Pressure-induced phase transition of 1,5-diamino-1: H -tetrazole (DAT) under high pressure

Article abstract of DOI:10.1039/d0ra06328b

Nitrogen-rich energetic materials have attracted certain interest as promising high energy density materials (HEDMs) in recent years. Pure N2 and nitrogen-based molecular crystals are ideal HEDMs that would polymerize under high pressure, as reported in previous literature. We selected a 1,5-diamino-1H-tetrazole (DAT) crystal, which has two kinds of molecular structures and hydrogen bonds, to study under high pressure by spectroscopy and diffraction due to its high nitrogen percentage and low sensitivities. Pressure-induced structure transitions occur at pressures of 2.3-6.6 GPa, ～8.5 GPa, and ～17.7 GPa. The phase transition at 2.3-6.6 GPa is related to the rotation of NH2, and the latter two transitions are caused by both the rotation of NH2 and the distortion of the heterocycle. Significantly, the reconstitution of the hydrogen bond may induce the rotation/distortion of the NH2/heterocycle in the second phase transition. There is no evidence showing a transformation between the two molecular structures in the whole pressure range studied. Our investigation uncovers the phase transition mechanism of DAT under pressure, which will help to find targeted HEDMs. This journal is

Full text of DOI:10.1039/d0ra06328b

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Pressure-induced phase transition of 1,5-diamino-
1H-tetrazole (DAT) under high pressure†
Cite this: RSC Adv., 2020, 10, 30069
Cheng Jin, ‡ Ying Liu,‡ Lijuan Wang, Weijing Zhang, Tonglai Zhang *c
a
b
a
c
ad
and Jinlong Zhu*
Nitrogen-rich energetic materials have attracted certain interest as promising high energy density materials
2
(HEDMs) in recent years. Pure N and nitrogen-based molecular crystals are ideal HEDMs that would
polymerize under high pressure, as reported in previous literature. We selected a 1,5-diamino-1H-
tetrazole (DAT) crystal, which has two kinds of molecular structures and hydrogen bonds, to study under
high pressure by spectroscopy and diffraction due to its high nitrogen percentage and low sensitivities.
Pressure-induced structure transitions occur at pressures of 2.3–6.6 GPa, ꢀ8.5 GPa, and ꢀ17.7 GPa. The
phase transition at 2.3–6.6 GPa is related to the rotation of NH
caused by both the rotation of NH and the distortion of the heterocycle. Significantly, the reconstitution
of the hydrogen bond may induce the rotation/distortion of the NH /heterocycle in the second phase
2
, and the latter two transitions are
2
Received 21st July 2020
Accepted 10th August 2020
2
transition. There is no evidence showing a transformation between the two molecular structures in the
whole pressure range studied. Our investigation uncovers the phase transition mechanism of DAT under
pressure, which will help to find targeted HEDMs.
DOI: 10.1039/d0ra06328b
rsc.li/rsc-advances
dimers, trimers, or new polymer polymorphs. Furthermore,
high-pressure may lead to the formation of hydrogen bonds,
Introduction
9
High energy density materials (HEDMs) are dened as those which will signicantly change the sensitivity of HEDMs.
that release tremendous heat and/or vast amounts of gas in Therefore, investigating the phase evolution and the decom-
a very short time by heat, impact, shock, spark, etc. stimulation. position of HEDMs under high pressure is critical to funda-
They can broadly be classied as pyrotechnics, propellants mentally understand explosion and combustion processes, and
(
including gas generators) and explosives, which are widely it will be helpful to nd a potential pathway for designing new
1–6
4,7
used in daily life, aerospace and the military. In general, energetic materials.
Recent experimental and theoretical
HEDMs are expected to have high energetic density, low sensi- efforts to nd a more stable crystal structure hosting higher
tivity and release less greenhouse gas than conventional energy energetic density and lower sensitivity have focused on aspects
1
,2,4,5
materials.
uene (TNT), 1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane (HMX), ular interactions.
