Temperature-dependent synthetic routes to and thermochemical ranking of α-and β-SrNCN

Article abstract of DOI:10.1021/ic902065q

White powdery β-SrNCN was obtained by the solid-state metathesis between Srl₂ and ZnNCN at 843 K, by the reaction of Srl₂, CsCN, and CsN₃ in tantalum cylinders at the same temperature, and from the direct reaction between

Full text of DOI:10.1021/ic902065q

Inorg. Chem. 2010, 49, 2267–2272 2267
DOI: 10.1021/ic902065q
Temperature-Dependent Synthetic Routes to and Thermochemical Ranking of
r- and β-SrNCN
†
†
‡
†
,†
Michael Krings, Michael Wessel, Wolfgang Wilsmann, Paul M €u ller, and Richard Dronskowski*
†
‡
Institut f u€ r Anorganische Chemie and Institut f u€ r Gesteinsh u€ ttenkunde, RWTH Aachen University,
52056 Aachen, Germany
Received October 19, 2009
White powdery β-SrNCN was obtained by the solid-state metathesis between SrI and ZnNCN at 843 K, by the
2
reaction of SrI , CsCN, and CsN in tantalum cylinders at the same temperature, and from the direct reaction between
elemental Sr and H NCN dissolved in liquid ammonia. The solid-state reactions carried out at a higher 973 K yield
2
3
2
white R-SrNCN. Both experimental data (X-ray diffraction (XRD), infrared spectroscopy, differential scanning
calorimetry (DSC)) as well as GGA density-functional phonon calculations show that the β-phase is thermochemically
more stable, by a minute 2 kJ/mol (electronic-structure theory) and about 6 kJ/mol (DSC), whereas the R-phase is
slightly more dense. In addition, both XRD and DSC measurements reveal two distinct (endothermic) steps for the
β-to-R phase transition, that is, first around 920 ( 20 K, then at 985 ( 15 K based on the X-ray data.
Thermochemically, the upper heat effect is larger by a factor of 20.
Introduction
reaction of SrCO with HCN at temperatures between 773
3
and 973 K. At the beginning of the 21st century, an alter-
native phase, dubbed β-SrNCN, was first synthesized by
As a fundamental class of compounds in the fields of solid-
state and also molecular chemistry, cyanamides and carbo-
diimides have gained increasing attention within the past
decade. Because of their -2 anionic charge, both cyanamide
and carbodiimide structural units allow the realization of a
Wissmann using a reaction between SrCO and flowing
3
10
NH3. The temperature dependence of that particular
reaction is striking: at 873 K, β-SrNCN is obtained whereas,
at 1173 K, the same reaction yields R-SrNCN. Interestingly
enough, and also a bit counterintuitive, the low-temperature
but high-symmetry (R3m) β-phase was estimated by primi-
tive Madelung lattice-energy calculations as being metastable
by about 9 kJ/mol if compared with the high-temperature but
low-symmetry (Pnma) and presumably stable R-phase. There
is no simple explanation for that obvious contradiction, but
we note that Wissmann highlights that β-SrNCN can only be
2-
nitrogen-based pseudo-oxide chemistry since NCN and
2
-
2-
2-
O
are isolobal entities; thus, NCN is able to replace O
1
in several novel materials. A number of alkali metal, alka-
line-earth metal, transition-metal,
metal cyanamides as well as carbodiimides were obtained
following different synthetic routes.
2
3-6
and main-group
7,8
With respect to the alkaline-earth carbodiimides, the
crystal structure of strontium carbodiimide, R-SrNCN, was
9
synthesized by starting with high-surface SrCO . Thus, it is
3
first reported by Strid and Vannerberg already more than
not fully clear at the outset whether R- or β-SrNCN is the
stable carbodiimide phase.
4
decades ago. They obtained this particular phase by the
Figure 1 reveals the crystal structures of both polymorphs
2þ
*
To whom correspondence should be addressed. E-mail: drons@
HAL9000.ac.rwth-aachen.de. Fax: þ49-241-80 92642.
