The Ternary Rare Earth Chromium Nitrides Ce2CrN3 and LnsCr^-^Nn
with Ln = La, Ce, Pr
Sascha Broil, Wolfgang Jeitschko*
A norganisch-C hem isches Institut, U niversität M ünster, W ilhelm -K lem m -Straße 8,
D-48149 M ünster, G erm any
Z. N aturforsch. 50b, 9 05-912 (1995); received O cto b e r 5, 1994
C rystal Structure, Flux G row th, Structural R elationships
The title com pounds have been p rep ared by annealing cold-pressed pellets o f the binary
nitrides LnN and CrN. Well developed crystals w ere o b tain ed by recrystallization of the
binary or prereacted ternary nitrides in a Li3N flux. T h eir structures w ere d eterm in ed from
single-crystal diffractom eter data. C e2C rN 3 has a U 2C rN 3 type structure: Im m m , a = 379.0(1),
b = 340.4(1), c = 1251.7(2) pm, Z = 2, R = 0.012 for 383 stru cture factors and 16 variables.
The atom ic positions of this structure are sim ilar to those of U 2IrC 2 and K2N iF4. The struc­
ture m ay be rationalized to a first approxim ation with the form ula (C e+4)2[C rN 3]8_. The
chrom ium atom s are in a distorted sq u are-p lan ar nitro g en co ordination. The C rN 4-squares
are linked via corner-sharing nitrogen atom s, thus form ing infinite, straight - N - C r N 2- N C rN 2- chains. The cubic structure of L a 3C r10_ jN n (a = 1298.2(1) pm ), C esC rjo -^N n (with
a small hom ogenity range; u = 1284.3(1)-1286.1(3) pm ), and Pr3Cr,0_vNn (a = 1289.1(2)
pm ) was determ in ed for the lanthanum com pound: F m 3 m , Z = 8, R = 0.027 for 189 F values
and 18 variables. O ne chrom ium site was found to have an occupancy o f only 80.9(5)%
resulting in the com position La3Cr9.24(1)Nn. The nitro g en atom s occupy four atom ic sites.
T hree of these have octahedral environm ents (6 La, 3 L a + 3 C r, 2 L a + 4 C r), the fo u rth one is
surrounded by eight chrom ium atom s form ing a cube. The chrom ium atom s are tetrah ed rally
coordinated by nitrogen atoms, and these C rN 4-te tra h e d ra are linked via com m on corners
and edges to form a three-dim ensionally infinite polyanionic netw ork. In add itio n the ch ro ­
m ium atom s with oxidation num bers of about 2 to 3 form n um erous C r - C r bonds, which
allow to rationalize the Pauli param agnetism of the com pound.
Introduction
In recent years a large number of ternary rare
earth transition metal carbides have been pre­
pared and characterized [1-8, and references
therein]. Because of the well known structural
similarity of binary transition metal carbides and
nitrides it could be expected that it should be pos­
sible to prepare the corresponding ternary lanthanoid transition metal nitrides. Especially with
chromium as the transition metal com ponent some
ternary nitrides have been prepared already some
time ago. M archand and Lemarchand [9] have re­
ported a nitride with the approximate composition
“La6Cr2iN23 ”, which was confirmed by Pollmeier
[10], who also prepared the isotypic nitrides with
cerium and praseodymium. He furtherm ore syn­
thesized a new nitride with the approximate com­
position “Ce3Cr2N5” [10]. However, because of
* R eprint requests to W. Jeitschko.
0932-0776/95/0600-0905 $06.00
the difficulty in obtaining single-crystals the struc­
tures of these nitrides could not be worked out
at that time. We have now determ ined the crystal
structures of these nitrides using single crystals,
which were obtained from a Li3N flux. A flux of
lithium nitride was used recently to grow crystals
of Ca2FeN2 and Sr2FeN2 [11], as well as other ter­
nary nitrides containing lithium, e.g. Li15Cr2N9,
Li6CrN4, and Li6MoN4 [12].