,3,5-trinitrohexahydro-s-1,3,5-triazine (RDX) and nitrogen-rich materials have attracted considerable interest due to their high
energetic materials chronologically. The instant pressure can go nitrogen content, low carbon content, heterocyclic structure
Typical HEDM representatives are trinitrotol- of chemical composition, molecular structure and intermolec-
2,4–6
Remarkably, nitrogen-rich energetic
1
2,4–6,9,10
up to 50 GPa and temperature can go up to 5500 K during the and hydrogen bond that exists among molecules.
Among
explosion and combustion of HEDMs, resulting in molecular the nitrogen-rich energetic materials, polymeric nitrogen char-
1
,7,8
polymorphs and/or crystal phase transitions.
tool, high-pressure technology can simulate a certain degree of attention because of the expected large energy release during
As an effective acterized by pure single nitrogen bonding has attracted lots of
ꢁ
1
explosion and combustion, such as the possible formation of the process of single-bond (ꢀ160 kJ mol ) rupture and triple-
ꢁ
1
11,12
bond (ꢀ946 kJ mol ) formation.
Moreover, the gaseous
product is pure N , an environmentally friendly counterpart.
2
a
Center for High Pressure Science and Technology Advanced Research (HPSTAR),
Eremets et al. reported that polymeric cubic gauche nitrogen (cg-
Beijing, 100094, China. E-mail: zhujl@sustech.edu.cn
b
N) was obtained using pure N as a precursor in a laser-heated
2
Xi'an Modern Chemistry Research Institute, Xi'an, 710065, China
diamond anvil cell (DAC) under the pressure 110 GPa and
c
State Key Laboratory of Explosion Science and Technology, Beijing Institute of
11
temperature higher than 2000 K at 2004. More recently, black-
phosphorus-structured nitrogen (BP-N) was synthesised by
Technology, Beijing, 100081, China. E-mail: ztlbit@bit.edu.cn
d
Department of Physics, Southern University of Science and Technology, Shenzhen,
13
14
5
†
1
‡
18055, China
Laniel et al. and Cheng Ji et al. under 140 GPa, ꢀ4000 K and
Electronic supplementary information (ESI) available. See DOI: 146 GPa, >2200 K in DACs. However, the phase transitions from
0.1039/d0ra06328b
N
2
to cg-N or BP-N require a pressure higher than 110 GPa and
RSC Adv., 2020, 10, 30069–30076 | 30069
Contributed equally to this work.
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26
temperature higher than 2000 K, while both of them transfer
In Lesnikovich's another work under high temperature,
back to N2 once they are released to ambient pressure. For molecular DAT will partially transform from structure II to
practical application, the primary task is to achieve the phase structure I when heated under 500 K by transferring the
transition at ambient pressure or at least pressure below hydrogen on the N4 atom to the –N5H bonded to the C atom. In
3
0 GPa, which could be realized in a large volume press. One of the progress of heating to 900 K, molecule in structure I would
the possible solutions could be hydrogen bonds formation rstly break the N1–N2 and N3–N4 bonds to produce an inter-
among molecules, as they play a key role of maintaining the mediate product CN , releasing N gas. Then, CN
stability of the crystallographic structure and leading to a higher decomposes to HCN, NH , and N through hydrogen trans-
More importantly, pressure ferring between amino, C–N&N–N bond rupture and C–H bond
4
H
4
2
4 4
H
3
2
15–20
density phase with insensitivity.
can easily tune the hydrogen bond by means of rearrange- formation. The remaining molecule in structure II will rst
ment, reconstruction and symmetrization. In this work, break the C–N4 and N1–N2 bonds and produce CN H and HN
3
26
21
22
23
3
3
1
,5-diamino-1H-tetrazole (DAT), a member of heterocyclic when heated to 900 K. CN H subsequently decomposes to
3 3
2 3 3 2 3
nitrogen, was selected for the highest nitrogen content (84%) by 1,2,4-triazole (C N H ) with releasing N and NH . Lesnikovich
weight compared with tetrazole and 5-amino-tetrazole (5-AT). It reported the possible DAT decomposition routes under high
ꢁ
1
is abundant of nitric single-bonds (ꢀ160 kJ mol ) and double- temperature, but the crystal structure transition of DAT under
ꢁ1
bonds (ꢀ419 kJ mol ), which is easier to polymerize compared high pressure still needs to be explored further.