1) Pulham, R. J.; Hubberstey, P.; Down, M. G.; Thunder, A. E. J. Nucl.
of SrNCN. In both phases, Sr is octahedrally coordinated
2-
by nitrogen atoms from the NdCdN carbodiimide groups,
(
˚
and one finds CdN double bonds of about 1.23-1.24 A in
Mater. A 1979, 85-86, 299.
(
(
2) Vannerberg, N.-G. Acta Chem. Scand. 1962, 16, 2263.
3) Liu, X.; Krott, M.; M u€ ller, P.; Hu, C.; Lueken, H.; Dronskowski, R.
both. The only structural difference, however, is given by the
orientation of the carbodiimide groups. Within hexagonal
Inorg. Chem. 2005, 44, 3001.
4) Liu, X.; Wankeu, M. A.; Lueken, H.; Dronskowski, R. Z. Natur-
forsch. 2005, 60b, 593.
5) Krott, M.; Liu, X.; Fokwa, B. P. T.; Speldrich, M.; Lueken, H.;
Dronskowski, R. Inorg. Chem. 2007, 46, 2204.
6) Liu, X.; Stork, L.; Speldrich, M.; Lueken, H.; Dronskowski, R. Chem.
Eur. J. 2009, 15, 1558.
7) Liu, X.; Decker, A.; Schmitz, D.; Dronskowski, R. Z. Anorg. Allg.
2-
β-SrNCN, all six NCN units are arranged parallel to each
other along the c axis such that the edge-sharing octahedra
form “layers” within ab which are separated by the carbo-
diimide units. In the orthorhombic crystal structure of
R-SrNCN, however, four carbodiimide units lie parallel,
(
(
(
(
Chem. 2000, 626, 103.
(
(
8) Dronskowski, R. Z. Naturforsch. 1995, 50b, 1245.
(10) Wissmann, B. Ph.D. Dissertation, Universit €a t T u€ bingen, Germany,
2001.
9) Strid, K. G.; Vannerberg, N. G. Acta Chem. Scand. 1966, 20, 1064.
r 2010 American Chemical Society
Published on Web 02/02/2010
pubs.acs.org/IC

2
268 Inorganic Chemistry, Vol. 49, No. 5, 2010
Krings et al.
Figure 1. Polyhedralprojections of the crystal structures ofR-SrNCN (left) and β-SrNCN (right) with the SrN
6
octahedra givenin orange, Sr atomsin red,
N atoms in green, and C atoms in gray.
but the remaining two are flipped against the other four. This
results in corrugated layers of edge-sharing octahedra which
also engage in corner sharing.
Synthesis No. 2. SrI2 (99.99%) and CsN3 (99.99%) were
purchased from Aldrich. CsCN was prepared as described in
the literature. The preparation is identical with the one
mentioned above, with the exception that the tube is not
evacuated upon heating. The reactions follow the equation
1
3
We have recently found a new synthetic route leading to
single crystals of R- and β-SrNCN using a reaction between
11,12
NaCN, NaN , and SrI in sealed tantalum containers
at
3
2
SrI2 þ CsCN þ CsN3 f R=β-SrNCN þ 2CsI þ N2
different temperatures. We now report on further synthetic
improvements such as a novel low-temperature route, struc-
tural investigations, total-energy and phonon calculations, as
well as DSC measurements, targeted at the thermochemical
ranking of the two polymorphs.
and yield CsI as a side-product which must be removed in
a subsequent step. Thus, the raw product was ground with 2 equiv
of CsBr and heated under vacuo at 923 K for 12 h. The CsBr CsI
3
mixture then evaporates and reappears at the cold part of the
tube.
Whenever the above reaction was performed in sealed
Experimental Section
(
instead of open) Ta containers at 843 K, the presence of
Three synthetic techniques were used for the temperature-
dependent preparations of the two SrNCN polymorphs.