Sample Preparation
Starting m aterials were ingots of lithium
(Merck, >99%) and the rare earth metals (RhonePoulenc, >99.5%), a mixture of the chromium nit­
rides CrN and Cr2N (Johnson Matthey, Cr: 99.8%),
and nitrogen (Messer Griesheim, 99.996%). Turn­
ings of the rare earth elements were prepared un­
der dried paraffin oil, which was washed away by
dried (sodium) /7-hexane. They were stored under
high vacuum and only briefly exposed to air prior
to the reactions with nitrogen. Cerium filings were
© 1995 Verlag d er Z eitschrift für N aturforschung. All rights reserved.
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S. B ro ll-W . Jeitschko • The T ernary R are E arth C hrom ium N itrides
used to purify the nitrogen at 600 °C, since, ac­
cording to our observations, C e 0 2 has a higher
stability than CeN. Lithium nitride was prepared
from the lithium ingots by slowly heating up to a
tem perature of 870 °C in a stream of purified
nitrogen.
The ternary rare earth chromium nitrides could
all be prepared in microcrystalline form by direct
reaction of the corresponding binary nitrides with­
out the Li3N flux. Cold-pressed pellets of the inti­
mate mixtures were wrapped in a molybdenum
foil, sealed in evacuated silica tubes and annealed
at 900 °C (Ce2CrN3) or 1160 °C (Ln3C r10_xN „ )
for about one week. After this treatm ent the sam­
ples were usually still contam inated by the binary
nitrides. The treatm ent of these samples with di­
luted hydrochloric acid only dissolved the rare
earth nitrides. The compounds with the composi­
tion Ln3C r]0_vN n could be prepared in pure form
by a repeated annealing of cold-pressed pellets. In
between these treatm ents the pellets were ground
to a fine powder, the decomposed rare earth ni­
tride was dissolved in hydrochloric acid and fresh
rare earth nitride was added again to the mixture
to maintain the proper composition.
A well crystallized sample of Ce2CrN3 was pre­
pared by annealing the binary nitrides in a flux of
Li3N in the molar ratio C eN :C rN :L i3N = 2:1 :5 0
in an alumina container under argon (sealed in a
silica tube) for 12 h at 900 °C. The crystals of
La3Cr9 24N n were obtained by annealing the pow­
der of the ternary compound with a lithium nitride
flux (La3Cr924N n :Li3N = 1:50) in an alumina
container for 3 days at 900 °C.
Properties and Lattice Constants
Well crystallized samples of the ternary nitrides
show metallic luster, the powders are black. They
are stable in air over long periods of time.
Ce2CrN3 is readily attacked (within minutes) by
diluted hydrochloric acid, while the nitrides
Ln3C r10_vN n , due to their higher chromium
content, are attacked at a considerably slower rate.
A sample of La3Cr9 24N ,, was investigated with a
SQUID m agnetom eter between 20 and 300 K. It
was contaminated by a very small am ount of an un­
known ferromagnetic impurity, as was concluded
from the field dependence of the magnetic suscep­
tibility. The susceptibilities as shown in Fig. 1 were
T[K ]
------ »
Fig. 1. M agnetic susceptibility of L a3C r 9 24N n as a func­
tion o f tem p eratu re.
obtained by extrapolation to infinite field strengths.
They are practically independent of the tem per­
ature with a susceptibility of 0.56 • 10~9 m3/mol at
room tem perature. This behaviour is typical for a
Pauli paramagnet. The slight upturn in the mag­
netic susceptibility at low tem perature may be as­
cribed to a very small amount of a paramagnetic
impurity or to magnetic surface states. U nfortu­
nately the samples of Ce2CrN3 were contaminated
by Ce-^Crjo-.vN)! and therefore their magnetic
properties could not be determined.
All samples were characterized by Guinier pow­
der patterns using an Enraf Nonius camera with
a-quartz (a = 491.30, c = 540.46 pm) as a standard.