ꢁ
1
to a triple-bond (ꢀ946 kJ mol ) under pressure. Generally,
In this work, the possible structural phase transition of DAT
26
27
theoretical and experimental researches reported two kinds polymorphs under pressure is investigated by in situ Raman,
of DAT molecule polymorphs at ambient conditions, which are Infrared-Ray (IR) spectra and X-ray diffraction (XRD) experiments
denoted as molecular structure I and structure II in Fig. 1(a), up to 40 GPa. The results indicate that three crystal structure phase
˚
respectively. The tetrazole ring is planar within 0.001 (1) A and transitions occurred at the pressure of 2.3–6.6 GPa, ꢀ8.5 GPa and
a p-system exists on the tetrazole ring. The N5–H or N5H
is lying in the tetrazole ring plane and conjugated with the p- phase transition. Both rotation of NH
system of the tetrazole ring, while N6H
2
group ꢀ17.7 GPa, respectively. The rotation of NH
2
induces the rst
and distortion of hetero-
2
is not the case. At cycle are involved in the second and third transitions. The tran-
2
24
ambient conditions, with hydrogen bonds of N–H/N in the sition at ꢀ8.5 GPa is closely related to the reconstitution of the
framework, DAT crystallized in a high-density structure with hydrogen bonds. No obvious evidence shows the molecular
˚
a space group P2 /c and lattice parameters of a ¼ 6.780(1) A, b ¼ transformation between the two molecular structures.
1
˚
˚
ꢂ
24
6
.112(1) A, c ¼ 10.694(1) A, and b ¼ 107.25(1) . The schematic
crystal structure is plotted in Fig. 1(b). The unique three-
dimensional network has hydrogen bonds between layers,
Experimental methods
resulting in a slight rotation of the molecule. This makes the DAT samples were synthesized by reacting thiosemicarbazide,
structure more stable and insensitive. Lesnikovich et al. re- sodium azide, and ammonium chloride in dimethylformamide
ported the melt temperature and decomposition temperature of in the presence of PbO. The detailed information can be found
DAT are 460 K and in the range of 470 K to 540 K, respectively. in ref. 28. Elemental analyses (C, H and N) were performed on
The enthalpy change of the decomposition of DAT is a Flash EA 1112 fully automatic trace element analyzer. The
ꢁ
1
ꢁ1
850 kJ mol
compared to 220 kJ mol
of 1-methyl-5- samples of DAT were dried in the water bath drying oven at
ꢂ
aminotetrazole (MAT), indicating that DAT has a better 60 C for 24 h and cooled to room temperature in dryer before
thermal stability and is more energetic releasing. Besides, DAT the test. Through elemental analyses, we got the percent of C
releases gas up to 550 cm g during the decomposition, while and N as 12.56% and 83.76%, respectively. X-ray powder
3
ꢁ1
3
ꢁ1
25
it is just 240 cm g for MAT.
diffraction (XRPD) measurements at ambient pressure were
performed on a Bruker D8 advance diffractometer at 60 kV, 300
Fig. 1 (a) Polymorphs of 1,5-diamino-1H-tetrazole are denoted as structure I and structure II, (b) crystal structure of 1,5-diamino-1H-tetrazole at
ambient conditions. The dashed blue lines represent the hydrogen bonds.
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mA and Cu Ka radiation (l ¼ 1.5406 A), with a scan speed of
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˚
ꢂ
ꢁ1
ꢂ
5
C min and a step size of 0.02 in 2q. The XRPD result and
the simulated XRD peaks from cif le of ref. 24 are plotted in
Fig. S1.† Therefore, we propose that the purity was higher than
95%. High-pressure experiments were carried out in a DAC with
diamond culets of 300 mm in diameter. A T301 stainless steel
gasket was used and pre-indented to 26 mm in thickness, and
then a hole of 100 mm in diameter was drilled into the centre as
the sample chamber. Ruby balls with a size of 10 mm in diam-
eter were used to monitor the pressure during the experi-
29–31
ments.
The measurement point is about 3 mm from the ruby
ball. No pressure transmission medium was used because DAT
is a so crystal with a bulk modulus of 23(3) GPa tted as below,
which can keep the sample chamber in a quasi-hydrostatic
condition.
