YCoC and Isotypic Carbides with a New, Very Simple Structure Type
M atthias H. Gerss and Wolfgang Jeitschko*
Anorganisch-Chemisches Institut der Universität M ünster. C orrensstraße 36, D-4400 Münster
Z. Naturforsch. 41b, 9 46-950 (1986); received March 12, 1986
Crystal Structure, Equiatomic Ternary Rare E arth Metal C obalt Carbides
The crystal structure of the new com pound Y CoC was determ ined from X-ray powder data. It
is tetragonal, space group P4:/mmc. with a = 0.36500(4) nm, c = 0.68636(9) nm and Z = 2
formula units per cell. The residual for a refinem ent of D ebye-Scherrer data is R = 0.048 for 22
structure factors and 3 variable param eters. The structure is of a new type with no variable
positional param eter. The arrangem ent of the metal atoms corresponds to that of the CsCl
structure. The tetragonal superstructure with a doubled c axis arises through the ordered insertion
of carbon atoms on octahedral sites formed by four Y and two Co atoms. The hydrolysis of YCoC
in hydrochloric acid yields mainly m ethane, propane, and ethane. The compounds LnCoC (Ln =
Gd —Tm. Lu) are isotypic with YCoC.
Introduction
During our investigation of the ternary system dysprosium-cobalt-carbon we have prepared DyCoC 2
and refined its crystal structure from single crystal
X-ray data [1], Here we report on DyCoC and sever­
al other new isotypic carbides, the crystal structure of
which we determ ined for YCoC from X-ray powder
data.
uli). None of these carbides is strongly attracted by a
magnet at room tem perature. The crushed samples
decompose in air during a period of several days.
The hydrolysis of YCoC in 2 N hydrochloric acid
yields 56 (weight)% C H 4, 24% C 3H 8, 20% C 2H6, and
about 0.5% C 2H4. This analysis was made in a gas
chrom atograph with a flame ionisation detector. To
control our experim ental equipment we hydrolyzed a
sample of A14C3, which released 99.99% methane.
Results
Sample preparation
Stoichiometric mixtures of rare earth metal chips
(99.9% ,
40 mesh),
cobalt
powder
(99.9% ,
325 m esh), and graphite flakes (99.5% , 20 mesh)
were coldpressed to pellets of about 0.5 g. A fter
reaction in an arc melting furnace under reduced
pressure (600 mbar) of purified argon, pieces of the
samples were wrapped in tantalum foil and annealed
for 10 d at 900 °C in evacuated silica tubes. E xam ina­
tion of the samples in a scanning electron microscope
showed their microcristallinity, both in the as cast as
well as in the annealed condition, and we were not
successful in isolating single crystals suitable for a
structure determ ination.
Lattice constants
Guinier powder patterns were recorded using
CuKct] radiation with a-quartz (a = 0.49130 nm, c =
0.54046 nm) as standard. By trial and error we were
successful to assign indices to these patterns on the
basis of tetragonal cells, which we refined by leastsquares fits (Tab. I). Excellent agreements were ob­
tained between observed and calculated [2 ] inten­
sities assuming the atomic positions obtained by the
structure determ ination of YCoC.
Table I. Lattice constants of tetragonal YCoC type com­
pounds. H ere and in the following tables standard devia­
tions in the least significant digits are given in parentheses.
Properties
Com pound
a [nm]
c [nm]
V [nm1]
The new equiatomic rare earth metal cobalt car­
bides are grey with metallic lustre. They have good
electrical conductivity (measured for compact reg-
YCoC
G dCoC
TbCoC
DvCoC
H oCoC
ErC oC
TmCoC
LuCoC
0.36500(4)
0.36613(5)
0.36523(7)
0.36468(3)
0.36443(3)
0.36361(4)
0.36309(5)
0.36206(3)
0.68636(9)
0.7025(1)
0.6931(2)
0.68642(7)
0.67967(6)
0.67328(8)
0.6676(1)
0.65803(7)
0.09144
0.09417
0.09245
0.09129
0.09027
0.08901
0.08801
0.08626
* R eprint req u ests to Prof. D r. W. Jeitschko.