The lattice constants were refined by least-squares
fits. To assure the proper assignment of the indices
the observed patterns were compared to the ones
calculated [13] using the positional param eters as
obtained from the structure determinations. The
orthorhom bic lattice constants of Ce2CrN3 are a =
379.0(1), b = 340.4(1), c = 1251.7(2) pm, V = 0.1615
nm3. For the cubic nitrides Ln3C r10_vN n the
following values were obtained: a = 1298.2(1) pm.
V = 2.188 nm3 for La3Cr9 24N n and a = 1289.1(2)
pm. V = 2.142 nm3 for Pr3C r10_^Nn . While these
values were reproducible for different samples
within the small standard deviations, the cerium
com pound seems to have a noticeable hom o­
geneity range, because slightly different lattice
constants were obtained for two different samples:
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S. B ro ll-W . Jeitschko • T he T ernary R are E a rth C hrom ium N itrides
Table I. C rystal data for C e2C rN 3 and La3C r924N n .
Space group
Formula weight
Formula units/cell
Calculated density [g/cm3]
Crystal dimensions [/<m '|
9/26 scans up to
Range in hk l
Total number of reflections
Inner residual
Unique reflections
Reflections with I > wj(I)
Number of variables
Conventional residual
Weighted residual
Ce2CrN3
La3Cr9 24Nu
Immm (No. 71)
374.3
2=2
7.70
15x20x4
20 =100°
± 8. ± 7. ±27
3439
/?, = 0.033
523
383 (« = 3)
16
R = 0.012
Rw = 0.014
Fm3m (No. 225)
1051
Z = 8
6.38
2 0x1 5 x 1 5
20 = 80°
±23, 0 -2 3 . 0 -2 3
3682
Ri = 0.057
542
189 (n = 2)
18
R = 0.027
Rh, = 0.023
1284.3(1) pm, V = 2.118 nm 3 and a = 1286.1(3) pm,
V - 2.127 n m \ This might be associated with a
mixed valence behavior of the cerium atoms in
this compound as can also be concluded from the
fact that the lattice constants of Ce3C r10_^Nn are
slightly smaller than those of Pr^Crjo-.vNn.
Crystal Structures
Intensity data were collected from single-crystals of Ce2CrN3 and La3Cr924N n in an Enraf-Nonius CAD 4 diffractom eter with graphite-monochrom ated M oKa radiation, a scintillation counter
with pulse-height discriminator, and background
counts at both ends of each 0/2 0-scan. Absorption
907
corrections were made using psi scan data. Further
details are summarized in Table I.
Ce2CrN3 was conjectured to be isotypic with
U 2CrN3 [14] once its lattice constants were known
and compared to those of related compositions
using Pearson’s handbook [15], This was con­
firmed during the full-matrix least-squares refine­
ments [16] with atomic scattering factors [17], cor­
rected for anomalous dispersion [18]. The
weighting scheme accounted for the counting sta­
tistics and a param eter correcting for secondary
extinction was varied as a least-squares param eter.
To check for deviations from the ideal composi­
tions, occupancy param eters were refined in one
series of least-squares cycles. The highest and low­
est occupancy param eters were 98.6(2)% for the
Cr position and 102(1)% for the N 2 position.
Thus, no serious deviations from the ideal occu­
pancies were observed and the ideal values were
assumed during the last least-squares cycles.
La3C r10_xN n was recognized to be face-centered cubic already from the powder patterns.