The in situ Raman experiments were performed using
a Renishaw Invia. The excitation wavelength of the laser was
5
32 nm. Raman spectra were collected through a holographic
grating of 2400 g per mm. The laser power range was from 0 to
0 mW (100%) to avoid damaging the DAT sample. The strong
2
ꢁ
1
peak of diamond was located at 1300–1400 cm and 2300–
ꢁ
1
2700 cm , which was cut off for all the data in Raman.
The IR spectra were collected on a Bruker VERTEX 80v FTIR
spectrometer with a HYPERION 2000 IR microscope. The IR
beam was set to the size of 20 ꢃ 20 mm by a pair of knife-edge
diaphragms. KBr pellet was used to optimize the thickness of
the sample. The IR spectra were collected in transmission mode
with a resolution of 4 cm and recorded with a nitrogen-cooled
broadband mercury cadmium telluride detector.
Fig. 2 XRD of DAT under pressure. The asterisks mark the new peaks.
The arrows highlight the peak vanishings. The black, orange, red and
blue lines stand for the phase I to phase IV.
ꢁ1
The high-pressure XRD experiments were conducted with
a symmetric DAC with same geometry and same culet of 300 mm
comparing to the DAC in Raman experiment, and similar
procedures to the Raman measurements were adopted. The
high-pressure XRD experiments were performed at the 4W2
bulk modulus B ¼ 23 (3), as shown in Fig. S2 and S3.† The
0
0
0
derivative of bulk modulus is set as B ¼ 4 and the original
3
˚
volume of crystal is 423.2 A in ref. 24. The detailed information
0
about renement of XRD data and tting of B for ambient
pressure phase is shown in the ESI.† The tting result shows
that DAT is a so crystal under low pressure, which can keep the
sample chamber in a quasi-hydrostatic condition.
High Pressure Station of Beijing Synchrotron Radiation Facility
˚
(
BSRF) where the X-ray wavelength was l ¼ 0.6199 A. The XRD
patterns were collected with a MAR 3450 image plate detector
and integrated from the images using the FIT2d soware.
Raman and IR spectroscopy under pressure
Results and discussion
X-ray diffraction of DAT under pressure
To investigate the source of possible phase transitions, in situ
Raman and IR experiments under pressure were carried out.
To investigate the phase transitions of DAT under pressure, They can provide the information of local vibration and bonding
synchrotron XRD pattern was collected up to 40 GPa, as plotted under pressure. Raman/IR spectra of DAT at near ambient pres-
in Fig. 2. Through checking the full width at half maxima sure (0.03 GPa) and decompression to near ambient pressure are
(FWHM) of peak at 002, a new peak near 002 is found at 2.3 GPa. displayed in Fig. S4 and S5.† The details of vibration modes at
At 6.2 GPa, there is a peak near 013 emerging. At 9.6 GPa, a new ambient conditions are listed in Table S1† and are compared
ꢂ
peak at 2 theta of 7.7 is observed. At 17.1 GPa and 19.7 GPa, with the IR spectra modes in ref. 26 and 32. Some modes are
ꢂ
ꢂ
a new peak at 2 theta of 7.2 and a shoulder at 2 theta of 7.5
recognized according to ref. 33 and 34. Totally, the new found
ꢁ
1
appear. The vanishings of peaks are also marked by black peaks at 50–200 cm are recognized as lattice modes. The modes
ꢁ
1
ꢁ1
arrows. From the above observation, there may exist three phase located in 200–1000 cm and 3000–3500 cm are related to
transitions roughly at 2.3–6.2 GPa, 9.6 GPa and 17.1–19.7 GPa. NH stretching (n(NH )) or N–H wagging (g(N–H)). The modes in
Under further compression, amorphization occurs when pres- 1000–1800 cm are mostly connected with heterocycle vibration,
sure is higher than 27.3 GPa. By adopting the ambient pressure plus the N–H bending modes (b(N–H)) at 1545 cm in Raman
phase, we rened the XRD data by Le Bail method and tted and at 1636 cm , 1659 cm in IR.
2
2
ꢁ1
ꢁ
1
24
ꢁ1
ꢁ1
34,35
In Fig. S4 and S5,† the
the bulk modulus using 2nd-order Birch–Murnaghan (BM) unique vibrational modes of structure I and structure II can both
equation of state (EOS) in low pressure phase, which got the be found, indicating the two typed of DAT polymorphs coexist in
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the sample. It should be noticed that XRD pattern shows a pure
phase at ambient phase because both of the DAT polymorphs can
26,27
exist in the same crystal.