V erlag d e r Z eitsch rift für N aturforsch u n s, D-7400 T iibingen
0 3 4 0 -5 0 8 7 /8 6 /0 8 0 0 -0 9 4 6 /$ 01.00/0
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947
M. H. G e rss—W. Jeitschko • Y C oC an d Isotopic C arb id es
Crystal structure o f Y CoC
Intensity data of the carbide YCoC were m easured
with an optical densitom eter first from a G uinier pat­
tern recorded with CuKct] radiation, then also from a
Debye-Scherrer diagram obtained with M n-filtered
Fe radiation. The Debye-Scherrer pattern had lower
resolution, but more data with better orientation
statistics of the microcrystals. The observed structure
factors F 0 were calculated after accounting for m ulti­
plicity and the Lorentz-Polarization and geometric
factors [2].
At first the positions of the metal atoms were de­
duced for the pseudocubic subcell (a ' = a, c' = c/2 )
by geometric arguments. They correspond to the
CsCl structure with one formula unit per cell. In this
cell it is not possible to place the carbon atoms at
reasonable distances to the metal atoms without re­
sorting to a solution with statistical distribution. A
careful reinspection of the powder diagrams then re­
vealed one weak reflection — the 101 reflection —
which requires a doubling of the c axis. At this stage
a least-squares fit of the Guinier data resulted in a
residual of R — 0.085 with the scale factor as the only
variable. The positions of the carbon atoms were
then located by difference Fourier syntheses. A
least-squares refinement including these carbon posi­
tions lowered the R value to 0.065.
Since we were not fully satisfied with this result,
we refined the structure using the larger data set ob­
tained from the Debye-Scherrer diagram. All leastsquares refinements were by full-matrix m ethods, us­
ing weights accounting for the estim ated standard
deviations, and atomic scattering factors [3] cor­
rected for anomalous dispersion [4]. A refinem ent
with individual atomic tem perature factors showed a
large value for the carbon atoms. T herefore, in the
final cycles we refined, besides the scale factor, an
occupancy factor for the carbon atoms together with
an overall tem perature factor. The final conventional
residual is R = 0.048 (weighted residual /?w = 0.044)
for 22 structure factors and three variable param e­
ters. The unweighted residual based on intensities is
R = 0.055. A final difference Fourier analysis
showed as highest peaks electron densities of
1 .3 x l0 3 e/nm ' and 1 .2 x l 03 e/nm 3, both too close to
the Co atoms to be suited for additional carbon sites.
The thus obtained structure has spa,ce group
P 42/m m c -D 4h (space group extinctions: h h t only
with € = 2 n; because of the special atomic positions
occupied, no intensities can be observed for the re­
ciprocal space positions 201, 203, 311, and 205) with
Z — 2 formula units per cell and a calculated density
g c = 5.81 g/cm’. The agreement between calculated
and observed structure factors for both, the Guinier
and the Debye-Scherrer data is shown in Table II.
The atomic param eters are listed in Table III,
interatom ic distances in Table IV.
Table II. X-ray powder diagrams of YCoC. On the left
side the evaluation of the Guinier pattern is shown; on the
right the structure factors obtained from the Debye-Scherrer diagram.
G uinier pattern
C uK a,
Fc
Io
F„
Ic
10
7
29
2
1
20
25
16
7
89
96
24
91
97
28
1
1
45
97
4
47
-
24
<
100
6
< 1
23
-
95
8
6
90
-
0
-
0
7
3
9
87
2
78
28
1
1
-
1
-
-
<
Q„
Qc
752
850
963
1501
1599
2351
751
849
963
1501
1600
2350
2661
3002
3215
3396
3753
3851
3965
4147
4602
4897
4913
5663
6005
6057
6398
6755
6854
6968
7149
7506
7604
7641
7718
8309
8355
8392
-
3001
-
3396
22
-
15
4
-
21
-
2
2
41
19
42
17
23
74
73
-
0
-
0
-
1
-
-
8
78
4
72
4
71
-
<
10
-
16
<
1
16
-
72
75
70
4603
4898
-
8 666
O =
100
/d : [nm -2]
9059
9142
9401
Debye-Scherrer
FeK a
h k /
F„
Fc
1
0
0
0
0
2
1
0
1
1
1
0
1
0
2
1
1
2
1
0
3
2
0
0
23
18
8
87
95
26
-
86
23
16
7
92
96
25
5
87
2
0
1
-
0
0
0
4
92
2
1
0
21
2
0
2
16
2
1
1
-
1
0
4
2
1
2
1
1
2
0
-
0
2
1
4
3
3
18
72
67
84
19
16
4
19
71
67
-
2
2
0
4
67
3
65
17
14
3
17
55
57
13
5
4
300
57
1
0
-
2
0
63
2
2
3
0
1
-
2
1
4
3
3
1
0
0
2
18
59
61
0
0
6
-
3
1
1
-
2
0
5
-
0
3
1
2
19
54
19
54
3
3
18
53
2
1
0
6
3
0
2
1
3
5
1
1
6
2
2
4
-
-
16
50
0
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M . H . G e rss—W. Jeitsch k o • Y C oC and Isotopic C arbides
948
Table III. Positional param eters of YCoC.
