RbBa2(N3)5: A new ternary azide

Article abstract of DOI:10.1107/S0108270106033981

Rubidium dibarium pentaazide, RbBa₂(N₃)₅, was prepared from an aqueous solution of the binary azides at room temperature. It crystallizes in the monoclinic system (space group P2/n). Two central atoms of azide groups occupy the 2c (1) and 2b (1) positions, another azide group lies completely on a twofold axis (2f), while Rb atoms are situated in 2e (2) positions. The crystal structure of RbBa₂(N ₃)₅ can be regarded as a distorted AlB₇-type arrangement of the metal atoms, with the azide groups occupying the voids between the cations. This results in coordination numbers of 8 (Rb) and 10 (Ba). The N-N distances are in the range 1.169 (8)-1.190 (5) A, typical for the azide group.

Full text of DOI:10.1107/S0108270106033981

inorganic compounds
Acta Crystallographica Section C
Crystal Structure
Communications
are the mixed azides of alkali and alkaline earth metals.
Anhydrous compounds with compositions Cs7Ca4(N3)15
(Krischner et al., 1984), CsSr(N3)3 (Krischner et al., 1981a),
Cs2Sr(N3)4 (Krischner et al., 1981b) and CsBa2(N3)5
(Krischner et al., 1982) have been reported so far. Addition-
ally, a number of hydrated azides have been described, viz.
K2Ca(N3)4Á4H2O (Krischner et al., 1980), Rb2Ca(N3)4Á4H2O
(Mautner & Krischner, 1988), CsCa(N3)3ÁH2O (Krischner,
Mautner & Kratky, 1983), Cs5Ca2(N3)9Á2H2O (Saracoglu et al.,
1983), Cs6Ca(N3)8Á2H2O (Krischner, Saracoglu et al., 1983)
and K2.24Sr1.34(N3)4.92Á6.16H2O (Walitzi & Krischner, 1978).
In this study, we focused on ternary azides of alkali metals
with barium. The synthesis of the hitherto only known azide of
this kind, CsBa2(N3)5, was reproduced and a new rubidium
compound with the same stoichiometry was obtained. Our
attempts to prepare ternary azides of barium with potassium
or sodium were unsuccessful.
ISSN 0108-2701
RbBa2(N3)5: a new ternary azide
Grigori V. Vajenine
Max-Planck-Institut fur Festkorperforschung, Heisenbergstrasse 1, 70569 Stuttgart,
È
È
Germany, and Institut fur Anorganische Chemie, Universitat Stuttgart, Pfaffen-
È
È
waldring 55, 70569 Stuttgart, Germany
Correspondence e-mail: g.vajenine@fkf.mpg.de
Received 27 June 2006
Accepted 23 August 2006
Online 12 September 2006
The crystal structures of orthorhombic CsBa2(N3)5 and
monoclinic RbBa2(N3)5 are similar. The arrangements of the
metal ions in both azides can be regarded as distorted variants
of the AlB2-type structure (Fig. 1); graphite-like sheets of
Rubidium dibarium pentaazide, RbBa2(N3)5, was prepared
from an aqueous solution of the binary azides at room
temperature. It crystallizes in the monoclinic system (space
group P2/n). Two central atoms of azide groups occupy the 2c
(1) and 2b (1) positions, another azide group lies completely
on a twofold axis (2f), while Rb atoms are situated in 2e (2)
positions. The crystal structure of RbBa2(N3)5 can be regarded
as a distorted AlB2-type arrangement of the metal atoms, with
the azide groups occupying the voids between the cations. This
results in coordination numbers of 8 (Rb) and 10 (Ba). The
Ê
NÐN distances are in the range 1.169 (8)±1.190 (5) A, typical
for the azide group.
Comment
The chemistry of metal azides has been developing fast
recently, exempli®ed by syntheses and characterization of new
binaries M(N3)6 (M = Mo or W) for the d metals (Haiges et al.,
3
2005) and U(N3)7 for the f elements (Crawford et al., 2005)
just in the past year.
Conversely, more stable azides of the s metals have been
known for a long time. Alkali metal azides, especially NaN3,
are classical sources of pure metals and/or nitrogen. Alkaline
earth metal azides have not been used as extensively for this
purpose owing to their somewhat more violent thermal
decomposition and the greater effort needed to obtain
solvent-free azides. Anhydrous barium azide can be relatively
easily obtained because of the large cation size; its crystal-
lization (Manno, 1965; Marinkas, 1966) and crystal structure
(Choi, 1969; Walitzi & Krischner, 1970) attracted some
attention previously because of its potential as a high-energy
material. Recently, it was shown that Ba(N3)2 can be employed
as a valuable precursor, e.g. for the preparation of barium
pernitride BaN2 by controlled thermal decomposition (Vaje-
nine et al., 2001). Moreover, anhydrous barium azide can also
be used as a barium and nitrogen source in the syntheses of
new intermetallic (Smetana et al., 2006a) and subnitride
(Smetana et al., 2006b) phases.
Figure 1
The crystal structure of RbBa2(N3)5 viewed along [010]. [Symmetry
codes: (i) x + 1, y + 1, z; (ii) x, y + 1, z.]
Figure 2
Tenfold coordination of barium (left) and (8+2)-fold coordination of
rubidium (right) by N atoms of the azide groups. [Symmetry codes: (iii) x,
1
y + 1, z; (iv) x, y + 2, z; (v) x + 12, y, z + 21; (vi) x + 21, y + 2, z
(vii) x, y 1, z; (viii) x + 21, y 1, z + 12; other codes as in Fig. 1.]
;
Therefore, the search for new complex azides is important
for the development of nitride chemistry. Especially numerous
2
Acta Cryst. (2006). C62, i85±i87
DOI: 10.1107/S0108270106033981
# 2006 International Union of Crystallography i85