Synthesis No. 1. SrI (99.99%) was purchased from Aldrich,
elemental nitrogen caused a further pressure dependence of
SrNCN formation (see below).
2
Synthesis No. 3. An entirely new method to synthesize
both polymorphs of SrNCN utilizes very low temperatures.
Elemental strontium (99.99%, Aldrich) and H NCN (98%,
and ZnNCN was freshly prepared. To do so, 1 equiv of ZnI
Merck) was dissolved in demineralized water, and 1 equiv of
NCN (98%, Aldrich), also dissolved in demineralized water,
was added to the well-stirred solution which was further stirred for
h. The white precipitate was filtered, washed with water and
2
(99%,
2
H
2
Aldrich) in a molar ratio of 1:1 were dissolved in liquid ammo-
nia at 195 K. The resulting blue solution was allowed to stir
for 2 h before the liquid ammonia was evaporated. The remain-
ing white powder was evacuated and heated to 773 K in a
semiopen Ta container to increase crystallization and remove
any remaining ammonia. The product was characterized as
β-SrNCN on the basis of X-ray diffraction (XRD). When the
resulting white powder was heated up to 1073 K, the product
changed into R-SrNCN. The reaction follows the simplest
possible equation
3
ethanol, and dried in high vacuum at 120 ꢀC for 12 h.
Unless otherwise noted, all following manipulations were
performed in a glovebox under a purified argon atmosphere.
Note that tantalum containers are mandatory because the
alkaline-earth iodides react with oxygen, water, glass and also
ceramic containers to form very stable hydroxides and oxides.
Because of the sensitivity against humidity and oxygen, the
metal containers and the silica tubes were heated at 1273 K in
vacuum for 12 h and then transferred into the glovebox. The
starting materials (1 mmol each) were ground together in an
agate mortar and placed in a semiopen Ta container. The
container was placed into a silica tube, and the silica tube was
closed with a glass valve. The tube was placed upright into a
furnace and heated up to the reaction temperature within 24 h.
The temperature was held for further 48 h, and then the furnace
was cooled at 30 ꢀC per hour to room temperature. During
the entire duration of the reaction the tube was evacuated to
Sr þ H2NCN f R=β-SrNCN þ H2
Characterization
X-ray Studies. X-ray analysis was performed on a
Huber G 670 Guinier powder diffractometer with strictly
monochromatized Cu-KR radiation. All measurements
were performed in an evacuated flat sample-holder to
protect the products from oxygen and moisture. Rietveld
refinements, depicted in Figure 2, were carried out with
the programs FullProf and WinPlotr.
details can be found in Table 1.
1
-
6
5
line powder. The metathesis reactions thus correspond to
ꢀ10 mbar. The product always appeared as a white crystal-
1
4,15
Numerical
SrI2 þ ZnNCN f R=β-SrNCN þ ZnI2
and only differ in the chosen temperatures (973 and 843 K). The
phase ZnI is achieved as a volatile side-product which is
2
removed during the reaction to maximize the yield.
(13) Markley, T. J.; Toby, B. H.; Pearlstein, R. M.; Ramprasad, D. Inorg.
Chem. 1997, 36, 3376.
(14) Rodriguez-Carvajal, J. Physica B. 1993, 192, 55.
(15) Roisnel, T.; Rodriguez-Carvajal, J. WinPLOTR: a Windows tool for
(
11) Liao, W. Ph.D. Dissertation, RWTH Aachen University, Germany,
005.
12) Liao, W.; Dronskowski, R. Acta Crystallogr. 2004, E60, i124.
powder diffraction patterns analysis, Materials Science Forum; Proceedings of
the Seventh European Powder Diffraction Conference (EPDIC 7) 2000; Delhez,
R., Mittenmeijer, E. J., Eds.; p 118.
2
(

Article
Inorganic Chemistry, Vol. 49, No. 5, 2010 2269
Figure 3. IR spectra of R-SrNCN (top) and β-SrNCN (bottom).
Figure 2. Powder XRD patterns of R-SrNCN (top) and β-SrNCN
(bottom).
equipped with a resistance heater and a linear position-
sensitive detector.