There were no additional space group extinctions
and of the three possible space groups F432,
F43m , and F m 3m with the high Laue symmetry
m 3m , the one with the highest symmetry was
found to be correct during the structure refine­
ments. The metal positions were obtained from a
Patterson synthesis and the nitrogen atoms were
located by difference Fourier calculations. The
T able II. A tom ic p aram eters of C e2C rN 3 and L a3C r9 24(i)N 11. S tandard deviations in the positions o f the least signifi­
cant digits are given in paren th eses throu g h o u t the paper. The anisotropic th erm al param eters U (p m 2) are defined
by e x p [ - 2 jr 2( U n ^ 2ö*2 + -" + 2 U 23Ä:/ö*c*)]. The last colum n contains the eq uivalent isotropic B values (xlOO, in
units of nm 2) of the anisotro p ic displacem ent param eters and the isotropoic displacem ent p aram eters of the nitrogen
atom s in L a3C r9 24(1)NnA tom
C e2C rN 3
Ce
Cr
N1
N2
Im m m
4i
2a
2b
4i
L a3C r9 24( j )N j j Fm3m
La
24 e
Cr 1
48 g
C r2*
32 f
48 h
N1
32 f
N2
4b
N3
N4
4a
X
y
z
Un
U22
u 33
u 12= u 13 U 23
Beq
0
0
0
0
0
0
1/2
0
0.35373(1)
0
1/2
0.1664(2)
31.6(4)
20(2)
36(10)
54(8)
35.8(4)
74(2)
72(11)
80(8)
28.6(4)
35(2)
53(5)
41(7)
0
0
0
0
0.253(2)
0.34(1)
0.42(4)
0.46(3)
0.30187(6)
0.1010(1)
0.0971(1)
0
0.3533(5)
1/2
0
0
1/4
0
1/4
68(2)
44(3)
U22
U22
X
X
37(3)
38(5)
42(3)
Uu
Un
0
0
28(5)
0.1716(4)
y
-
-
-
-
X
X
-
-
-
-
1/2
0
1/2
0
-
-
-
-
-
-
-
-
0
0
0
0
0.455(7)
0
-0 .1 (1 ) 0.33(1)
U ,2 0.33(3)
0.6(1)
0.5(1)
0.6(5)
0.8(5)
-
* The C r2 position is occupied to only 80.9(5)% .
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S. B ro ll-W . Jeitschko • The T ernary R are E a rth C hrom ium N itrides
Table III. Interato m ic distances (pm ) in the structures
of C e2C rN 3 and L a3C r9 24(1)N n . All distances sh o rter
than 390 pm (L n - L n . L n - C r , L n - N ) , 340 pm ( C r- C r .
C r - N ) , and 280 pm ( N - N ) are listed.
C e2C rN 3
Ce:
Cr:
N l:
N 2:
1N 2
2N 1
4N 2
4Cr
2C e
4C e
1 Ce
2Ce
2N 1
2N 2
8Ce
2C r
4Ce
1 Cr
1 Ce
4C e
L a 3C r9,24( 1)N 11
234.5(2)
250.0(1)
255.9(1)
313.7(1)
340.4(1)
363.7(1)
366.2(1)
379.0(1)
189.5(1)
208.3(2)
313.7(1)
189.5(1)
250.0(1)
208.3(2)
234.5(2)
255.9(1)
La:
1N 3
4N 2
4N 1
4 C r2
8C r 1
4 La
C r 1: 2N 1
2N 2
1Cr 1
4C r 1
2 C r2
4 La
C r2: 3N 1
1N 4
3 C r2
3C r 1
3 La
N l:
2 C r2
2C r 1
2 La
N2: 3 C r 1
3 La
N 3: 6 La
N4: 8 C r2
257.2(1)
277.4(2)
279.7(4)
320.1(1)
356.5(1)
363.8(1)
194.7(3)
198.7(3)
262.3(2)
273.5(1)
280.8(1)
356.5(1)
186.0(3)
218.3(1)
252.0(1)
280.8(1)
320.1(1)
186.0(3)
194.7(3)
279.7(4)
198.7(3)
277.4(2)
257.2(1)
218.3(1)
least-squares refinem ent of the occupancy param ­
eters (in percent) led to the following values:
La-100.1(2), Cr 1-99.8(4), C r2-80.9(5), N 1-99(2),
N 2-101(2), N 3-108(9), N4-96(9). Thus, no signifi­
cant deviations from the full occupancy values
were observed except for the C r2 position, and
in the final refinement cycles only the occupancy
param eter of this position was allowed to vary,
while the others were assumed to be ideal. The
final atomic param eters and interatom ic distances
are given in Tables II and III. Listings of the struc­
ture factors are available from the authors [19].