There are no direct connection
between crystal structure and molecular structure. The detailed
identication is shown in the supplemental materials. Certain
unique vibrational modes of two molecule structures can be
observed with pressure up to 40 GPa, such as nendo(C]N) (in-
ꢁ1
phase) at 1473 cm in molecular structure I and nexo(C]N) at
ꢁ
1
1
669 cm in molecular structure II. No obvious evidence shows
molecular structure transformation occurs accompanying the
crystal structure phase transitions. Finally, almost all the modes
recover when decompressing to ambient pressure in Fig. S4 and
S5,† indicating the local chemical environment is preserved.
Fig. 3 and 4 plot the representative Raman/IR spectra of DAT
under pressure up to 40 GPa, and the relative peak shis of the
Fig. 4 IR spectroscopy of DAT as a function of pressure up to
Raman and IR modes as a function of the pressure are plotted in ꢀ40 GPa. The black, orange, red and blue lines stand for the phase I to
Fig. 5 and 6, respectively. It has to be noticed that the Raman phase IV. The dash lines mark the new modes and their evolution. And
ꢁ
1
other marks of the vibrational modes are same to the Raman spectra in
Fig. 3.
peak at 791 cm is too strong to be plotted in Fig. 3, we only
plotted its pressure evolution in Fig. 5. Furthermore, the Raman
ꢁ
1
ꢁ1
ꢁ1
ꢁ1
ꢁ1
modes at 90 cm , 147 cm , 328 cm , 687 cm , 1134 cm
ꢁ
1
and IR mode at 1730 cm vanish once increasing pressure, so between the nendo(C]N)(out-of-phase) and b(N–H) mode can be
they are not plotted in Fig. 5 and 6. Under pressure, the lattice found. Summarily, all the above anomalies occur in the vibra-
ꢁ1
modes in 50–200 cm
have signicant anomalies at 2.3– tional range of NH
stretching or just near the bending mode of
2
6
7
.6 GPa. In Fig. 3(a), at 2.3 GPa two new modes located at N–H. The interaction between molecules becomes stronger with
ꢁ1
ꢁ1
5 cm and 105 cm emerge. When compressing to 3.5 GPa, compression, as expected, the amino attached to the hetero-
ꢁ
1
ꢁ1
ꢁ1
two shoulders at 84 cm , 104 cm and a peak at 178 cm
cycle is easily to be tuned and rotate under pressure. This is
ꢁ
1
appear. At 4.8 GPa, a shoulder is observed at 96 cm . These because the amino group attached on the N1 in DAT is not
24
indicate the rst phase transition from phase I to phase II in the conjugated with the p-system, as that in a tetrazole ring.
pressure range between 2.3 GPa and 6.6 GPa, which is consis- Therefore, at 2.3–6.6 GPa the phase transition is related to the
tent with the XRD results. Meanwhile, molecular vibrations also rotation of NH , much like the reported rotation of NO of in
show complex changes. In Fig. 3, at 2.3 GPa a new mode HMX that induced the phase transition at ꢀ5.0 GPa in ref. 7.
2
2
ꢁ
1
ꢁ1
emerges at 689 cm . The n(C–NH
2
) mode at 495 cm broadens This phase transition is sluggish over a broad pressure range. It
ꢁ
1
ꢁ1
and splits into two peaks at 490 cm , 498 cm under pressure may be caused by their Gibbs free energy difference or the slow
ꢁ
1
7,9
of 3.5 GPa. At 6.6 GPa, n(NH ) mode at 697 cm splits into two transition kinetics.