Infrared Spectroscopy. Infrared spectra (Figure 3) were
recorded at room temperature using a Fourier trans-
form Avatar 360 ESP spectrometer (Nicolet) in the range
Table 1. Results of the Rietveld Refinements of R- and β-SrNCN
R-SrNCN
β-SrNCN
-
1
4
00-4000 cm . To do so, the samples were dispersed
space group, No., Z
lattice parameter
Pnma, 62, 4
R3m, 166, 3
in strictly anhydrous KBr and pressed into pellets (Ø =
1.3 cm).
˚
a (A)
12.4089(10)
3.9598(2)
5.3878(3)
3.20
39.888
0.059
0.041
3.9827(1)
a
15.0260(1)
3.08
41.442
0.042
0.058
˚
b (A)
Differential Scanning Calorimetry (DSC). Differential
scanning calorimetric measurements were performed
with the aid of a DSC 404 apparatus (Netzsch) in the
temperature range between 313-1023 K at a heating rate
of 50 K/min. To shield the sample (9 mg) against oxida-
tion, it was kept under a protective argon atmosphere
throughout the measurement.
˚
c (A)
-
3
calculated density, (g cm
3
)
3
molar volume (cm mol
-1
)
3
R
R
Bragg
p
The results of the structural refinement of the two
polymorphs are in good agreement with the literature.
Both lattice parameters and averaged bond lengths
match, within three standard deviations, with the details
Theoretical Calculations
1
6
Density functional theory (DFT) total-energy calculations
were performed using plane waves and pseudopotentials by
means of the computer program VASP (Vienna Ab Initio
from preceding contributions. Berger and Schnick re-
˚
ported averaged d(Sr-N) = 2.637(6) A and d(NdC) =
˚
1
.225(11) A for R-SrNCN, which compare well with our
1
7,18
˚
˚
Simulation Package)
employing the generalized-gradient
results: d(Sr-N)=2.644(4) A and d(NdC)=1.203(8) A.
For β-SrNCN, Liao and Dronskowski
19
1
2
approximation (GGA) of PBE type and the projector-aug-
mented wave (PAW) method; the cutoff energy was set to
00 eV. Both unit cells of R- and β-SrNCN were allowed to
specified
20
˚
˚
d(Sr-N)=2.623(2) A and d(NdC)=1.232(5) A, in good
5
agreement with this work which arrives at d(Sr-N) =
˚
.621(2) A and d(NdC) = 1.246(5) A.
˚
2
In addition, temperature-dependent X-ray studies were
(
17) Kresse, G.; Furthm u€ ller, J. Comput. Mater. Sci. 1996, 6, 15. Kresse,
G.; Furthm €u ller, J. Phys. Rev. B 1996, 55, 11169.
18) Kresse, G.; Hafner, J. Phys. Rev. B 1993, 47, 558. Kresse, G.; Hafner, J.
Phys. Rev.B 1994, 49, 14251.
19) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77,
865.
(20) Bl o€ chl, P. E. Phys. Rev. B 1994, 50, 17953.
performed to precisely determine the temperature of the
β-to-R phase transformation. The measurements were
done using a Huber G644 Guinier powder diffractometer
(
(
3
(
16) Berger, U.; Schnick, W. J. Alloys Compd. 1994, 206, 179.

2
270 Inorganic Chemistry, Vol. 49, No. 5, 2010
Krings et al.
change in volume and shape, and all atomic positions
were allowed to relax, too. Because of the expected small
total-energy differences the convergence criterion of the
-
6
electronic structure calculation was chosen as 10 eV. The
bulk modules were obtained by fitting a Murnaghan-type
equation of state to the energy-volume data in the proximity
of the equilibrium volumes. In addition, theoretical phonon
densities-of-states and spectra were calculated using the
quasi-harmonic approximation by means of the FROPHO
21
utility together with VASP, followed by subsequent ther-
22
modynamic calculations targeted at theoretical Gibbs free
energies.
