Journul
of the Less-Common
Metals,
(’ Elsevier Sequoia
S.A., Lausanne
RARE-EARTH
MIXED
GIN-YA
TORU
ADACHI,
Department Osaka-fir
o.f Applied
301
32 (1973) 301-306 - Printed in The Netherlands
DICARBIDES
NISHIHATA
Chemistry.
Fuculty
and JIRO of
SHIOKAWA
Engineering,
Osuka
Unicersity,
Yamadakami,
Suita-shi.
(Japan)
(Received
February
20. 1973)
SUMMARY
Solid solutions of two different rare-earth carbides have been prepared and the relation between their lattice constants and compositions has been discussed. Body-centered tetragonal solid solutions were formed in the whole range of LaC,-(CeTb)C,, CeC,-(Pr - Ho,Y)C, and PrC,
INTRODUCTION
The dicarbides, with the exception of ThC,, are b.c.t. (CaC, type) at room temperature but possess f.c.c. modifications at higher temperatures’. The temperature at which the tetragonalcubic-transformation occurs, varies significantly from one dicarbide to another, e.g., CaC, at 450°C and UC, at 176551820°C. Bowman et al.’ also examined the high-temperature neutronand X-raydiffraction and found the same transition (tetragonalcubic at 1060°C and 1320°C for LaC, and YC,, respectively). McColm et a1.3 investigated the UC,-LaC, and UC,CeC, systems and concluded that the cubic modification was able to exist down to room temperature. The present authors have also reported, briefly, some phase relations in solid solutions of two different rare-earth dicarbides4. The purpose of the present paper is to communicate the results of a study in detail of the solid solutions of two different rare-earth dicarbides. EXPERIMENTAL
Mixed dicarbides were prepared by the Vickery method5. Two kinds of rare-earth oxides (with the exception of CeO,), of 99.9% purity (Shin-etsu Chemical Corp.), were weighed, mixed and dissolved in 2N HCl and a saturated solution of oxalic acid was added. The precipitate formed was filtered, oven-dried at 110°C for 1 day, and ignited. In the case of praseodymium and terbium, any
302
G.-Y. ADACHI,
T. NISHIHATA,
J. SHIOKAWA
Pr4+ or Tb4+ present in the resulting mixed oxides were reduced by hydrogen to the trivalent state. The mixed oxides obtained were thoroughly blended with a spectroscopic electrode-grade graphite powder (300 mesh) in stoichiometric proportions by grinding together, and were then pressed into pellets under vacuum. In the case of the cerium series, an aqueous solution of CeCl, of known concentration was used as the source of cerium ion. Carbonation of the oxides takes place according to reaction (1) or (2). mLn?jO,+nLn$O,$7(m+n)C pCeO,+qLn:O,+(4p-t-7q)C
3 2mLn’C2.2nLn2C2+3(m+n)C0. + pCeC,~2qLn2C,+(2p+3q)C0.
(1) (2)
The pellets were placed in a molybdenum container and heated at 1600°C by an induction furnace under a vacuum of 10m4 mmHg. The heating time was varied from 3 h to 20 h according to conditions. Each of the products was divided into three parts. One was analyzed for total carbon content by igniting the weighed product to the corresponding oxide. The second part was hydrolyzed in 3.0 N HCl in order to determine the combined carbon6. The third part was preserved under paraffin oil for further examination.
3.95-
6.60
32
Molefraction
Ir’
PrC2
Mole
Fig. 1. Lattice
constants
vs. composition
for the system
LaC,-PrC,.