2
ꢁ
1
ꢁ1
modes at 719 cm , 729 cm and a new peak near the b(N–H) is
With the pressure increasing, obvious changes of vibration
ꢁ
1
observed at 1556 cm . In Fig. 4, when compressed to 2.7 GPa, modes are observed at ꢀ8.5 GPa. In the lattice modes, the peak
ꢁ
1
ꢁ1
ꢁ1
a new mode at 662 cm appears and a new peak at 1608 cm
at 104 cm gets broadening with an obvious red shi and splits
ꢁ
1
ꢁ1
ꢁ1
Fig. 3 Raman spectroscopy of DAT as a function of pressure up to ꢀ40 GPa. (a) 50–500 cm , (b) 500–800 cm , (c) 800–1300 cm , (d) 1400–
ꢁ1
ꢁ1
1
800 cm , (e) 2950–3450 cm . The black, orange, red and blue lines stand for the phase I to phase IV. The asterisks and dash lines mark the
new modes and their evolution. The subscript “exo” refers to the mode attached to the five-membered nitrogen heterocycle. The subscript
endo” refers to the modes inside the five-membered nitrogen heterocycle. n refers to stretching of bond. b refers to bending of bond. g refers to
wagging of bond. The ring with bracket behind modes, such as g(N–H)(ring), means that H is attached to the heterocycle directly.
“
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ꢁ1
Fig. 5 Evolution of Raman vibration modes upon increasing pressure. (a) The lattice mode in 50–300 cm , (b)–(e) contain the evolution of
ꢁ
1
ꢁ1
ꢁ1
ꢁ1
molecular modes, (b) 200–6500 cm , (c) 650–1250 cm , (d) 1400–1750 cm , (e) 2950–3350 cm . The dash lines mark the phase transition
pressure points. The red squares mark significant discontinuities at ꢀ8.5 GPa, which can be the evidence of the reconstitution of hydrogen
bonds.
ꢁ
1
ꢁ1
ꢁ1
into two peaks at 147 cm , 155 cm under the pressure of
nendo(C]N)(out-of-phase) at 1578 cm vanishes when pressure
9
9
.5 GPa, corresponding to the changes in XRD pattern near is higher than 8.9 GPa. There are also new modes emerging in
.5 GPa. This indicates the second phase transition from phase the range of heterocycle vibrations. In Fig. 3, under the pressure
ꢁ1
ꢁ1
II to phase III. Meanwhile, anomalies of molecular vibrational of 8.5 GPa two new modes emerge at 1017 cm and 1695 cm
.
modes can also be found. In Fig. 3, under pressure of 8.5 GPa At the same pressure, the nendo(C]N)(out-of-phase) mode at
ꢁ
1
ꢁ1
ꢁ1
ꢁ1
ꢁ1
two new modes at 222 cm and 294 cm emerge, and the 1622 cm splits into 1628 cm mode and 1646 cm mode. A
ꢁ
1
ꢁ1
ꢁ1
n(ring-NH
2
) at 322 cm and n(N–H)(ring) at 3154 cm broaden new peak at 1461 cm is observed at 10.5 GPa. These anoma-
ꢁ
1
and split. At 9.5 GPa, a new peak is observed at 3098 cm and lies lying in the range of heterocycle vibrations indicate that the
ꢁ
1
ꢁ1
n(N6H ) mode at 3246 cm splits into two peaks at 3267 cm
second phase transition is also accompanied by the heterocycle
2
ꢁ
1
and 3298 cm . Under pressure up to 10.5 GPa, a shoulder and distortion. As further increasing pressure, the repulsive force
a new peak emerge at 317 cm , 546 cm , respectively. Overall, occurring between p-system of tetrazole rings may make the
the emergence and splitting of modes stated above are in the heterocycle distortion. A similar phenomenon is also reported
ꢁ
1
ꢁ1
9
region of NH
2
stretching or N–H wagging, indicating NH
2
4 4
in ref. 7 where the distortion of the C N ring in HMX induces
7
rotation involves in the second phase transition. However, phase transitions under pressure up to 40 GPa. Another
different from changes at 2.3–6.6 GPa, the anomalies at example is that the distortion of heterocycle in tetrazole also
ꢀ
8.5 GPa are not only related to the NH vibrations but also occurs in the phase transition under pressure of 3.1–7.8 GPa,
located in the range of heterocycle vibrations. Several modes which is induced by the formation of hydrogen bonds. The
2
9
related to heterocycle disappear. For instance, in Fig. 3(c), inuence of hydrogen bonds on this transition will be discussed
ꢁ
1
n(ring) mode at 998 cm disappears above 8.5 GPa. In Fig. 4,
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ꢁ1
ꢁ1
Fig. 6 Evolution of IR vibration modes upon increasing pressure. (a) 650–1250 cm , (b) 1300–1850 cm . The dash lines mark the phase
transition pressure points. The red squares mark the significant discontinuities at ꢀ8.5 GPa.
