High-pressure synthesis of crystalline carbon nitride imide, C 2N2(NH)

Article abstract of DOI:10.1002/anie.200603851

(Figure Presented) Extreme chemistry: The carbon nitride imide C ₂N₂(NH) is synthesized from the single-source precursor 1-cyanoguanidine under high-pressure and high-temperature conditions in a laser-heated diamond-anvil cell. Characterization of single crystals recovered at ambient conditions reveals that the new compound adopts a defect wurtzite structure (see picture).

Full text of DOI:10.1002/anie.200603851

Communications
DOI: 10.1002/anie.200603851
Nitrides
High-Pressure Synthesis of Crystalline Carbon Nitride Imide,
C N (NH)**
2
2
Elisabeta Horvath-Bordon, Ralf Riedel,* Paul F. McMillan, Peter Kroll, Gerhard Miehe,
Peter A. van Aken, Andreas Zerr, Peter Hoppe, Olga Shebanova, Ian McLaren,
Stefan Lauterbach, Edwin Kroke, and Reinhard Boehler
The main-group-element nitrides Si N and Ge N crystallize
are amorphous or nanocrystalline, and their structures and
chemical compositions are not well characterized. Further-
more, the compounds prepared under HP-HT conditions are
not generally recovered to ambient conditions.
3
4
3
4
in at least four polymorphs, including the technologically
important a and b forms, a new spinel-type g phase synthe-
sized at high pressure and temperature, and a high-pressure
[
1,2]
[1,2]
d phase.
Sn N also forms a cubic spinel-type phase.
Herein, we report the first synthesis of a well-crystallized
compound with an N:C ratio of 3:2, in which all of the carbon
atoms are tetrahedrally coordinated. Crystals of the com-
pound are formed from the single-source precursor 1-
cyanoguanidine (dicyandiamide (DCDA), C N H ) under
3
4
There is still no reliable evidence for analogous dense phases
of C N . High-density C N polymorphs were predicted to
3
4
3
4
have very high bulk modulus and hardness values comparable
[
3–11]
with or exceeding that of diamond.
Many experimental
2
4
4
studies have attempted to produce dense crystalline C Ny
HP-HT conditions in a laser-heated diamond-anvil cell
(Scheme 1). Single crystals of the new dense carbon nitride
phase were recovered to ambient conditions for structural and
chemical analysis.
x
phases, by using various techniques, including high-pressure,
high-temperature (HP-HT) synthesis. Solids with N:C ratios
[
1,2]
of 1.3–1.5 have been reported.
However, these materials
[
+]
[*]Dr. E. Horvath-Bordon, Prof. Dr. R. Riedel, Dr. A. Zerr
Disperse Feststoffe, Material- und Geowissenschaften
Technische Universität Darmstadt
Petersenstrasse 23, 64287 Darmstadt (Germany)
Fax: (+49)6151-166-346
E-mail: riedel@materials.tu-darmstadt.de
Scheme 1. Synthesis of C N (NH).
2
2
Prof. Dr. P. F. McMillan, Dr. O. Shebanova
Christopher Ingold Laboratories
Department of Chemistry, University College London
The precursor was embedded in an NaCl pressure
medium along with ruby chips for pressure determination.