Fig. 2. The crystal structure of C e2C rN 3 and its relation
of the structures o f U 2IrC 2 and K 2N iF4.
completely filled structure of K2NiF4 crystallize
with the tetragonal space group I4/mmm, the par­
tial and ordered filling of the corresponding site in
Ce2CrN3 lowers the symmetry to the orthorhombic space group Immm.
The cerium atoms in Ce2CrN3 are surrounded
by seven nitrogen atoms at an average distance of
251.2 pm, two chromium atoms at 313.7 pm, and
nine cerium atoms at distances covering the range
from 340.4 to 379.0 pm (Fig. 3). While the C e -N
interactions are undoubtedly bonding, the bonding
character of the C e -C r and C e -C e interactions
might be regarded as questionable. The chromium
Discussion
Ce2CrN3 is isotypic with U 2CrN3 [14]. This
structure may be regarded as a “filled” U 2CrC2
type structure [20]. It may also be considered as a
defect variant of the well known structure of
K2NiF4 (Fig. 2), which occurs also for many
related compositions, e.g. Cs2CrCl4, K2U 0 4,
La2N i0 4 [21], R b2CaH4 [22], as well as for the
“anti”-type structure Eu4A s20 [23]. While the
completely unfilled structure of U 2IrC2 and the
0
c ß * x P
O
N1 (mmm)
Fig. 3. N ear-neighbor coordinations in the stru ctu re of
C e2C rN 3. The site sym m etries are given in parentheses.
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S. B ro ll-W . Jeitschko • The T ernary R are E arth C hrom ium N itrides
atoms have two nitrogen neighbors at 189.5 pm
and two more at 208.3 pm. These four nitrogen
atoms form a rectangle. Eight cerium atoms in a
cube-like arrangem ent complete the coordination
of the chromium atoms. The two nitrogen atoms
have distorted octahedral coordination with two
chromium plus four cerium neighbors for N 1 and
one chromium plus five cerium neighbors for the
N2 atom.
Since cerium is the only rare earth element
forming a U 2C rN 3 type structure, and since this
structure is also found for Th2CrN3 [14], it seems
likely that cerium is (at least predominantly) tetra­
valent in Ce2C rN 3, and the formula of the com­
pound may be written as (Ce+4)2[CrN3]~8, where
the superscripts are oxidation numbers (formal
charges). Since there are no nitrogen-nitrogen
bonds and since nitrogen is the most electronega­
tive com ponent of the compound, one may ascribe
the oxidation num ber - 3 to the nitrogen atoms.
This leads to the formula (Ce+4)2Cr+1(N _3)3, i.e.
the chromium atoms obtain a d"1 system, still as­
suming cerium to be tetravalent, which may not
be entirely the case.
The chromium together with the nitrogen atoms
might be considered to form a polyanionic chain,
which extends along the x direction (Fig. 4). There
Fig. 4. T he stru ctu re of C e2C rN 3 view ed along the direc­
tion of the infinite chrom ium -nitrogen chain.
N2
N2
Si
1
°\
1
Cr ! £ 5 M H i Cr —
I
.
oM1
C
N2
N2
N2
I
1
N 1 ----- Cr ------1
1
N2
Fig. 5. A cu to u t of the polyanionic chrom ium -nitrogen
chain of the C e2C rN 3 structure. The C r - N distances are
in units o f pm.
909
are two C r-N distances in the chain. The one with
the terminal nitrogen of 208.3 pm is greater than
the distance of 189.5 pm within the chain (Fig. 5).
In molecular chromium nitrogen compounds
C r-N distances vary between a distance of 157 pm
for a formal triple bond in a nitrido(porphyrinato)chromium(V) complex [24] to apparent single­
bond distances in the range of 204-225 pm [25].