Fig. 2. Lattice
constants
vs. composition
for the system
LaC,-NdC,.
fraction
NdCZ
RARE-EARTH
MIXED
303
DICAR~I~ES
The metal content was determined by means of an X-ray fluorescence method using a Rigaku-Denki X-ray fluorescence spectrometer. X-ray diffraction data of the samples were obtained with a Rigaku-Denki “Geiger-flex” using Ni-filtered Cuba radiation. Silicon, 99.999% purity (a= 5.4301 A), was employed as an internal standard. RESULTS
AND
DISCUSSION
A golden-coloured mixed dicarbide was present in all the reaction products, but in the systems which contained europium or ytterbium a mixed carbide could not be formed since these carbides would be very volatile at high temperatures. Cerium-thulium dicarbide also could not be obtained because of the difference in reactivity between both ceric and thulium oxides’with graphite. Chemical analyses of the products indicated that these possessed almost exactly the compositions of dicarbides. The relationship between the lattice constants and compositions in the systems La-Pr-C, La-Nd-C, Ce-Y-C and La-Y-C are shown in Figs. 14. It is clear that the first three formed b.c.t. solid solutions since Vegard’s law held over 3.QOr
CCC2
Mole
fraction
YC2
3.001 LaC*
Fig. 3. Lattice
constants
us. composition
for the system CeC,-YC,.
Fig. 4. Lattice
constants
t’s. ~on~position
for the system
LaC,-YC2.
Mole
fraction
‘I
304 TABLE
G.-Y. ADACHI,
T. NISHIHATA,
J. SHIOKAWA
I
POWDER
X-RAY
PATTERNS
OF THE FACE-CENTERED
CUBIC
LaErC,
a = 5.6854 A
3 4 8 11 12 16 19 20 24 21
TABLE LATTICE
3.2919 2.8321 2.0058 1.7154 1.6405 1.4224 1.3048 1.2711 1.1595 1.0953
100 100 38 20 9 3 5 4 3
3.2825 2.8427 2.0101 1.7142 1.6412 1.4214 1.3043 1.2713 1.1605 1.0941
*
L
II CONSTANTS
OF F.C.C. OR HEXAGONAL
La
(A) DY
5.72
Ho Y Er Tm Yb
5.69 5.75 5.68 5.62 *
LU
a 5.78** c 8.32
;i,
MIXED
DICARBIDES
Pr
(4 -
5.63 * *
*
5.62
5.62
* The mixed dicarbide was not formed ** Hexagonal modification.
because
of vaporization
of Tm or Yb.
the whole range. On the contrary, the curve for the system La-Y-C is broken in the neighborhood of 50 mole% LaC, and at this point the existence of a f.c.c. phase was observed4. The same phenomenon was observed in the systems La-Dy-C, La-Ho-C, La-ErC, La-TmC, Ce-Er-C, Ce-Lu-C and Pr-Lu-C. X-ray diffraction data of a typical cubic phase, LaErC,, and the lattice constants of other cubic modifications, are given in Tables I and II, respectively. It can be deduced, from the facts described above, that the effect of sizedifference is of importance in the formation of the cubic phase. Table III shows the relationship between ionic radii and phases which appear in the La-Ln-C, Ce-Ln-C and Pr-Ln-C systems. The ionic radii quoted are from Templeton’s’ paper for the co-ordination number 6, except in the case of Y3+. For yttrium, Zachariasen’8 value of the ionic radius has been used, since a value for Y3 + does not appear in Templeton’s table.
RARE-EARTH TABLE
III
IONIC
RADII
MIXED
AND STRUCTURES
Ionic radius
Ld+
30s
DICARBIDES
OF THE MIXED Phase
La-Ln* La
La Ce Pr Nd Pm Sm EU Gd Tb DY Ho Y Er Tm Yb LU * Relative
TABLE
102 11 1 003 112 113 114 115 006 304 223 116 117
Phase
b.c.t. S.S.
f.c.c.
IlO
hexagonal
(1: 1) Pr-Ln*
Phase
Pr
0.000 ’ 0.020 0.038 (0.053) 0.067 (0.08 1) ’ 0.093 0.107 0.123 0.135 0.150 J 0.150 (0.160) (0.171) 0.180
b.c.t. S.S.
f.c.c. no f.c.c.