20 Gordon Street, London WC1H0AJ (UK)
Heating was carried out with a CO laser (l = 10.6 mm), and
and
2
Royal Institution of Great Britain
Davy Faraday Research, Laboratory
2
1 Albemarle Street, London W1S4BS (UK)
+
[
]Present address:
Laboratoire des PropriØtØs MØcaniques et Thermodynamiques des
MatØriaux, CNRS
Institut GalilØe, UniversitØ Paris 13
99 avenue J. B. Clement, 93430 Villetaneuse (France)
Priv.-Doz. Dr. P. Kroll
Institut für Anorganische Chemie, RWTH Aachen
Professor-Pirlet-Strasse 1, 52056 Aachen (Germany)
[
++]
Dr. G. Miehe, Dr. I. McLaren
Strukturforschung, Material- und Geowissenschaften
Technische Universität Darmstadt
Petersenstrasse 23, 64287 Darmstadt (Germany)
++
[
]Present address:
Department of Physics and Astronomy
University of Glasgow, Glasgow G128QQ (Scotland)
[
+++]
Priv.-Doz. Dr. P. A. van Aken,
Dr. S. Lauterbach
+++
[
]Present address:
Institut für Angewandte Geowissenschaften
Material- und Geowissenschaften
Technische Universität Darmstadt
Max-Planck-Institut für Metallforschung
Heisenbergstrasse 3, 70569 Stuttgart (Germany)
[
**]The work was financially supported by the Deutsche Forschungs-
gemeinschaft (Germany), partially in the form of a Heisenberg
Fellowship (P.K.). The P.F.M. group in London is supported by the
Engineering and Physical Science Research Council (UK) under
Portfolio Grant EP/D504872. P.F.M. is also a Wolfson–Royal Society
Research Merit Award Fellow. We thank Elmar Gröner (MPI für
Chemie) for his support with the nanoSIMS, and Denis Machon
(UniversitØ Claude Bernard, Lyon) and Dominik Daisenberger
(University College London) for help with the diamond-anvil-cell
and synchrotron experiments.
Schnittspahnstrasse 9, 64287 Darmstadt (Germany)
Dr. P. Hoppe
Abteilung Partikelchemie, Max-Planck-Institut für Chemie
55020 Mainz (Germany)
Prof. Dr. E. Kroke
Institut für Anorganische Chemie
Technische Universität Bergakademie Freiberg
09596 Freiberg (Germany)
Dr. R. Boehler
Hochdruck-Mineralphysik, Max-Planck-Institut für Chemie
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
55020 Mainz (Germany)
1
476
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2007, 46, 1476 –1480

Angewandte
Chemie
[
12]
the temperature was determined by thermal emission. The
products were examined in situ by Raman spectroscopy.
After recovery to ambient conditions, the samples were
cleaned with distilled water and studied using transmission
electron microscopy (TEM) techniques, including selected-
area electron diffraction (SAED), electron energy-loss spec-
troscopy (EELS), and energy-dispersive X-ray spectroscopy
(
EDS). Synthesis attempts at pressures below 27 GPa and
temperatures below 17008C yielded black amorphous prod-
ucts (Figure 1, part A). Treatments at higher pressures and
temperatures resulted in transparent samples (Figure 1,
part B).
Figure 1. Pressure and temperature conditions for the synthesis of
carbon nitride phases from DCDA by laser-heating. Typical optical
micrographs of the products after heating at temperatures of 1100–
1
2008C under pressures less than 27 GPa (part A), and after heating at
temperatures of 1700–23008C under pressures higher than 27 GPa
part B). The diameter of the sample is 70–80 mm; P indicates the
(
I
laser-heated part of the sample, P the cold rim of the sample, and M
II
the pressure medium (NaCl).
Dark-field TEM images of the recovered materials
revealed nanocrystals (diameters of 10–30 nm) embedded in
an amorphous matrix. Extended heating (episodes of several
minutes each) at temperatures higher than 17008C under a
pressure of 41 GPa resulted in the formation of crystals of
Figure 2. a) Bright-field TEM image of a crystal of C N (NH) synthe-
sized from DCDA by laser-heating at a pressure of 41 GPa and a
temperature higher than 17008C. b) Representative C K and N K EELS
2
2
edges of the reference compound C
N Cl (top) and of
6
7 3
C N (NH) (middle). The shaded areas indicate the 60-eV integration
1
.0–1.5 mm in length, which were recovered to ambient
2
2
windows that start at the edge onsets. Selected orbital transitions are
labeled. At the N K edges, the extrapolated power-law background is
shown (bgd). The ratio of the intensity of the C K edge to that of the
N K edge is 3.02 for C N Cl and 2.18 for C N (NH). The C K and N K
conditions (Figure 2a). No bands characteristic of C=N
ꢀ1
bonds (ca. 2180 cm ) are visible in the Raman spectra of
ꢀ
1
the products. However, broad bands at 2900–3150 cm
6
7
3
2
2
indicate the presence of NꢀH or CꢀH groups. Further
spectra calculated for C
2
2
N (NH) using FMS theory are shown below
structural characterization of the new compound excluded
the experimental spectra (see Supporting Information for details).
