid and liquid alloys. The values of AH for the RIn, compounds [7] and for some Nd-In alloys containing less than 95 at.% Nd [S] have been reported. 0 Elsevier Sequoia/Printed

in The Netherlands

282

The magnetic, superconductive and electronic properties of R-In compounds, particularly RIn,, and their possible applications have been reported in many papers (see for example refs. 9 and 10). The results of investigations of the Sm-In system, of which only the compositions and structures of the intermediate phases were known, are reported. The crystal data are summarized in Table 1 which contains both the values reported in the literature for the intermediate phases and the values obtained in this work. TABLE 1 Crystal structure Phase

Sm,In ’ Sm,In

Sm, +,In Sm,In,

data for Elm-In phases

Structure type

Cubic; cP4, AuCu,, or cF4, Cu Hexagonal; hP6, Ni,In

Cubic; cP2, CsCl, or cI2, W Orthorhombic; oC32, related to the Pu,Pd, type

SmIn,

Cubic; ‘cP4, AuCu,

Unit cell dimensions

v,, a

AVIV”

p

(A)

(A?

(%I

(g cm-?

a = 4.900

29.4,

-6.2,

7.99

This work

5.454 6.806 = 1.248) 5.4500 c = 6.7850 (c/a = 1.245) a = 3.815

29.2,

-5.1,

7.87

This work

29.0,

-5.5,

7.91

Cl11

27.7,

-6.3,

7.93

This work

a = 10.01 b = 8.13, c = 10.39 (b/a = 0.813) (c/a = 1.038) a = 4.628 a = 4.6259

26.4

-8.1

8.05

PI

24.7, 24.7,

-11.1, - 11.3,

8.29 8.30

This work

a = c = (c/a a =

Reference

[12, 131

aAverage atomic volume. b A V/V = { lOO(Vc.1,- z V,,)/z V,,}%. ‘This phase has never been observed alone but only together with the subsequent

2. Experimental

phase Sm,In.

details

2.1. Samples The metals used were samarium (99.9% pure) and indium (99.99% pure). About 50 samples, each weighing 1 - 2 gf, were prepared. The metals were enclosed in small tantalum crucibles sealed by welding and were heated in an induction furnace. No appreciable weight losses were observed during the preparation and the subsequent differential thermal analysis (DTA). 2.2. Differential thermal analysis After suitable heat treatment the samples were subjected

to several

283

DTA cycles. The heating and cooling rates were generally kept between 4 and 8 “C min-‘. The details of this procedure are given elsewhere 1143. 2.3. Metallographic analysis In several cases the samples were examined both in the as-cast condition and after several thermal treatments. After polishing with diamond paste, the alloys were etched in air or in acid solutions diluted with glycol and alcohol. 2.4. X-ray examination The X-ray examinations were carried out on powder samples annealed for a few hours at 250 “C. The Debye method was used with Cu K% or Fe Kr radiation. The measured crystallographic constants were refined by a leastsquares interpolation and the values of d were corrected using the Nelson Riley function. In most cases the observed diffraction intensities were compared with the values calculated using a program prepared for the HP 9825 A calculator.

3. Results The results are summarized in the phase diagram shown in Fig. 1. Some typical micrographs are shown in Figs. 2 - 5. The characteristics of the different phases and sections of the phase diagram are discussed below. 3.1. Samarium-rich alloys This region, as in other R-In systems, is characterized

by ranges of

2 Y.

Fig. 1. Phase diagram of the Sm-In system: A, thermal effects ohserved on heating; effects observed on cooling.

V. thrrmal

284

Fig. 2. Micrograph of as-cast Sm-lOat.%In, etched in air showing the a phase (black) and a eutectoid structure. (Magnification, 350x .) Fig. 3. Micrograph of as-cast Sm_26at.%In etched in air showing primary crystals of Sm,In (black) and the Sm-Sm,In eutectic. (Magnification, 260x .)

Fig. 4. Micrograph of as-cast Sm-59at.%In (black). (Magnification, 70x .)

etched in air showing Sm,In,

(white) and Sm, +Jn

Fig. 5. Micrograph of as-cast Sm66at.%In SmIn, eutectic. (Magnification, 87.5x .)

etched in air showing almost pure Sm,In,

(black)-

solid solutions formed by samarium in its c( and p modifications. The liquidus temperatures decrease from the melting point of samarium towards the eutectic at 905 f 5 “C and 16.5 f 1 at.% In. At this temperature the solubility of indium in p-Sm is about 13 - 14 at.% In. The p phase decomposes by a eutectoid transformation at 800 + 10 “C; the’ p eutectoid phase contained about 12 at.% In.

285

The liquidus and solidus curves for the l32 liq equilibrium were obtained from the DTA results. However, the measurements for the a t’ l3 equilibrium were much less clear; in particular the cx solvus could not be evaluated. Therefore the solubility of indium in the CLphase was extrapolated from the behaviour of other rare earths and was found to be equal to or greater than 5 at.% In at the eutectoid temperature (as shown in Fig. 1). It was possible to evaluate the JL function defined by J

L

=

AH,,,“(T- T,,,“) + T f”SO

RTln

where AH,,,” and T,,,” are the characteristic melting parameters of pure samarium for the samarium-rich alloys at the liq 2 p equilibrium. It is known that the difference of this function from zero can be related to the deviation of the system from ideal behaviour [4, 15, 161. Values of JL were calculated for various temperatures T using the xrf and xnL values shown in the phase diagram (Fig. 1). This function is compared in Fig. 6 with those of some R-In and R-T1 systems and with that of Sm-Tl which is currently under investigation. Very similar behaviour is evident for all the rare earths examined and it should be particularly noted that a negative deviation always follows a positive deviation. The light rare earths behave as ideal solvents for indium over a wider temperature range than the heavier rare earths do. The deviation from ideality and the derivative of JL with respect to temperature are both smaller for thallium than for indium. More detailed thermodynamic analysis of this behaviour will be possible only when data for other R-In and R-T1 systems are available.

