CP620, Shock Compression of Condensed Matter - 2001
edited by M. D. Furnish, N. N. Thadhani, and Y. Horie
2002 American Institute of Physics 0-7354-0068-7
STRUCTURAL STUDIES AND EOS OF
DIAMINODINITROETHYLENE (DADNE, FOX-7)
UNDER STATIC COMPRESSION
S. M. Peiris, G. I. Pangilinan, F. J. Zerilli and T. P. Russell
Energetic Materials Research and Technology Department
Naval Surface Warfare Center, Indian Head, MD 20640
Abstract. Structural and molecular changes in diaminodinitroethylene compressed to static pressures up to
4.2 GPa were investigated. Angle-dispersive x-ray diffraction experiments were performed with
synchrotron radiation to monitor the compression and any phase changes. The results indicated higher
compression along the b-axis than along the a- or c- axis. In addition, the ambient temperature isothermal
equation of state of FOX-7 was generated from this data. Raman spectroscopy covering a 300 to 3400 cm"1
range showed expected hardening of most vibrational modes. However, two modes in the energy regions
corresponding to N-O stretching and H wagging, softened with pressure. This indicates the possible
increase of intermolecular H bonding within the zigzagging planes of FOX-7 at increased pressures.
crystal structure is stable to 4.2 GPa. Our Raman
studies show two vibrational modes softening with
pressure. Gaussian calculations of vibrational
modes were performed to understand the
implications of the mode softening.
INTRODUCTION
Diaminodinitroethylene (DADNE or FOX-7) is
a newly developed energetic material with low
sensitivity comparable to TATB and high
performance. The structure of FOX-7 under
ambient conditions is similar to the structure of
TATB, with strongly hydrogen bonded intra-layer
molecules connected to neighboring layers with
weak Van der Waal bonds. [1]
Since detonation of energetic materials occur in
the high pressure and temperature regime, modeling
of the detonative properties of any explosive
requires high-pressure equations of state and other
thermodynamic parameters. Therefore, we studied
the effects of compression on the structure of FOX7 using both x-ray diffraction methodology and
Raman spectroscopy.
We report here the isothermal compression
curve of FOX-7 at room temperature. Our x-ray
studies show that the ambient pressure monoclinic
EXPERIMENTAL METHOD
The FOX-7 samples used in this study were
obtained from Richard Gilardi at NRL. Samples for
angle-dispersive x-ray diffraction were loaded into
Merrill-Bassett cells with diamond anvils that had
0.6-1mm culets. [2] Stainless steel gaskets O.lmm
thick, drilled with 0.3-0.6 mm holes, were used to
contain the sample. A 5:1 mixture of finely ground
FOX-7 :NaCl was used. Pressures were determined
from the measured compression of NaCl with
diffraction collected simultaneously with FOX-7.
The pressure reported is the average of the values
obtained from the modified Decker equation of
181
1.00
state [3] and the Spetzler[4]-Fritz[5] EOS as
analyzed by Birch. [6]
Angle-dispersive x-ray diffraction experiments
were carried out at the B2 line of the Cornell High
Energy Synchrotron Source (CHESS). Diffracted x-rays were collected on an x-ray-sensitive image
plate for 15-40 minutes at each pressure. The image
plate was then scanned and analyzed using SIMPA
software. [7] The d spacing associated with each
diffraction peak was estimated by fitting a Gaussian
to the peak. The 5-15 peaks observed were indexed
using the monoclinic P2(l)/n space group.
For Raman spectroscopy, FOX-7 powder was
loaded into Merrill-Bassett cells with cubic zirconia
anvils. The gaskets were made from 0.2 mm
tantalum with a gasket hole diameter of 200-250
um. A small ruby sphere (10 urn in diameter) was
added to each sample, and the frequency shift of the
ruby Rl line was used as a pressure gauge. [8]
The Raman system consisted of an Ar ion laser
operating at 514.5 nm with 150 mW of power, and
a SPEX 1m double-mate spectrometer employing a
CCD detector. Peak positions were determined by
fitting a Gaussian function to the observed Raman
vibrations.
0.98
Q.
I
o
r
c
0
10
1
2
3
4
Pressure (GPa)
FIGURE 2. Compression of the unit cell dimensions a, b and c.
phase changes were observed to 4.2 GPa. Analysis
of the diffraction patterns shows that the b axis of
the unit cell compresses faster than the a or c axis.