inorganic compounds
barium ions are separated by alkali metal cations. Azide
anions occupy the voids between the cations, whereby a slight
difference in their coordination environment leads to a lower
crystal symmetry in the case of RbBa2(N3)5.
The coordination polyhedra of rubidium and barium in
RbBa2(N3)5 are shown in Fig. 2. The barium cations are
surrounded by ten terminal N atoms of azide groups in the
form of a trigonal prism with four additional caps. The BaÐN
away (cf. the rubidium environment above) in the case of the
N6/N5/N7 azide group, resulting in a (4+2)-coordination. The
nearly side-on coordination of the azide group to rubidium is
particularly evident for the N9/N8/N10 group. The latter is
also the only azide group not restricted to be linear by
symmetry, though this anion is almost linear, with a bonding
angle of 179.6 (5)ꢀ. The NÐN distances range from 1.169 (8)
Ê
to 1.190 (5) A and are thus in good agreement with the
structures of other known azide compounds.
Ê
distances range from 2.834 (3) to 3.255 (5) A, which is
comparable to the ranges for ninefold coordinated barium in
In comparison to CsBa2(N3)5, in which three of four crys-
tallographically inequivalent azide groups are in a ®vefold
coordination and only one is in a sixfold coordination by metal
cations, the average coordination number of the azide groups
in RbBa2(N3)5 is slightly higher (six for three azide groups and
four for one) owing to the smaller size of Rb+ compared with
Cs+. This minute difference is apparently suf®cient to favor the
monoclinic structure for the rubidium compound.
Ê
CsBa2(N3)5 [2.816 (6)±3.087 (6) A] and Ba(N3)2 [2.883 (3)±
Ê
2.985 (3) A]. The rubidium coordination can be described as a
polyhedron intermediate between a cube and a square anti-
prism. Here the distances to the terminal N atoms [3.047 (5)±
Ê
3.251 (4) A] are only slightly shorter than the closest contacts
to the central atoms of the azide groups [3.165 (4)±
Ê
3.308 (4) A]. Two additional capping azide groups lie much
Ê
farther away at 4.352 (5) A, favoring the thermal motion of
the rubidium cations along the a axis. Eightfold coordination
of rubidium is also found in RbN3, with RbÐN distances of
An alternative description of the crystal structure of
RbBa2(N3)5 is instructive. If the positions of the central atoms
of the azide groups alone are considered (i.e. the centers of
mass of the anions) then the ternary azide may be described as
a slightly distorted variant of the Ga3Pt5 structure (Bhan &
Schubert, 1960), with the metal cations in the gallium positions
and the azide groups in the platinum sites. The latter inter-
metallic phase is in turn an ordered 2 Â 2 Â 1 superstructure
of the fcc packing with 16 atoms in the unit cell [Z = 2, as in
RbBa2(N3)5]. The observed axis ratio of a:b:c = 1.54:1:2.08 for
the azide group illustrates the degree of distortion compared
with the expected value of 2:1:2 for the ideal fcc lattice.
Interestingly, the crystal structure of CsBa2(N3)5 can also be
reduced to the Ga3Pt5 structure type.
Ê
3.099 A (Choi & Prince, 1976), and in Rb2Ca(N3)4Á4H2O [six
Ê
RbÐN contacts of 2.967 (7)±3.133 (7) A, with two additional
water molecules].
The coordination environments of the four crystal-
lographically inequivalent azide groups in RbBa2(N3)5 are
shown in Fig. 3. The arrangement of the metal cations can be
described as distorted octahedra of four barium and two trans
rubidium cations. These two rubidium positions lie farther
According to our preliminary results, thermal decomposi-
tion of the ternary azides RbBa2(N3)5 and CsBa2(N3)5 leads to
BaN2 accompanied by evaporation of the alkali metal.
Experimental
Barium azide (98.5%, Schuchardt) was recrystallized twice from
aqueous solution by adding ethanol. RbN3 was obtained by mixing
aqueous solutions of Ba(N3)2 and Rb2CO3 (99%, Merck) in a 1:1
molar ratio followed by ®ltration and recrystallization. Transparent
colorless single crystals of RbBa2(N3)5 were obtained by slowly
evaporating concentrated aqueous solutions of Ba(N3)2 with a sixfold
excess of RbN3.
Crystal data
Ba2N15Rb
Mr = 570.30
Z = 2
Dx = 3.310 Mg m
Ag Kꢁ radiation
ꢂ = 5.91 mm
T = 293 (2) K
Ellipsoid, colorless
0.16 Â 0.07 Â 0.05 mm
3
Monoclinic, P2=n
1
Ê
a = 8.6681 (10) A
Ê
b = 5.6287 (4) A
Ê
c = 11.7330 (14) A
ꢀ = 90.199 (14)ꢀ
Ê
V = 572.45 (10) A
3
Data collection
Stoe IPDS-I diffractometer
' scans
Absorption correction: numerical
[X-RED (Stoe & Cie, 2001) and
X-SHAPE (Stoe & Cie, 1999)]
Tmin = 0.663, Tmax = 0.815
16393 measured re¯ections
2812 independent re¯ections
2178 re¯ections with I > 2ꢃ(I)
Rint = 0.075
Figure 3
The azide groups in RbBa2(N3)5, each surrounded by four Ba atoms in
the horizontal plane and two Ru atoms above and below the plane.
[Symmetry codes: (ix) x + 1, y + 2, z; (x) x + 12, y,
12; (xi) x + 12,
ꢄmax = 28.0ꢀ
z
1
2
y
1,
z
12; (xii) x
,
y + 2, z + 12; other codes as in Figs. 1 and 2.]
ꢁ
i86 Grigori V. Vajenine
Ba2N15Rb
Acta Cryst. (2006). C62, i85±i87