the presence of CꢀH bonds (see below). Raman spectra taken
c) SAED pattern of the [110]zone of the orthorhombic cell of dwur-
¯
2
C N (NH). The kinematic intensities jFj calculated on the basis of
in situ at high pressure following laser-heating exhibit at least
2
2
the atomic parameters in Table 1 and scaled to 100 for reflection 002
are indicated. The intensity of reflection 400 (marked (*)) is enhanced
owing to secondary diffraction from the 200 reflection.
1
4 bands (Figure 3). No changes were detected in the spectra
during decompression to 6 GPa, before background fluores-
[
13]
cence obscured the Raman signal. The presence of hydro-
gen in the recovered samples was confirmed by nanoscale
[
14]
secondary ion mass spectrometry (nanoSIMS). The H:C
The carbon and nitrogen contents of the recovered
crystals were determined by EELS in combination with
ratio was determined to be 0.63 ꢁ 0.18.
Angew. Chem. Int. Ed. 2007, 46, 1476 –1480
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
1477

Communications
coordination geometry of the nitrogen sites. In the dwur-C N
3
4
3
structure, the nitrogen sites are pyramidal (sp ), rather than
2
trigonal-planar (sp ) as would be expected in the b- or a-C N
3
4
structures. The observed lattice spacings of the new structure
pffi ffiffi pffi ffiffi
are very close to a tripled ( 3 ” 3) lonsdaleite cell; however,
we were unable to reconcile the dwur-C N structure with the
3
4
experimentally determined cell parameters and space group.
Once the presence of structural hydrogen atoms was
recognized (by nanoSIMS) and a composition of C N (NH)
2
2
was determined, all of the experimental information could be
integrated in a structure analogous to that adopted by
Si N (NH), Si N O, and a high-pressure modification of
2
2
2
2
[
24–26]
B O .
2
The structure of the new dwur-C N (NH) phase
2 2
3
(Figure 4) is derived from the hexagonal lonsdaleite structure
Figure 3. In situ Raman spectrum of C N (NH) synthesized from
2
2
DCDA by laser-heating at a pressure of 41 GPa and a temperature
ꢀ1
higher than 17008C. The bands between 1200 and 2700 cm stem
from the diamond-anvil cell.
theoretical calculations (Figure 2b). This technique requires
the comparison of the edge intensities due to the ionization of
characteristic inner-shell electrons to the partial ionization
cross sections of a suitable reference compound (e.g.,
[
15]
C N Cl ). The C:N ratio was determined to be 0.62 ꢁ 0.06.
6
7
3
No oxygen bands were detected in the EELS spectra,
[
16]
indicating an oxygen concentration less than 0.1–1 at%.
From the nanoSIMS and EELS studies, a composition of
C N H (C N (NH)) could be derived.
2
3
2
2
Figure 4. Defect wurtzite (dwur) structure of C N (NH).
2
2
The energy-loss near-edge structure (ELNES) was used to
obtain information on local coordination environments
[
17]
(
Figure 2b). Pre-edge features at 285 eV at the C K edge
and is in excellent agreement with the electron-diffraction
and EELS results. With respect to the wurtzite structure,
nitrogen atoms occupy one set of tetrahedral sites, and carbon
atoms fill 2/3 of the tetrahedral sites of the other sublattice.