This has been illustrated in Fig. 2. The b axis lies in
the direction perpendicular to the zigzagging layers
within the FOX-7 crystal. These layers are bonded
by Van der Waal's bonds and separated by 3.31 A
at ambient P and T. Near a pressure of 1.2 GPa,
there appears to be a discontinuity in the
compression of the b axis.
Elastic parameters were calculated by using the
Birch-Murnaghan (BM) [6] equation of state
formalism to model the P,V data obtained from x-ray diffraction. The second order BM equation of
state yields a zero-pressure isothermal bulk
modulus (Ko) of 20.6±0.5 GPa, where the pressure
derivative of the bulk modulus (Ko') is held
constant at a value of four. The third order BM
EOS gives a K0 of 17.9±1.4 GPa, and a KO' of
6.6±4.9.
Figure 3 shows the Raman spectra observed at
different pressures. Again, there is some small
change in the Raman pattern by 1.2 GPa (indicated
by the ovals), however most of the modes stiffen or
shift to higher frequencies at higher pressure as
expected. Two modes show anomalous mode
softening, or shift to lower Raman frequencies with
pressure. These two modes at 1025 cm"1 and 1342
cm"1 at ambient pressure are denoted by thick black
arrows in Fig 3.
The diffraction pattern originating from the
monoclinic lattice structure of FOX-7 was
indexable to 4.2 GPa, the highest pressure
Figure 1 shows selected x-ray diffraction
patterns for FOX-7 as a function of pressure. No
Angle 26
0.94
0.92
RESULTS AND DISCUSSION
5
0.96
15
(degrees)
FIGURE 1. X-ray diffraction patterns of FOX-7 under
compression.
182
I
3
400
600
800
1200
1000
1400
1600
Raman Shift (cm )
FIGURE 3. Raman spectra of FOX-7 at different pressures. The ovals indicate changes at 1.1 GPa and the arrows indicate the two modes
that soften with pressure.
investigated here. Therefore, the observed changes
in the Raman spectrum and the discontinuity of the
b-axis compression around 1.2 GPa perhaps
indicate some subtle molecular change that does not
affect the monoclinic lattice.
Calculations were performed with the Gaussian
98 [9] quantum chemistry program to investigate
the Raman modes and symmetries softening with
pressure. The B3-PW91 density functional method
was used. This method combines Becke's threeparameter hybrid method [10] with the PerdewWang
generalized
gradient
approximation
functional [11]. The basis set used was the 631+G** basis set of Pople et al. [12] with "d"
polarization functions for heavy atoms and "p"
polarization functions for hydrogen, supplemented
with diffuse functions for heavy elements [13]. The
vibrational frequencies computed by the B3-PW91
methods are scaled by a factor of 0.9573 to correct
for systematic errors in the calculations. [14]
The calculated vibrational frequencies and
intensities were matched to the experimentally
obtained frequencies and intensities. The two
modes that soften were matched to two calculated
modes which include intense H-wagging and slight
N-O bond stretching. This together with the
extensive hydrogen bonding network within the
layers of FOX-7 suggest that, with pressure,
intermolecular hydrogen bonds become strong
enough to weaken N-H bonds and N-O bonds
within each molecule.
CONCLUSION
FOX-7 exhibits some changes in the Raman
vibrational spectrum and x-ray diffraction pattern
when compressed to 1.2 GPa. The monoclinic
lattice structure found at ambient pressure is
indexable to 4.2 GPa, the highest pressure used in
this study. Under compression most of the Raman
modes stiffen with pressure, other than two modes
at 1025 and 1342 cm"1. Gaussian calculations
indicate that these two modes consist of H-wagging
vibrations together with slight N-O stretching. This
indicates that N-H and N-O bonds within each
molecule are weakening under pressure to increase
the intermolecular hydrogen bonding. Further
investigations will be performed to study the
changes observed at about 1.2 GPa.
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ACKNOWLEDGEMENTS
We thank Dr. Richard Gilardi of the Naval
Research Lab, Washington DC, for his assistance
and input.
The x-ray diffraction work reported here is
based upon research conducted at the Cornell High
Energy Synchrotron Source (CHESS) which is
supported by the National Science Foundation
under award DMR 97 13424.
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