Thus, the C r-N 2 distance of 208.3 pm in Ce2CrN3
might be classified as a single-bond distance and
certainly not as a triple-bond distance, as could be
concluded from the term inal position of the N2
atoms in the chromium -nitrogen chain. The N2
atom has, in addition to the chromium neighbor,
five cerium neighbors. In CrN with NaCl type
structure [26], where the nitrogen atoms also have
six metal neighbors in octahedral arrangement,
the C r-N distance is 207.4 pm, and this distance
compares well with the C r - N 2 distance of 208.3
in Ce2CrN3. The shorter C r - N 1 distance of 189.5
pm within the chromium -nitrogen chain, however,
might possibly be assigned some double-bond
character. The therm al param eters U 22 of the
chromium and nitrogen atoms are larger than the
corresponding U n and U 33 param eters, indicating
that the chemical bonding is weakest in that direc­
tion of the chain, which has the smallest extension.
The structure of La3C r9 24N n contains 192 atom
positions in the face-centered cubic cell and seems
to be novel. It may be built up of three different
building blocks as is outlined in Fig. 6. These
building blocks have the compositions La6N9, Cr6,
and Cr8N 13, and there are four, eight, and four,
respectively, of these in the cubic cell in an ar­
rangem ent as it occurs in the M nCu2Al type struc­
ture. This structure is also known as a filled fluo­
rite (CaF2) structure, where the Ca positions
(Cr8N 13 cubes) correspond to the Mn positions,
the F positions (Cr6 octahedra) correspond to
those of the Cu atoms, and the voids of CaF2 are
filled by Al atoms (La6N9 octahedra). A lterna­
tively the M nCu2Al structure may also be re­
garded as a filled NaCl structure, where the posi­
tions of the Mn and Al atoms correspond to the
atomic positions in NaCl, and the Cu atoms (in
La3Cr924N n represented by the small Cr6 octahe­
dra) fill tetrahedral voids formed by the Cl atoms.
The Cr6 octahedra shown in Fig. 6 are formed
by the Cr 1 atoms. The distances from the center
of a Cr6 octahedron to the C rl atoms are 193.4
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S. B ro il-W . Jeitschko • The Ternary R are E arth C hrom ium N itrides
Cr1 (mm)
La (4mm)
-4^3
N1 (mm)
Cr2 (3m)
O
La6N9
N3 (m3 m)
N2 (3m)
Cr8N13
N4 (m3m)
Fig. 6. The face-centered cubic cell of L a3C r9 24N 11 as
built up by three d ifferen t building blocks of the co m p o ­
sitions L a6N 9, C r6, and C r8N !3. T hese blocks are show n
in a shrunken form to m ake their relative arran g em en t
m ore transparent. Single-digit n u m b ers co rresp o n d to
the atom designations.
pm. Considering the cubic C r2 environm ent of the
N 4 atoms (Fig. 7) with N 4 - C r 2 distances of
218.3 pm, or the environm ent of the N 1 atom with
four Cr neighbors at an average distance of
190.3 pm these Cr6 octahedra might seem to be
suited for the accommodation of an additional ni­
trogen atom. We considered this possibility and re­
fined the structure with a nitrogen atom at this
site. The resulting occupancy, however, was negli­
gible: 9% with a standard deviation of 5%.
Fig. 7. A tom ic
L a3C r9 24Nn-
coordinations
in
the
stru ctu re
of
The composition of La3Cr 9 .24 N 11 with 12.9:39.8:
47.3 at% , as obtained from the structure refine­
ment, compares well with the composition
12.0:41.2:46.8 at% found by chemical analysis for
“La6Cr2iN23 ’' by M archand and Lem archand [9].