The remaining sites in this sublattice are filled with hydrogen
atoms bound to nitrogen. The structural parameters of dwur-
C N (NH) were refined using a combination of the electron-
and at 398 eV at the N K edge of C N Cl are identified as
6
7
3
transitions to p* and s* (Figure 2) molecular orbitals (MOs),
2
indicating trigonal-planar sp bonding. In contrast, the spectra
3
for C N (NH) indicate tetrahedrally-directed sp hybrid
2
2
orbitals for both atoms. The first C K-edge peak at 290 eV
is attributed to transitions to MOs with strong s* character,
2
2
3
like those of sp -bonded diamond.
diffraction and theoretical results (Table 1). Two thirds of the
nitrogen atoms (N1) in dwur-C N (NH) have pyramidal
The crystal structure of the new compound was deter-
mined by combining electron-diffraction results with first-
principles theoretical studies. The experimentally determined
composition and SAED data were used first to select a set of
possible crystal structures for the C N H phase. Ab initio
2
2
coordination environments (Table 2). The dense packing of
nitrogen and carbon atoms in dwur-C N (NH) can be
2
2
described as hexagonal close-packed. Alternatively, the
C N (NH), Si N (NH), and Si N O structures can be de-
x
y
z
2
2
2
2
2
2
calculations then allowed us to refine the structure and
confirm its composition. Using density functional theory
scribed in terms of corrugated SiN or CN layers comprising
Si N or C N six-membered rings connected by NH groups or
oxygen bridges.
3
3
3
3
[
18–22]
(
DFT),
we first examined several potential C N struc-
3
4
[
4,6,23]
tures.
The electron-diffraction data suggested a relation-
The bulk modulus calculated for dwur-C N (NH) is K =
2
2
o
ship to a hexagonal close-packed motif, as found in the
lonsdaleite or wurtzite structures. Various hypothetical struc-
tures were immediately excluded on the basis of high internal
energies or inherent elastic and dynamic instabilities (calcu-
lated using ab initio molecular dynamics (MD) simulations),
or by their incompatibility with the observed lattice param-
eters and SAED intensities. Only a defect-wurtzite (dwur)
structure for C N gave good agreement with the electron-
277 GPa, which is significantly lower than those of diamond
and the hypothetical dense polymorphs of C N (K = 430–
3
4
o
460 GPa). This increased compressibility is due to the
presence of NꢀH groups that do not contribute to the solid-
state network. However, the bulk modulus of dwur-C N (NH)
2
2
[
12]
exceeds that of b-Si N (K = 256 GPa).
3
4
o
The ELNES at the C K and N K edges were modeled
using self-consistent full multiple scattering (FMS) theory
3
4
[
27]
diffraction and EELS data, in particular, with respect to the
(Figure 2b). The excellent agreement between the energy
1
478
www.angewandte.org
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2007, 46, 1476 –1480

Angewandte
Chemie
[
a]
Table 1: Comparison of the structural parameters of C N (NH) and
reflection conditions (h0l: l = 2n) consistent with space group
C2cm (Ama2), Cmc2 , or Cmcm. The structure of Si N O, in
2
2
[
26]
Si N (NH).
2
2
1
2
2
[
26]
Parameter
C N (NH)
Si N (NH)
space group Cmc2 , is the prototype for the dwur structure;
1
2
2
2
2
[
26]
Si N (NH) also adopts this structure type (Table 1). The
2
2
Space group
Cmc21
Cmc21
reflection conditions, axial ratios, and relative intensities of
the spots in the SAED pattern, as well as the chemical
composition indicate that the new compound is C N (NH)
Lattice parameters
a [Š]7.5726
b [Š]4.4425
c [Š]4.0036
a:b:c
9.1930
5.4096
4.8190
2
2
with a Si N O-type dwur structure.