in the following section. In a word, the phase II to phase III time, a number of new peaks related to heterocycle vibration
ꢁ
1
transition is from both NH
2
rotation and heterocycle distortion. can be found: in Fig. 3, two shoulders at 1048 cm
and
ꢁ
1
Further compression to 13.5 GPa, some changes occur in 1198 cm emerge at 16.7 GPa; near 19.1 GPa, a peak appears at
ꢁ1
ꢁ1
molecular vibrations. For example, new peaks at 271 cm
,
1048 cm marked by an asterisk. In Fig. 4, a new peak is
ꢁ1
ꢁ1
ꢁ1
1
151 cm , 1411 cm appear in Fig. 3. On the contrary, XRD observed at 1441 cm around 16.8 GPa; two modes emerge at
ꢁ1
ꢁ1
ꢁ1
peaks in Fig. 2 and lattice modes in Fig. 3 have no obvious 1168 cm , 1416 cm and the n(ring) mode at 1305 cm splits
changes at this pressure point. Therefore, these anomalies can near 19.5 GPa. These changes about heterocycle vibrations
only be recognized as local vibration changes without phase indicate that distortion of heterocycle also involves in this phase
7,9
transition. By further compression, signicant anomalies of transition. We conclude that the third phase transition is
7,9
molecular vibrational modes are observed at ꢀ17.7 GPa. induced by both rotation of NH
2
and distortion of heterocycle.
Although no obvious changes are observed in lattice modes, the Further increasing the pressure, some more anomalies in
anomalies in XRD peaks and molecular vibrations conrm the molecular modes occur at ꢀ24.9 GPa, including a new peak at
ꢁ
ꢁ1
1
phase transition from phase III to phase IV. Firstly, several NH
vibration modes disappear. In Fig. 3, the peak at 3098 cm
2976 cm near 24.9 GPa in Fig. 3 and another new peak at
2
1
ꢁ
1134 cm under 24.0 GPa in Fig. 4. These may be from the
emerging at 9.5 GPa disappears above 19.1 GPa. The shoulder at disordering entangled amorphization or local molecular vibra-
ꢁ
1
2
94 cm which is observed at 8.5 GPa vanishes when pressure tion changes.
ꢁ
1
is higher than 19.1 GPa. In Fig. 4, the peak at 662 cm which
appears at 2.7 GPa and the n(NH ) mode at 747 cm both
2
vanish when pressure is higher than 16.8 GPa. Meanwhile, new
modes of NH vibrations are observed. In Fig. 3, two shoulders
emerge at 400 cm and 669 cm around 16.7 GPa. When
compressed to 17.7 GPa, the mode at 689 cm which appears
under the pressure of 2.3 GPa splits into two peaks. At 19.1 GPa,
a new peak can be found at 3166 cm . A shoulder at 514 cm
and a peak at 752 cm appear at 20.2 GPa. In Fig. 4, a new
ꢁ
1
The inuence of hydrogen bonds
Just as stated above, hydrogen bond has a signicant impact on
2
ꢁ
1
ꢁ1
the structure and phase transition of HEDMs under pres-
15–23
ꢁ
1
sure.
Coincidently, large amount of hydrogen bonds exist in
the crystal of DAT as shown in Fig. 1(b). Under compression,
ꢁ1 most modes in both vibrational spectra exhibit blue shis with
the increasing of pressure before phase transitions, as the intra-
ꢁ
1
ꢁ1
9
ꢁ
1
and inter-atomic distances shrink under compression.
