432 JOURNAL OF THE LESS-COMMON METALS

Short Communication

Melting points of La&, CeRus, and PrRuz*

In a current program dealing with the electronic properties of 4f and 5f transition metals and alloys it was necessary to know the melting points of the LaRuz, CeRuz, and PrRuz cubic Laves phases. Because of the growing interest in rare-earth metal alloys, especially the rare-earth Laves phase compounds, it was felt that publication of the melting points of these three compounds might be of interest to other workers in this area.

The specimen, in each case, consisted of roughly half of an arc-melted button. None of the specimens was single-phase, but the phase sought constituted 80 to 90% of the specimen by volume. Present in all the samples was a dark structure, probably a eutectic. Also present were small amounts of what appeared to be a ruthenium-rich terminal solid solution, judging from its apparent metallographic continuity, in one instance, with a particle of ruthenium which had been incompletely melted. Through X-ray diffraction analysis it was possible to identify the Laves phase in each case, but not to identify the other phases. The weighted-average

TABLE I

LATTICE CONSTANTS OF LaRuz, CeRuz, AND PrRuz

Compound No. determinations Temp.

(“C1

LaRuz 7.7034 * 0.0004 4 18 & 2

CeRuz 7.5364 * 0.0002 3 I9 z!z 1 PrRuz 7.6223 * 0.0001 4 21 + 3

lattice constants of the three Laves phases are given in Table I. The X-ray data were obtained by use of a 114.6 mm Debye-Scherrer camera, using Cu KOL radiation. The lattice constants were calculated from Koli and KOLZ doublets in the back-reflec- tion region, using a #tan$ extrapolation method. These results are in good agree- ment with the previously-published lattice constantsi*s.

The specimens were suspended by a fine tungsten wire at the midpoint of a vertical, self-resistively-heated tungsten tube 25 cm long by 2.5 cm in diameter. They were observed with a telescope for changes in shape while the temperature was read on the inside surface of the heater tube with a calibrated optical pyrometer through a 2.5 mm lateral hole in the radiation shielding and the heater tube. The furnace was maintained at a pressure of the order of 3 10-s torr or less throughout each deter- mination. The melting point was taken to be that temperature at which the specimen was observed to coalesce into a spheroidal shape, wetting the loop of tungsten wire passed around it. Although the wire was wet by each sample, there was no metallo-

* Work performed under the auspices of the United States Atomic Energy Commission.

J. Less-Common Metals, 5 (1963) 432-433

SHORT COMMUNICATION 433

graphic evidence for a reaction between these materials. Window-corrections were ap- plied to the pyrometer readings. Further temperature corrections, based on subsequent work with thermocouples, were made to allow for the fact that the equilibrium tem- perature is lower than the brightness temperature of the inside surface of the heater tube adjacent to the specimen in a furnace of this type.

The observed melting points of the three samples were: LaRuz = 1431 f 3o”C, CeRuz = 1539 f 3o°C, and PrRu2 = 1681 f 15°C. After these data had been obtained, OBROWSKI~ published the constitutional diagram for cerium-ruthenium alloys. He found that CeRu2 melts at 1570 & IO%, which is in agreement with the upper limit of our value for the melting point of CeRuz. Because of experimental difficulties, as noted below, we believe that the melting point of CeRu2 is probably closer to OBROWSKI’S value than to our own.

Owing to the intrinsic coarseness of adjustment of the furnace power, the coales- cence of each sample was observed to take place between one furnace setting and the next. The reported melting points are the arithmetic means of the equilibrium temperatures observed at the two adjacent settings, accounting for the principal experi- mental error. Whether the observed melting points are congruent or incongruent cannot be decided on the basis of the present data. We are inclined to believe that these Laves phases melt incongruently because of our observation that the three- phase structures in samples held in a partly-melted condition for several minutes and slowly cooled are essentially the same as those observed in the arc-melted buttons. This conclusion is in agreement with the data presented by OBROWSKI for cerium- ruthenium alloys.

It is interesting to note that the increase in the melting points of these RRu2 compounds with increasing atomic number (2) of R is much larger than that observed for other intermetallic compounds containing La, Ce, and Pr. For these RRuz compounds the melting point increases by approximately IZO’C for an increase of one in 2, but for other RM, compounds (where M is a non-rare-earth metal) the in- crease is usually less than 50°C.

Los Alamos Scientific Laboratory,

University of California, Los Alamos, N. M. (U.S.A.)

ROBERT D.REISWIG

KARL A.GSCHNEIDNER,J~.*

1 W. B. COMPTON AND B. T. MATTHIAS, Acta Cryst., IZ (1959) 651-654 2 A. E. DWIGHT, Trans. Am. Sot. Metals, 53 (1961) 479-500. 3 W. OBROWSKI, 2. Metallk., 53 (1962) 736.

Received February 18th, rg63

* On leave from the Los Alamos Scientific Laboratory from September, 1962 through August, 1963. Present address: Department of Metallurgy and Institute for Atomic Research, Iowa State University, Ames, Iowa.

J. Less-Common Metals. 5 (1963) 432-433