Y
b
c
P4Vmmc
Occupancy
2
1
1
2 c
0.81(14)
X
0
1 /2
0
y
0
1 /2
1 /2
1/4
0
0
z
e
Co
2
Overall isotropic therm al param eter. B = 0.0097(17) nm 2
Table IV. Interatom ic distances (nm) in YCoC. All dis­
tances shorter than 0.44 nm (for the Y atoms) and 0.34 nm
(for the Co and C atoms) are listed. Standard deviations
are all less than 0 . 0 0 0 1 nm.
Y:
4C
8 Co
2Y
4Y
0.2505
0.3099
0.3432
0.3650
Co:
C:
2C
8 Y
2 Co
4Y
0.1825
0.3099
0.1825
0.2505
Discussion
The crystal structure of YCoC is shown in Fig. 1.
The positions of the metal atoms correspond to those
of a CsCl structure, which is slightly compressed
along the c axis. The carbon atoms occupy one third
of the (distorted) octahedral voids in an ordered
m anner. The doubling of the c axis is solely due to
the ordered arrangem ent of the carbon atoms: if the
carbon atoms were statistically distributed over
those four faces of the subcell (formed by eight Y
atoms at the corners of the cube with one Co atom
Fig. 1. Crystal structure and near neighbor environm ents
of Y C o C .'
in the center) which contain the c direction, the c
axis of the true tetragonal cell would be halved.
Those faces of the subcell, which are perpendicular
to the c axis, cannot be occupied by carbon atoms,
because the C o—C distances would become too
short (0.1716 nm). Thus, given the metal positions
and the cell dimensions, the arrangement of the car­
bon atoms found during the structure determination
is the only possible one.
The distorted octahedra formed by four rare earth
metal and two cobalt atoms are ideally suited to ac­
com m odate a carbon atom. The Y —C distance of
0.2505 nm compares well with the corresponding dis­
tance of 0.2551 nm in Y3C [5], and the C o—C dis­
tance of 0.1825 nm is close to the distance of
0.1871 nm found for the short C o—C distance in
C o2C [6],
The occupancy param eter obtained for the carbon
position is 81%. A lthough it is well known, that car­
bides frequently have hom ogeneity ranges, resulting
from partially occupied carbon positions, we do not
consider this result of great significance, in view of
the standard deviation of 14%. We prefer to describe
the com pound with the ideal formula YCoC, which
in any case might well be within the homogeneity
range.
The Y atoms have four carbon neighbors forming
a slightly distorted tetrahedron, and the Co atoms
have two carbon neighbors in linear coordination. In
addition, each metal atom has eight metal neighbors
of the other kind, forming a slightly compressed
cube. The Y —Co distances of 0.3099 nm are some­
what greater than the average Y —Co distance of
0.2870 nm in the trigonal CoY 6 prisms of the binary
com pound Y 8Co 5 [7], Thus the metal-carbon bond­
ing is more im portant than the metal-metal bonding.
In view of their electropositivity the Y atoms will
largely have transferred their valence electrons, and
the formula may be written with the oxidation num­
bers as Y 3+[CoC]3~. The [CoC]3- polyanions form
infinite linear chains .. . C o—C —C o—C — . . . , which
extend parallel to the a direction at z = 0 and parallel
to the b direction at 2 = 1/2. Within this polyanion
the carbon atoms are the more electronegative com­
ponent. Thus the form ula may also be written as
Y 3+C o 1+C4~, where the numbers indicate the oxida­
tion states of the atoms. This does not imply com­
plete transference of electrons, however, it can be
assumed that the 2 s and 2 p orbitals of the carbon
atoms are fully participating in the formation of crys-
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M. H . G e rss—W. Jeitschko • Y C oC an d Isotopic C arb id es
949
tal orbitals which will be filled. Thus the Co atoms
obtain a d 8 system, which is ideally suited to form the
two strong linear bonds to the carbon atom s via the
dz: orbitals. From this simplistic rationalization of the
V[A
chemical bonding one could expect diamagnetism or
Pauli param agnetism , which is not supported by our
crude magnetic m easurem ents. The sample of YCoC
is weakly attracted by a magnet. This, however, may
be caused by minor amounts of magnetic impurities.