mode at 973 cm
appears near the g(N–H)(ring) mode at
ꢁ1
However, in Fig. 5(e) n(N–H)(ring) at 3154 cm , n(N6H
2
) at
1
9.5 GPa. Similar to the phase transition at ꢀ8.5 GPa, the
ꢁ
1
ꢁ1
3
271 cm and n(N5H ) at 3323 cm exhibit red shis as the
2
changes above show that NH rotation is mainly responsible for
2
26
7
result of the hydrogen bond interaction. Pressure shortens the
distance of H/N in the hydrogen bonds and strengthens
electrostatic attraction, therefore extending the N–H bond
the phase transition at ꢀ17.7 GPa. Also, heterocycle vibration
vanishings are observed in this transition, such as the n(ring) at
ꢁ
1
1
332 cm disappearing near 19.5 GPa in Fig. 4. At the same
37
2
length and resulting in the red shis. On the contrary, n(N6H )
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at 3246 cm and n(N5H
RSC Advances
ꢁ
1
ꢁ1
2
) at 3193 cm exhibit blue shis. The transitions at 2.3–6.6 GPa and ꢀ17.7 GPa, and the new peaks
9
same phenomena occur in the Cui's work on tetrazole and with red shis may be inuenced by the original hydrogen
36
Zou's work on urea nitrate under pressure. This is because the bonds.
attractive and repulsive interactions coexist in the proton donor
9,36
and acceptor of hydrogen system. So the blue and red shis
are determined by the percentage of attractive and repulsive
interaction.
Conclusions
In conclusion, DAT has phase transitions at the pressure of 2.3–
During compression, signicant discontinuities occur in the 6.6 GPa, ꢀ8.5 GPa and ꢀ17.7 GPa. From the results of vibra-
second phase transition at ꢀ8.5 GPa. Several sudden droppings tional spectra and XRD experiment under high pressure, it is
of frequency and red shis are observed in Fig. 5 and 6, which indicated that the rst phase transition at 2.3–6.6 GPa is rele-
are marked by red squares. More specically, in Fig. 5, the vant to the rotation of NH ; the second phase transition at
2
ꢁ
1
n(NH ) mode at 729 cm , which is split from the n(NH ) mode ꢀ8.5 GPa and the last phase transition at ꢀ17.7 GPa are related
2
2
ꢁ1
at 697 cm under 6.6 GPa, transforms from blue shi to red to both distortion of heterocycle and rotation of NH . Signi-
2
shi near 8.5 GPa. At the same pressure, the new peak at cantly, the anomalies in the second phase transition may be
ꢁ
1
1
628 cm which is split from nendo(C]N)(out-of-phase) mode induced by the reconstitution of hydrogen bonds. Besides, no
ꢁ
1
at 1622 cm shows a huge reduction of frequency comparing to obvious evidence of the molecular transformation between the
the original mode and exhibits a red shi. When compressed to two molecular structures is found in the vibration spectroscopy
ꢁ
1
9
1
3
.5 GPa, the n(NH ) mode at 791 cm , the nexo(C]N) mode at and XRD pattern.
2
ꢁ1
669 cm show a sudden frequency dropping, and the peak at
267 cm splitting from n(N6H ) mode at 3246 cm not only
has a large frequency dropping comparing to the original mode
but also shows a red shi. In Fig. 6(b), the mode at 1608 cm
ꢁ
1
ꢁ1
2
Conflicts of interest
ꢁ1
There are no conicts to declare.
emerging at 2.7 GPa gradually transforms from blue shi to red
shi near 8.9 GPa.
Acknowledgements
Generally speaking, sudden drops of frequency in the
vibration modes indicate the releasing of vibrational energy. This work was supported by National Science Foundation of
This may be relevant to the new interaction among individual China (Grant No. U1930401, No. 11904281) and by Funda-
molecules, for instance the hydrogen bond here. When pressure mental Research Funds for the Central Universities (Grant No.
increases, the interaction energy grows with the compression of 2017CX10007). We acknowledge BSRF, which is supported by
distance between molecules. The Gibbs free energy increases Chinese Academy of Sciences. J. Zhu acknowledges the support
rapidly. When it reaches a critical point, the original hydrogen- of open funding of Xi'an Modern Chemistry Research Institute
bond networks cannot be maintained. Then the reconstitution (Grants No. SYJJ200303).
of hydrogen bond occurs in the purpose of reducing the Gibbs
9,36
free energy. The red shis aer frequency drops indicate the
bonds soen and the vibrational energy reduces above 8.5 GPa,
which are induced by the attractive interaction of new hydrogen
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This journal is © The Royal Society of Chemistry 2020