Results and Discussion
10
In comparison with the preceding synthetic approach,
the present solid-state routes yield both phases of strontium
carbodiimide at 30-200 K lower reaction temperatures.
While R-SrNCN can be prepared phase-pure using all three
methods, the synthesis of β-SrNCN is accompanied by side-
products in the case of the solid-state routes, in part explain-
able by the low transition temperature of β-SrNCN to
R-SrNCN. Within synthesis no. 1, the low reaction tempera-
ture results in an unknown side-product whose scattering
contribution is on the order of 15% if based on integrated
X-ray intensities. Unfortunately, we have been unable to
separate this unknown phase from β-SrNCN. Within synthe-
sis no. 2, the removal of CsI is achievable at 923 K which is
slightly above the first transition temperature such that this
approach must result in a mixture of both phases. Lower
temperatures, however, lead to an incomplete CsI removal.
Fortunately enough, synthesis no. 3 gives the best results,
leading to phase-pure β-SrNCN (Figure 2) on the basis of
XRD.
Figure 4. Theoretical (DFT-GGA) phonon spectra of R-SrNCN (top)
and β-SrNCN (bottom) as calculated using the quasi-harmonic approxi-
mation.
In accordance with the XRD data, infrared measurements
indicate that the carbodiimide structure is present. Both IR
spectra (Figure 3) show the characteristic frequency sequence
inset) while none such are found for β-SrNCN (bottom).
Thus, the structure of R-SrNCN must be considered an
unstable transition state which will decay into the stable
β-SrNCN ground-state structure, in harmony with the fact
that the R-phase occurs at higher and the β-phase at lower
temperatures.
of the linear three-atoms group with D symmetry, typical
¥h
for the carbodiimide unit. For R-SrNCN (top) the ν
as
-
1
vibration occurs at 1987/2024 cm and the δ vibration at
-
1
6
76/664 cm . For β-SrNCN (bottom) the ν vibration is
as
1
-
-1
Figure 5 displays a temperature-dependent X-ray pattern
of β-SrNCN starting at room temperature. There is an
obvious phase transition toward R-SrNCN beyond a tem-
perature of about 900 K which, surprisingly, occurs 280 K
lower than the one given in ref 10. Nonetheless, we note
that the latter was not determined using pure SrNCN but,
instead, using a reaction mixture. A closer look into our
X-ray data (Figure 5) also yields that the phase transforma-
tion seemingly occurs through two distinct steps: there is the
new (201) reflection of R-SrNCN which defines, first, a
transformation temperature of 920 ( 20 K (lower dashed
horizontal line). In addition, there is another change in
intensities, at approximately 985 ( 15 K (upper dashed
horizontal line), with an additional but weaker reflection
around 25.5ꢀ in 2Θ.
The temperature range of the above two intensity changes
is in good agreement with the first theoretical (GGA
density-functional) estimate (see below). In addition, it is
worthwhile noting that the β-to-R phase transition is irre-
versible in the XRD experiment because R-SrNCN does not
recrystallize into β-SrNCN upon cooling once it has been
found. Such a transformation would require the reorienta-
tion of two of the six coordination carbodiimide groups
observed at 1981/2028 cm and the δ vibration at 663 cm
.
These experimental frequencies are in very good agreement
10,16
with both the literature
and also those that have been
calculated on the basis of DFT and the quasi-harmonic
approximation. According to the theoretical phonon spectra
depicted in Figure 4 the asymmetric carbodiimide vibration
-
1
can be found at 2010 cm while the δ vibration is located
-
1
at 650 cm for both structures. In addition, the Raman-
active but IR-inactive symmetric vibration is visible in the
-
1
.
theoretical phonon spectra, namely, at 1260 cm
Furthermore, the first-principles phonons give a first con-
clusive hint concerning the relative thermodynamical stabi-
lities of both phases. Only for the case of R-SrNCN (top) the
calculated spectrum contains negative [Note that the calcula-
tions yield, in fact, imaginary frequencies which are typically
displayed in negative form.] low-energy frequencies (see
(
21) Togo, A. FROPHO: a tool to compute phonon band structures and
thermal properties of solids; RWTH Aachen University: Aachen, Germany,
008. Available at: http://fropho.sourceforge.net. Togo, A.; Oba, F.; Tanaka, I.
Phys. Rev. B 2008, 78, 134106.