2 2
The calculated density of dwur-C N (NH) is 1 =
calcd
2
2
1.705:1:0.901
1.699:1:0.891
ꢀ3
(
3
(
3.21 ꢁ 0.3) gcm , close to that of diamond (1 =
ꢀ3
.52gcm ). The atom density of dwur-C N (NH)
2 2
Atomic positions
ꢀ
3
172.7 atomsnm ) is very high. The cubic diamond and
Atom
C/Si
N1
x
y
z
0
x
y
z
0
0.3295 0.3390
0.3020 0.3651 0.3582
0.3263 0.3440
hexagonal lonsdaleite polymorphs of carbon have the highest
ꢀ
3
0.287 0.363 0.35
0 0.258 0.43
atomic density of any material (176.5 atomsnm ). The
atomic densities of the cubic zinc-blende-type and hexagonal
wurtzite-type BN structures are slightly lower (169.3 and
N2
H
0
0
0.2851 0.4240
0.4032 0.1956
ꢀ
3
[
a]The C N (NH) structure was determined by DFT calculations. The
167.5 atomsnm , respectively).
2
2
lattice parameters determined by electron diffraction are a=7.536(15),
b=4.434(8), and c=4.029(8) Š. The LDA (local density approximation)
follow the usual trend for such compounds of underestimating lattice
parameters by approximately 1%.
If all of the interatomic interactions involved covalent
bonding, dwur-C N (NH) would have a low compressibility
2
2
and a very high hardness. The bulk modulus K is related to
o
the cohesive (binding) energy E and the molar volume V by
c
m
[
28]
3
K = cE /V , where c ꢂ 2–4. The hardness of sp -bonded
o
c
m
[
12]
materials correlates well with K . The compressibility of
o
Table 2: Selected interatomic angles and distances in C N (NH).
2
2
dwur-C N (NH) is increased because of the presence of
2
2
Angle [8]Distance [Š]
hydrogen atoms that do not contribute to cross-linking in the
structure. However, the available NꢀH groups could undergo
N1-C-N1
107.32
CꢀN1
1.45
further condensation reactions leading to dense C N poly-
1
1
08.11
09.84
1.46
1.43
1.05
3
4
morphs.
CꢀN2
N2ꢀH
The DCDA precursor used in our HP-HT experiments
N1-C-N2
C-N1-C
113.15
115.70
was chosen as a source for a C N compound with alternating
x
y
CꢀN units and an N:C ratio of greater than 4:3. We supposed
that condensation into C N would occur by elimination of
3
4
1
1
16.19
20.37
4
/3 equivalents of NH (or 1/2N + 3/2H ). Under our reac-
3 2 2
tion conditions, only 1 equivalent of NH was eliminated to
3
C-N2-C
C-N2-H
127.78
112.12
give C N (NH) (Scheme 1).
2
2
In conclusion, we laser-heated DCDA in a diamond-anvil
cell at temperatures higher than 17008C under pressures
higher than 27 GPa and obtained a novel carbon nitride phase
with an N:C ratio of 3:2. Single crystals (1
= (3.21 ꢁ
calcd
ꢀ
3
and intensity distributions of the calculated and observed
spectra for the leading C K and N K edge maxima and for
post-maximum features confirmed the structural assignment
as dwur-C N (NH). None of the spectra calculated for other
0.3) gcm ) of the product were recovered to ambient
conditions. Quantitative nanoSIMS analysis revealed the
presence of hydrogen, and a composition of C N (NH) was
2
2
determined. The new compound crystallizes in a dwur
structure analogous to that of Si N (NH).
2
2
candidate structures for C N or C N H compounds matched
3
4
x
y
z
2
2
the experimental ELNES as closely.
Structural analysis was carried out by collecting SAED
patterns with different zone orientations for individual single
Received: September 19, 2006
Revised: October 24, 2006
Published online: January 15, 2007
crystals of 1–1.5 mm in length. The cell parameters of a =
3
7
.536(15), b = 4.434(8), c = 4.029(8) Š, and V= 139(1) Š
Keywords: carbon · density functional calculations ·
determined (from 7 zones) for a C-centered orthorhombic
cell were similar to those of an equivalent hexagonal cell (a
ꢂ 4.4, c ꢂ 4.0 Š). The hexagonal c/a ratio and the distribution
.
high-pressure chemistry · nitrides · structure elucidation
of SAED intensities suggested a tripled wurtzite- or lonsda-
pffiffiffi
[
1] E. Horvath-Bordon, R. Riedel, A. Zerr, P. F. McMillan, G.
