Journal of the ~e~-Common Metafs, 43 (1976) 205 - 209 @ Elsevier Sequoia S. A., Lausanne - Printed in Switzerland
A. IANDELLI and A. PALENZONA Istituto di Chimica fisica, Universitd di Genava, Genoa (Italy) (Received February 13, 1975)
Summ~y The ytterbium-platinum system has been studied using differential thermal analysis for the alloys up to 45 at.% Pt, and micrographic and X-ray methods over the whole range of composition. Eight intermediate phases are formed, with the compositions: Yb,Pt,, YbzPt, YbsPts, Yb,P&, YbPt, Yb3Pt4, YbPte and YbPts, and their crystal structures have ,been determined. Little solid solubility exists for YbPt, or for ytterbium in platinum. From preliminary magnetic measurements, Yb behaves as a divalent element in Yb,Pts; in the others it is partially or wholly trivalent.
Introduction Three intermetallic phases are known to exist in the alloys of platinum with ytterbium, namely, YbPt crystallizing with the FeB type of structure [l] , an MgCuz-type Laves phase, YbPts  and YbPts with the AuCu, structure type [3,4], As part of a systematic investigation of the behaviour of europium and ytterbium with the 8th Group elements, we have studied the binary phase diagram of ytterbium with platinum and we report here the results obtained on this system.
The ytterbium-platinum phase diagram has been studied by means of differential thermal, X-ray and micrographic analysis and by magnetic measurements. As the melting point of the alloys, starting from pure ytterbium, rises rapidly well above 1500 “C for the equiatomic composition, thermal analysis with our instrumentation has only been possible for alloys from Yb up to 45 at.% Pt. These alloys have been prepared, in molybdenum crucibles welded under argon, by direct synthesis of turnings of Yb and Pt powder, following an already reported method [ 51. Commercial ytterbium was purified from hydrogen by the method reported by *Dedicated to Professor Dr. E. Raub in celebration of his 70th birthday.
McMasters and Gschneidner , and a metal obtained whose thermal characteristics (m.p. = 815 “C; t.p. = 765 “C) were near to those for “pure ytterbium” (m.p. = 816 ‘C; t.p. = 792 “C). Platinum at 99.9% was obtained from Johnson-Matthey and Co., Ltd. Alloys with a higher Pt content have been studied by preparing several samples of differing composition and examining them in the as-cast or “after-annealing” conditions. The preparation was effected in two steps: first the two metals were allowed to react in welded MO containers at a temperature not higher than the melting point of Yb, and then the temperature was raised to, and maintained at, 1100 “C for several days. The products of this first reaction were then pressed into cylindrical tablets and melted in a copper-cooled hearth arc furnace, under ultra-pure argon. The arc-fusion is made possible without loss of Yb because its vapour pressure is very low if Yb is present in a compound in the trivalent state, as is the case for the Ptrich phases. Both powder, and single-crystal X-ray diffraction methods were used to study the structure of these alloys. Copper, chromium and molybdenum Ka radiations were employed; no special care was necessary in handling the specimens as they are unreactive in the atmosphere. Samples for microscopic examination were prepared with the aid of silicon carbide papers and diamond polishing compounds; dilute alcoholic iodine solution was used as the etching reagent for the Yb-rich compounds; for Pt-rich samples, “aqua regia” at different concentrations was necessary. Results Figure 1 shows the phase diagram of the Yb-Pt system in the composition range thermally explored. Starting from Yb, the melting points of the alloys decrease to 655 “C at 12.5 at.% Pt and then rise rapidly to above 1500 “C for the 1:l composition. In this first part of the diagram one eutectic and four compounds, all formed by peritectic reactions, are observed. Yb,Pts shows a solid-state transformation at 1200 OC, and the data reported in Table 1 refer to the low-temperature modification. The alloys in this range of composition need prolonged heat treatment in order to reach equilibrium conditions. Nevertheless, single crystals are frequently formed and can be examined by X-rays, thus enabling the crystal structure and the composition of the single phases to be determined. The second part of the diagram mentioned above has not been studied thermally. Nevertheless, it was possible to identify the other phases by preparing and examining samples of the various compositions. Table 1 contains the crystallographic data for all the phases of the system. Some indication of their stability can be deduced from the X-ray results and from the micrographic examination; these studies indicate that YbPt and YbPts probably do not have congruent melting points, whilst YbsPt, and YbPt, are formed directly from the melt. The various phases do not show
1400 1300 1200 1100 1003 ? z u
Fig. 1. Partial ytterbium~latiuum
TABLE 1 Compound
Lattice constants (A)
6.476 p = 97” 37’
Pu3Pd4 bR4 2-R3
AuCu3 cP4-Pm 3m
*The symbols of the space group are preceded by the Pearson notation [S 1.