This compound as compared to Ce2CrN3 has a
much higher chromium content and this is re­
flected in the near-neighbor invironments of all
atoms. The lanthanum atoms are coordinated by
nine nitrogen atoms at an average distance of
276.2 pm and twelve chromium atoms at the
average distance 344.4 pm. The four lanthanum
neighbors completing this coordination polyhe-
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S. B ro ll-W . Jeitschko • T he T ernary R are E a rth C hrom ium N itrides
dron are at a distance of 363.8 pm. Considering
the small lanthanum content and the fact that lan­
thanum is the most electropositive com ponent of
the com pound (and therefore has almost com­
pletely transferred its valence electrons to the
chromium-nitrogen network) it m akes little sense
to compare this distance to the L a -L a distance of
about 375 pm in the elem ental modifications of
lanthanum [27], Certainly the L a - L a interactions
are weak. There are two chromium sites in
La3Cr924N n and even though the C r2 position is
occupied to only 80.9(5)%, the average inter­
atomic distances (for C rl/C r2 in pm units) of
196.7/194.1, 274.0/266.4, and 356.5/320.1 for the
C r-N , C r-C r, and C r-L a interactions reflect the
fact that the C rl atoms (with 4N + 7C r + 4La)
have a somewhat higher coordination number
than the C r2 atoms (with 4N + 6C r + 3La). Of the
four nitrogen atoms N 3 has perfect octahedral
lanthanum coordination, N 4 is surrounded by a
perfect cube of chromium atoms and the other two
nitrogen atoms have mixed, distorted octahedral
coordination of 2 L a + 4 Cr and 3L a + 3Cr.
Both chromium sites have tetrahedral nitrogen
coordination with C r-N bond distances between
186.0 and 218.3 pm. While the shorter limit might
correspond to a single-bond distance, possibly with
some double-bond character - as was discussed
above for Ce2CrN3 - the distance of 218.3 pm
must have a bond order of much less than one.
This can easily be seen from the viewpoint of the
N 4 atom, which forms eight N 4 -C r2 bonds of
that length.
The CrN4 tetrahedra are linked via edges and
corners (Fig. 8), thus forming a three-dimensionally infinite polyanion. Again one may assign oxi­
dation numbers to the lanthanum and nitrogen
atoms of +3 and -3 , respectively. The difference
in the formal charge balance of La3Cr924N t j musl
come from the chromium atoms. Assuming the
Cr2 site to be fully occupied, several assignments
for the oxidation numbers of the chromium atoms
Fig. 8. S tereoplots for one cell of
the face-centered cubic
L a3C r9 24N n structure. In the u p ­
p e r p art the w hole stru ctu re is
show n. In the low er p a rt lan th a ­
num atom s (large circles) are
o m itted for clarity. The c h ro ­
m ium (m edium sized circles) and
m ost n itrogen atom s (sm all cir­
cles) form a three-dim ensionally
infinite n etw ork of corner- and
edgesharing C rN 4 tetrah ed ra.
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912
S. B ro il-W . Jeitschko • The T ernary R are E a rth C hrom ium N itrides
are possible, for instance
(La+3)3(Cr l +3)6(Cr 2+1-5)4(N _3) 11,
(L a+3)3(Cr l +2)6(C r2+3)4(N~3)1T, and
(La+3)3(Cr l +1)6(C r2+4-5)4(N _3) 11. Of these the
second one is most plausible, not only because this
formula avoids large differences in the oxidation
numbers of the chromium atoms, but also because
it accounts for the fact that the C r2 atoms have a
somewhat smaller coordination number and
shorter average bond distances. Thus in this ideal­
ized formula the C rl and C r2 atoms obtain d4 and
d3 systems - enough electrons to form the num er­
ous C r-C r bonds, which cover the range from
252.0 to 280.8 pm. All spins are compensated that
way, as is indicated by the Pauli paramagnetism of
the compound.
Acknowledgements
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We thank Dipl.-Ing. U. Rodewald and Dr. M. H.
Möller for the collection of the single-crystal dif­
fractom eter data, Dipl.-Phys. K. Hartjes for the
magnetic susceptibility m easurem ent, and Mr. K.
Wagner for the work at the scanning electron
microscope. We are also indebted to Dr. G. Höfer
(Heraeus Quarzschmelze), Dr. Th. Lauterbach
(Chemetall), and the Rhöne-Poulenc company for
generous gifts of silica tubes, lithium, and rare
earth metals. This work was supported by the
Deutsche Forschungsgemeinschaft and the Fonds
der Chemischen Industrie.
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