The structure of YCoC is very simple with only six
Fig. 2. Cell volumes of equiatomic rare earth metal cobalt
atoms per cell and no variable positional param eter.
carbides with YCoC type structure.
Nevertheless, it seems to be a new structure type. It
is, however, difficult to do a comprehensive litera­
ture search for the “anti”-types. In the case of YCoC
although it has only isolated carbon atoms. In view of
an anti-type might have the composition A A 'O ,
the rather large variety of hydrolyzation products en­
where A and A ' are m onovalent metals, occuping
countered for the complex carbides, it is also rem ark­
the Co and C positions of YCoC with oxygen on the
able that YCoC yields almost no unsaturated hydro­
Y positions. Anti-types are known for the perovskite
carbons (only about 0.5% C 2H 4).
carbides T3MC [8], where the transition metal T and
Fig. 2 shows a plot of the cell volumes of the
the main group metal M atoms take the positions‘of
Y
CoC
type compounds. So far we were not success­
the oxygen and Ca atoms of C a T i0 3. A nother exam ­
ful
in
preparing YCoC type compounds with the
ple are the H-phase carbides T 2MC [9], where the T
early
lanthanoids
probably because of their larger
and M atoms occupy the positions of the oxygen and
size.
YbCoC
may
form
under different conditions or
Nb atoms in L iN b 0 2 [10] with Li on the carbon sites.
not
at
all,
because
of
the tendency of Yb for the
The structure of YCoC may also be described as a
divalent
state.
The
cell
volume of the Y compound
“filled” PtS type structure. In PtS [11, 12], which has
fits in between that of the Tb and the Dy compound.
the same space group and similar unit cell dim en­
This is also the case for other carbides e.g. Y 2Cr 2C3,
sions as YCoC, the Pt and S positions correspond to
YM oC2, YW C 2 [15], and for phosphides e.g. Y Pd 2P 2
those of carbon and Y of YCoC.
[19], Y Fe 5P 3 [20], In more ionic solids the cell vol­
The hydrolysis of rare earth metal carbides is a
umes of the Y compounds are relatively smaller and
complex process. LaC2, which contains carbon pairs,
m ore similar to those of the Ho or Er compounds
yields besides the expected com pounds C 2H 6 and
C 2H 4, also minor amounts of the hydrocarbons C 3H V [2 1 , 22 ],
with x — 4, 6 , and 8 [5]. The hydrolyses of the car­
We want to thank Dipl.-Chem. A. Ellmann and
bides DyCoC? [1], U C oC 2 [13], and U 2NiC 3 [14],
Prof.
D r. J. G robe for advice and help with the gas
which have carbon pairs with C —C distances varying
chrom atographic analysis. Special thanks are due to
between 0.137 nm and 0.148 nm, yield between 11
D r. P. Seidel and Prof. Dr. W. Hoffmann of the
and 14 wt-% propane and propene. U C rC 2, which
Institut für Mineralogie for their hospitality and for
has isolated carbon atoms [15], yields 95% C H 4 and
their introduction to the use of the optical den­
only 5% C 2 species [16], while the hydrolyzation p ro ­
sitom eter. We are also indebted to Dr. G. Höfer
ducts of UM oC 2 and U W C 2 — both are isotypic with
(H eraeus Q u a r z s c h m e l z e ) for a gift of silica tubes.
U C rC 2 [15, 17, 18] — contain about equal am ounts of
This work was supported by the Deutsche For­
schungsgemeinschaft and the Fonds der Chemischen
CH 4 and C 2 species and very little of the C 3 species
Industrie.
[16]. YCoC is unique in that it yields 24% propane,
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[3] D. T. C rom er and J. B. M ann, A cta Crystallogr. A 24,
321 (1968).
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1891 (1968).
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950
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M. H . G e r s s - W . Je itsch k o • Y C oC and Isotopic C arbides
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