**SC = single crystal, p = powder.
appreciable solid solubility; for YbPts the limiting values of the lattice constant, obtained from samples of different composition, are reported in Table 1. The isomorphous compound, LuPts, also shows a little solid solubility: we have found a value of 4.039 A for the lattice constant of the phase as compared with the reported values of 4.027  and 4.030 A . YbPt, is the most platinum-rich compound of the diagram, in accordance with the results of Bronger . Micrographic examination of a sample of the composition YbPt, (for other R.E. with Pt, 1: 5 phases are formed) indicates the presence of two phases, and the X-ray diffraction pattern shows the reflections of YbPts with those of a face-centered structure with u = 3.944 K, which corresponds to a little solid solution of Yb in Pt. Discussion The results already obtained in other binary alloy systems of Yb with the 8th Group elements, namely, the formation of phases with di- or trivalent Yb, or with an intermediate valency, have also been found in the Yb-Pt system. From preliminary magnetic measurements, and from a consideration of the lattice dimensions, Yb appears to be divalent in the Yb-richest compound, Yb,Pts, in agreement with its isomorphism with the already studied compounds Yb,Pd, and EusPds, containing either divalent Yb or Eu. The compounds YbPt, YbsPt,, YbPts and YbPts contain Yb(III), while YbsPt, Yb,Pts and Yb5Pt4 appear to consist of a mixture of Yb(II) and Yb(II1). The lattice constant of the compound YbPts (MgCus-type), reported in Table 1, has been obtained from samples quenched from high temperature. The value reported by Moriarty et al.  for the same compound (7.381 h) is rather different from the present value, and shows no regularity in the general trend of the lattice constants of the R.E.Pts compounds. Samples of YbPts, annealed at 1000 - 1100 “C for some days show powder patterns completely different from the preceding ones, and they could not be assigned to other known 1:2 structures. Considering the structures of the compounds YbRe,, YbOss(MgZns), YbIrs(MgCus)  and YbAus  (MoSis), all containing Yb(III), we find that between the last two phases there is a change from the Laves phase structures to the MoSis-type. Therefore, the high-temperature form of YbPts is consistent with this structural sequence, while at low temperatures, the MgCu, structure is no longer stable and transforms to another type, which is, perhaps, intermediate between the Laves phases and the MoSi, types. An analogous behaviour is observed in the Second Long Period for YbRus(MgZn,), YbRh,(MgCu,) and YbPd,, where this last compound exists only in a narrow temperature range and has a complex crystal structure. Samples of Lu-Pt alloys around the 1:2 composition have been prepared: in this case, no LuPts (MgCus-type) compound is formed and the
TABLE 2 Compound
WPz Yb2Pt Yb5Pt3 YbgPtg YbPt
18.2 23.1 25.2 25.5 26.6
2 2-3 2-3 2-3 3
Yb7Aug YbZAu YbgAu3 Yb5Aq YbAu(ac)
10.6 15.5 15.7 16.6 20.0
2 2 2 2 2-3
V = ytterbium valency.
powder pattern resembles that of the low-~mperature form of YbPtz. Finally, we may compare the two systems Yb-Pt and Yb-Au. A great number of phases are formed in both systems [ 51. Some of them show isomorphism (YbsPt, YbaAu; Yb5Pt4, YbsAu*; YbPt, a-YbAu), others have the same composition (YbsPts, YbS Au3; YbPts, YbAu,; YbPt,, YbAu3 ) and some have different compositions (Yb,Pta, Yb,Aus; Yb3Pt4, YbAu,). The differences between the Yb-Au and Yb-Pt systems appear to be linked both with the different dimensions of the Au and Pt atoms and with the ease with which Yb behaves as a di- or a trivalent element in the two cases. The alloys with more than 50 at.% Au or Pt contain Yb in the trivalent state, the others in the Yb-Au systems are formed by YbfII), which is different from the shalom in the Yb-Pt system already discussed. A factor which appears related to the possibi~ty of Yb becoming trivalent is the volume contraction in the formation of its phases from the elements. Table 2 gives the values for the Yb-richest compounds in the Pt and Au systems, and for this group of compounds, the trivalency of Yb appears only if the volume contraction is greater than 20%. References 1 Q, Johnson, R. G. Bedford and E. Catalano, J. Less-Common Met., 24 (1971) 335. 2 J. L. Moriarty, J. E. Humphreys, R. 0. Gordon and N. C. Baeniiger, Acta Cry&, 21 (1966) 840. 3 W. Bronger, J. Less-Common Met., 12 (1967) 63. 4 B. Erdmann and C. Keller, J. Solid State Chem., 7 (1973) 40. 5 A. Iandeili and A. PaIenzona, J. Less-Common Met., 18 (1969) 221. 6 0. D. McMasters and K. A. Gschneidner, Jr., J. Less-Common Met., 8 (1965) 297. 7 U.S. A.E.C. Rep. IITRI-5’78, 1964, P19-13. 3 W. B. Pearson, A Handbook of Lattice Spacings and Structures of Metals and Alloys, Vol. 2, Pergamon Press, Oxford, 1967.