inorganic compounds
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Re®nement
Re®nement on F2
R[F2 > 2ꢃ(F2)] = 0.034
wR(F2) = 0.084
S = 1.14
2812 re¯ections
87 parameters
w = 1/[ꢃ2(F2o) + (0.054P)2]
where P = (F2o + 2Fc2)/3
(Á/ꢃ)max < 0.001
3
Ê
Áꢅmax = 1.81 e A
3
Ê
1.25 e A
Áꢅmin
=
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The slight deviation of the ꢀ value from 90ꢀ was con®rmed by
powder X-ray diffraction on a single-phase sample, where splitting of
relevant re¯ections, especially for 412 and 412, is evident.
Data collection: EXPOSE in IPDS Software (Stoe & Cie, 1998);
cell re®nement: CELL in IPDS Software; data reduction: INTE-
GRATE in IPDS Software; program(s) used to solve structure:
SHELXS97 (Sheldrick, 1990); program(s) used to re®ne structure:
SHELXL97 (Sheldrick, 1997); molecular graphics: DIAMOND
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È
The author thanks T. Scholkopf for preparative work, C.
Hoch and H. Garttling for collecting and handling the single-
crystal diffraction data, Professor A. Simon for support, and
the Alexander von Humboldt Foundation for funding through
a Sofja Kovalevskaja Research Award.
È
È
Sheldrick, G. M. (1997). SHELXL97. University of Gottingen, Germany.
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Stoe & Cie (1998). IPDS Software. Version 2.89. Stoe & Cie, Darmstadt,
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Supplementary data for this paper are available from the IUCr electronic
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ꢁ
Acta Cryst. (2006). C62, i85±i87
Grigori V. Vajenine Ba2N15Rb i87