0.000 0.018 (0.034) 0.048 (0.062) 0.074 0.089 0.104 0.117 0.131 0.131 0.142, (0.143) 0.163
\
b.c.t. S.S.
no tc.c.
in ionic radius.
IV
POWDER a=5.78
differences
Ce-Ln* Ce
0.000 0.025 0.045 0.062 (0.077) ) 0.092 (0.105) 0.116 0.130, 0.144 0.157 0.171 0.171 0.181 I (0.191) 0.20 1
1.061 (A) 1.034 1.013 0.995 (0.979) 0.964 0.950 0.938 0.923 0.908 0.894 0.88 0.88 1 0.869 0.858 0.848
DICARBIDES
X-RAY
PATTERNS
A and c=8.32
3.26 2.82 2.76 2.38 2.00 1.69 1.44 1.38 1.30 1.29 1.26 1.10
OF THE HEXAGONAL
LaLuC,
8,
3.20 2.77 2.73 2.37 2.00 1.69 1.44 1.39 1.30 1.28 1.25 1.10
86 100 38 29 29 24 12 6 5 5 1 6
In the La-Ln-C systems, only a b.c.t. solid solution was obtained in the region from Ce to Tb, while a f.c.c. phase was formed in the region from equimolar Dy h Tm, i.e., the phase has changed at a 14% difference in ionic radius La3 +-Ln3 +/La3 +. In the Ce-Ln-C systems, a f.c.c. phase appeared at Ln=Er, or at a 15%
306
G.-Y. ADACHI,
T. NISHIHATA,
J. SHIOKAWA
radius difference. McColm et aL3 have suggested from a measurement of the magnetic moment, that some of the cerium ions in CeC, must be in the Ce4+ state. That is, the ionic radius of the cerium ion in the dicarbides would be smaller than that of a normal terpositive ion and the point of phase-change in relation to the radius difference with Ce-Ln< should be slightly smaller than 15%, and probably very close to 14%. A similar value of the limiting value (14~ 15%) is expected also in the systems Pr-Ln< and falls on the curve corresponding to Yb3+ but the mixed dicarbides of ytterbium have not been obtainable, as mentioned above. A mixed dicarbide of lanthanum with lutetium (La: Lu= 1: 1) appeared to be hexagonal and the X-ray diffraction result is shown in Table IV. Details of this phase will be reported elsewhere’. The structure of the high-temperature cubic phase of single lanthanide dicarbides has also been investigated and the space group Fm3m is proposed with metal atoms in ($, 3, f) and C, group with centers at (0,0,O) oriented at random along {11l} planes 2. The mixed dicarbides in the cubic phase seem to possess the same structure as described above. REFERENCES 1 N. H. Krikorian, T. C. Wallace and M. G. Bowman, Properties Thermodynamiques Physiques et Structurales des Devices Semimetallique, C.N.R.S., Paris 1967, p. 487. 2 A. L. Bowman, N. H. Krikorian, G. P. Arnold, T. C. Wallece and N. G. Nereson, U.S. Ar. Energy Comm. LA-DC-8451, CESTI, 1967, p. 7. 3 I. J. McColm, I. Colquhoun and N. J. Clark, J. Inorg. Nucl. Chem., 34 (1972) 3809. 4 G. Adachi, H. Kotani, N. Yoshida and J. Shiokawa, J. Less-Common Metals, 22 (1970) 517. 5 R. C. Vickery, R. Sedlacek and A. R$en, J. Chem. Sot., (1959) 498. 6 R. G. Gebelt and H. A. Eick, Inorg. Chem., 3 (1964) 335. 7 D. H. Templeton and C. H. Dauben, J. Am. Chem. Sot., 76 (1954) 5237. 8 W. H. Zachariasen, in G. T. Seaborg and J. J. Katz (eds.), The Actinide Elements, McGraw-Hill, New York, 1954, Chap. 18. 9 G. Adachi and J. Shiokawa, Chem. Letters, to be published.