22) Stoffel, R., dissertation under preparation, RWTH Aachen Univer-
sity.
2
(

Article
Inorganic Chemistry, Vol. 49, No. 5, 2010 2271
Figure 5. Temperature-dependent powderXRDpatternofSrNCN startingfromroomtemperature(bottom) toslightlyabove1000K (top). Highandlow
absolute intensities are displayed in yellow and blue, respectively. Note the two distinct intensity changes around 920 ( 20 K (lower dashed horizontal line)
and 985 ( 15 K (upper line).
Table 2. DFT-GGA Total-Energy Results for R- and β-SrNCN
3
0
B
0
ΔE (kJ/mol) ΔV (cm /mol)
B
0
(GPa)
R-SrNCN (Pnma) 2.0
β-SrNCN (R3m) 0.0
-1.19
0.00
53.4
54.5
4.17
3.83
Figure 6. Theoretical Gibbs free energy-temperature diagram for
R-SrNCN and β-SrNCN as calculated by DFG-GGA and the quasi-
harmonic approximation.
together with bond breaking and formation (see Figure 1
and structural discussion above), not just a simple bond-
length adjustment.
Figure 7. DSC of β-SrNCN in going from T = 850-1050 K; see also
text.
If synthesis no. 2 is carried out in a sealed tantalum
container at 823 K, the reaction results in a mixture of the
R- and β-phase, a result which is easily explainable by their
thermodynamically favored. As said before, the difference
between the experimental transition temperature (beyond
900 K) and the theoretically calculated one (760 K) is more
than 140 K which corresponds to an error of only 1 kJ/mol
using the GGA, which is quite remarkable. In addition, the
larger density of R-SrNCN (Table 1) favors this phase’s
existence at higher pressures, as explained above. The bulk
moduli arrive at almost the same value (about 54 GPa,
Table 2), with β-SrNCN being slightly stiffer.
In addition, DSC (Figure 7) reveals that β-SrNCN changes
into R-SrNCN at higher temperatures in an endothermic
manner, as expected for this kind of transformation. Puz-
zlingly enough, and in good agreement with the XRD
different molar volumes V
and Le Chatelier’s principle:
mol
3
Because Vmol of R-SrNCN is smaller by almost 1.2 cm /mol,
the formation of this high-temperature phase may be en-
forced by a moderate nitrogen pressure (about 6 bar) at lower
temperatures already, as is observed experimentally.
The results of the DFT phonon calculations corroborate
the prior experimental findings, as seen from Figure 6 which
displays a theoretical Gibbs free energy-temperature dia-
gram. The theoretical T = 0 K data already reveal (Table 2)
that β-SrNCN is the thermochemically favored phase, but
only by a mere 2 kJ/mol. Upon increasing the temperature
above 760 K (Figure 6), R-SrNCN will eventually be

2
272 Inorganic Chemistry, Vol. 49, No. 5, 2010
Krings et al.
measurement, one first observes a very small endothermic
peak, on the order of 0.3 kJ/mol, close to 950 K. In addition,
one finds a much larger endothermic peak on the order of
temperature (ca. 2 kJ/mol) taking into account the presently
possible accuracy of first-principles theory.
5
.9 kJ/mol at about 995 K, which also mirrors the preceding
Acknowledgment. It is a pleasure to thank the Deutsche
Forschungsgemeinschaft for financial support. We also
thank the computing center of RWTH Aachen for having
provided their computing resources.
XRD observation. Thus, β- and R-SrNCN energetically
differ from each other by roughly 6 kJ/mol, and this is in
good agreement with the DFT result for absolute zero