Some face-centered-cubic mixed rare-earth dicarbides

Journal of the Less-Common Metals Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

517

Short Communication Some face-centered-cubic mixed rare-earth G. ADACHI, Department

H. KOTANI,

ofAppliedChemistry,

N. YOSHIDA Faculty

dicarbides

AND J. SHIOKAWA

of E%gineering, Osaka Cnivzwity.

Suita, Osaka-fu

(,Japan)

(Reccixred July 7th, 1970)

Several rare earth dicarbides which possess a face-centered-cubic symmetry (f.c.c.) have been synthesized. Single rare earth dicarbides like CaCz possess a body-centered tetragonal (b.c.t.) symmetryl. Rare earth dicarbides which contain two different rare-earth elements, whose ionic radii are similar, also form b.c.t. solid solutions and a relation between the lattice constants and mole fractions of rare earths can be represented by the Vegard law over the whole region.

3.001 LaC2

Mole fraction

yc2

Fig. I. Relations between lattice constants and composition

of the system LaCz-YCZ.

Double dicarbides, however, containing two different rare-earth ions, with very different radii, were found to be face-centered cubic at a certain mole ratio of the rare-earth elements. In the system La-Y-C, the f.c.c. phase was observed at a I : I mole ratio of La&-YC2 while in other regions the b.c.t. phase was formed. J. Less-Common Metals,

22 (1970) 517-518

SHORT

518

COMMUNICATIONS

The cubic unit cell dimension of this phase determined by X-ray diffraction A, but those of the single phase in tetragonal La& and YC2 were a= 3.g34& c=6.572 A, anda=3.664& c=6.16g Ai, respectively (Fig. I). A tetragonal-to-cubic transformation has been observed to occur at 1060°C for La& and at 132o’C for Y&2. In a double dicarbide such as La&. YC2, a reversetransformation to tetragonal symmetry during cooling seems not to occur. In the system La-Dy-C (a=5.72 A) and Pr-Lu-C (a=5.62 A) the f.c.c. phases were also formed at the same mole ratio. The limit of the ionic radius difference for the formation of a f.c.c. phase, namely (RL&~+--RL~s+)/R~~s+ or (Rrr3+RRLn3+)/Rpr~+,seems to be about 14%. Preparative details, for example, of the reaction: was

a=5.75

1600°C

La&

+

Yz03 +

2LaCz eYCz + 6CO r4C e “BCUum

were similar to those described in the literatures. REFERENCES I F. H. SPEDDING, K. GSCHNEIDNER JR., AND A. H. DAANE, J. Am. Chem. Sot., 80 (1958) 4499. 2 A. L. BOWMAN, N. H. KRIKORIAN, G. P. ARNOLD, T. C. WALLACE AND N. G. NERESON, U.S.At. Energy Comm., LA-DC-8451 CESTI’, 1967, p. 7. 3 R. C. VICKERY, R. SEDLACEK AND A. RUBEN, ,I. Chem. Sot., (1959) 498. J. Less-Common

Metals,

zz (1970)

517-518