Auffermann, Y. Prots, W. Bronger, R. Kniep, P. Kroll, Chem.
Soc. Rev. 2006, 35, 987.
leite-type structure with a’ = a ꢀa (a’ = 3a) and c’ = c. In
2
1
terms of such a cell, the SAED pattern in Figure 2c is of the
¯
[
100] zone (equivalent to the [110] zone of the orthorhombic
[
[
[
2] E. Kroke, M. Schwarz, Coord. Chem. Rev. 2004, 248, 493.
3] M. L. Cohen, Phys. Rev. B 1985, 32, 7988.
4] A. Y. Liu, M. L. Cohen, Science 1989, 245, 841.
cell). However, in several orientations, the SAED intensities
violate the hexagonal (or trigonal) symmetry. The SAED
pattern of the [010] zone of the orthorhombic cell displays
[5] A. Y. Liu, M. L. Cohen, Phys. Rev. B 1990, 41, 10727.
Angew. Chem. Int. Ed. 2007, 46, 1476 –1480
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
1479

Communications
[
[
[
[
6] D. M. Teter, R. J. Hemley, Science 1996, 271, 53.
[17] L. A. J. Garvie, A. J. Craven, R. Brydson, Am. Mineral. 1994, 79,
411.
[18] P. Hohenberg, W. Kohn, Phys. Rev. B 1964, 136, B864.
[19] W. Kohn, L. J. Sham, Phys. Rev. 1965, 140, 1133.
[20] G. Kresse, J. Hafner, Phys. Rev. B 1993, 47, 558.
[21] G. Kresse, J. Hafner, Phys. Rev. B 1994, 49, 14251.
7] A. Y. Liu, R. M. Wentzcovitch, Phys. Rev. B 1994, 50, 10362.
8] Y. J. Guo, W. A. Goddard, Chem. Phys. Lett. 1995, 237, 72 .
9] P. Kroll, J. Solid State Chem. 2003, 176, 530.
[
[
[
10] S. Muhl, J. M. Mendez, Diamond Relat. Mater. 1999, 8, 1809.
11] T. Malkow, Mater. Sci. Eng. A 2001, 302, 309.
12] R. Riedel, Handbook of Ceramic Hard Materials, Wiley-VCH,
Weinheim, 2000.
[
[
22] G. Kresse, J. Furthmüller, Phys. Rev. B 1996, 54, 11169.
23] E. Kroke, M. Schwarz, E. Horath-Bordon, P. Kroll, B. Noll, A. D.
Norman, New J. Chem. 2002, 26, 508.
[
13] E. D. Miller, D. C. Nesting, J. V. Badding, Chem. Mater. 1997, 9,
[
24] J. Sjoberg, G. Helgesson, I. Idrestedt, Acta Crystallogr. Sect. C
18.
1
991, 47, 2438.
25] C. T. Prewitt, R. D. Shannon, Acta Crystallogr. Sect. B 1968, 24,
69.
[
14] A. Benninghoven, F. G. Rüdenauer, H. W. Werner, Secondary
Ion Mass Spectrometry: Basic Concepts, Instrumental Aspects,
Applications, and Trends, Wiley, New York, 1987.
15] R. F. Egerton, Electron Energy-Loss Spectroscopy in the Elec-
tron Microscope, 2nd ed., Plenum, New York, 1996.
16] R. Leapman, EELS Quantitative Analysis, Wiley-VCH, Wein-
heim, 2004.
[
8
[
[
26] D. Peters, H. Jacobs, J. Less-Common Met. 1989, 146, 241.
27] A. L. Ankudinov, B. Ravel, J. J. Rehr, S. D. Conradson, Phys.
Rev. B 1998, 58, 7565.
[
[
[
28] V. V. Brazhkin, A. G. Lyapin, R. J. Hemley, Philos. Mag. A 2002,
82, 231.
1
480 www.angewandte.org
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2007, 46, 1476 –1480