AT.K

Fig. 6. The JL functions of lanthanum-rich, praseodymium-rich and samarium-rich alloys with indium and thallium plotted against the difference between the temperature at which J,_ was calculated and the melting point of the rare earth.

The DTA results and the microstructures of the as-cast alloys indicate the formation of the Sm,In phase when eutectic and eutectoidal decompositions occur. However, the powder patterns of some alloys containing 12 - 18 at.% In are characteristic of phases of the cF4 copper or

286

cP4 AuCu, types. This suggests that an Sm,In compound exists with stoichiometry and structure analogous to the R,X compounds formed by different rare earths with indium, thallium etc. In particular the Ce,In [2] and Pr,In [S] phases have both ordered and disordered structures. The data available at present do not allow the mechanism of formation and/or the melting behaviour of Sm,In to be defined precisely. By analogy with the phase diagrams of the light rare earths and in view of the behaviour of the reduced temperatures (Fig. 7) the existence of a peritectic (or peritectoid) temperature extremely close to the eutectic temperature cannot be eliminated. In fact, when samples in the 20 - 25 at.% In range are heated after annealing at 850 “C, a splitting of the 905 “C thermal effect that might correspond to two very close invariant transformations is observed. This double effect is not explicitly shown in the diagram in view of the di~culty of resolution and its sluggish behaviour.

La

Ce Pr Nd Pmsm Atomic number

Fig. 7. Characteristic reduced temperatures of R-In alloys plotted against the atomic number of the rare earth: m, melting: p, peritectic formation, A large uncertainty must be attributed to the hypothetical peritectic formation of the Sm,In phase.

3.2. &n&z This phase has previously been identified by Palenzona [ll]; its structure is of the hI% NiJn type. Our powder photographs agree with the data reported in the literature. Significant variations in the lattice parameters with varying composition were observed on samples that had undergone DTA. This fact, together with the appearance of the photomicrographs, suggests the existence of a homogeneity range at high temperatures. We propose that this phase forms by congruent melting at 1090 f 10 “C (in contrast with the La-In, Ce-In and Pr-In systems) although we assign a

287

melting point that is only slightly greater than the adjacent eutectic temperature. The different melting behaviour agrees with the greater ease of preparation of the compound by cooling from the molten phase: homogeneous brittle samples are obtained much more easily than with the lighter rare earths. This phase forms a eutectic with Sm, +.In at a temperature of 1080& 10 “C (about 10 “C lower than the melting point of Sm,In) and a composition of 35.OkO.5 at.?; In. 3.3. Sm, + Jn This phase has a composition close to 1: 1 that, as in other systems, is slightly richer in the rare earth. It is formed by congruent melting with a maximum at about 48 at,% In and 1210 & 20 X. ~etallographic examination of this phase shows that it is characterized by a rather wide homogeneity range, particularly at high temperatures, extending towards compositions that are richer in the rare earth. As in the Pr-In and Ce-In systems the alloys with exact 1: 1 stoichiometry appear to be heterogeneous, and DTA clearly confirms a shift of the maximum towards compositions that are richer in the rare earth. As with Ce , +,In and Pr 1+,In, some effects were observed in this phase at temperatures close to melting that could be interpreted as a solid state transformation. Powder photographs that appear to confirm the CsCl-type structure proposed for R-In phases [17 - 191 have been obtained for Sm, +,In. Other structures have been suggested for these phases on the basis of neutron diffraction analysis: for HoIn and TbIn these structures were described in terms of a tetragonal cell corresponding to a magnetic orthorhombic cell at low temperatures [20]. When all the data available in the literature are considered, however, the possibility that the C&l-type structure in the R-In systems corresponds to a metastable or a high temperature stable phase cannot be eliminated. This phase forms a eutectic with Sm,In, at a temperature of 1090 + 10 “C and a composition of 59.5 i: 1 at.‘%, In. 3.4. Sm,In5 This phase also melts congruently at a temperature of lllO& 10 C. As with other R,In, phases, a small solubility range and a slight shift in stoichiometry towards compositions richer in the rare earth were observed. The structural data are given in Table 1. The structure and general properties of this phase have recently been described and discussed together with those of other R,In, and R,Tl, phases [S]. This phase forms a eutectic at 1075 _t 10 “C and 67 2 1 at.% In with SmIn,. 3.5. SmIn, The RIn, compound has the highest indium content in the system. It forms congruently at 1130 & 10 “C and, like the other RIn, phases, is

characterized by a very narrow solubility range. The existence and crystal structure of this phase are well known; the structural data are reported in Table 1. SmIn, forms a eutectic with indium at about 154 “C and more than 99.5 at.% In.

4. General remarks The Sm-In diagram is compared with other R-In diagrams in Fig. 8. The sequence of intermediate phases is very similar in all the diagrams. However, a distinct R,In compound is only present for the lighter rare earths and the RIn, compound is stable only for the first two rare earths. This progressive reduction in the number of compounds seems to correspond to their subsequent conversion into congruently melting phases. The increasing relative stability of the R, +Jn compounds has been discussed in a previous paper [6] where the dependences of the melting temperatures and of the reduced temperatures of different phases on the atomic numbers of the rare earths were correlated with the dependences of the molar volumes, heats of formation etc. on atomic number.

-at.%

In-

Fig. 8. A comparison of the alloying behaviour of the light rare earths with indium. The R-In diagrams are plotted against the atomic number of the rare earth.

289

References

6 7 8 9

10 11 12 13 14 15 16 17 18 19 20

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