# High-Pressure Synthesis and Study ofNO+NO3− and NO2+NO3− Ionic Solids

Authors: A. Yu. Kuznetsov; L. Dubrovinsky; A. Kurnosov; M. M. Lucchese; W. Crichton; C. A. Achete
Journal: Advances in Physical Chemistry
 (2009)

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## Abstract

Nitrosonium-nitrateNO+NO3− and dinitrogen pentoxide NO2+NO3− ionic crystals were synthesized by laser heating of a condensed oxygen-rich O2-N2 mixture compressed to different pressures, up to 40 GPa, in a diamond anvil cell (DAC). High-pressure/high-temperature Raman and X-ray diffraction studies of synthesized samples disclosed a transformation of NO+NO3− compound to NO2+NO3− crystal at temperatures above ambient and pressures below 9 GPa. High-pressure experiments revealed previously unreported bands in Raman spectra of NO+NO3− and NO2+NO3− ionic crystals. Structural properties of both ionic compounds are analyzed. Obtained experimental results support a hypothesis of a rotational disorder of NO+ complexes in NO+NO3− and indicate a rotational disorder of ionic complexes in NO2+NO3− solid.


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## Body

## 1. Introduction

Application  of high pressure  can convert simple  molecular structures of elemental
and heteronuclear systems into nonmolecular solids with extended or “infinite”
atomic lattices
[1]. Examples of such extended solids are
nonmolecular nitrogen [2–4],
metalic oxygen  [5],
polymeric form of carbon dioxide [6],
and carbon   monoxide
[7]. Such
pressure-induced molecular-to-nonmolecular phase transitions are one of the ways to reduce a total energy
of the molecular crystal via local (polymerization) or complete (metallization)
delocalization of intramolecular electrons between adjacent molecules. Another
alternative of a more stable form of chemical bonds perturbed by pressure and
temperature variation is ionization with the onset of charge-transfer
interactions. The first reported example of the neutral-to-ionic solid
transformation concerns the organic charge-transfer compound
tetrathiafulvalene-chloranil
[8, 9].
In this material, the pressure or temperature variation promotes either neutral
or ionic form, which is determined by the electrostatic energy contribution to
the total energy of the donor-acceptor molecules. The similar neutral-to-ionic
transformation has been reported in several simple molecular systems as well.
The formation of H2+H2− dimmer at
150 GPa in phase III of solid hydrogen [10]
was proposed to explain the discontinuous changes of the Raman and infrared vibron
frequencies [11, 12].
The nitrogen dioxide dimmer, N2O4, transforms to the
ionic NO+NO3− complex by temperature variation
at ambient pressure [13, 14]
and by pressure variation after laser heating  [15, 16].
Recently, the same ionic solid was synthesized from N2O and N2O4  molecular crystals by laser heating at high pressures  [17–21].
From the latter studies, one can infer a possibility of a direct synthesis of NO+NO3− crystal from oxygen and nitrogen reactants. In particular, the primary
dissociation of N2O on N2 and O2 at high
temperatures and pressures below 30 GPa was evident from the Raman spectra [17, 18]. On the other hand, X-ray diffraction studies showed that
nitrosonium nitrate is denser than other nitrogen-oxygen assemblages [19]. Direct confirmation of the O2+N2→NO+NO3− reaction has been done during high-pressure synthesis of CrO2 from
Cr and O2,where
addition of nitrogen into the Cr+O2 reactants environment resulted
in the formation of NO+NO3− compound  [22]. Recent studies confirmed the transformation
of a compressed mixture of O2 and N2 to NO+NO3− induced either by laser light [23]
or by X-ray radiation  [24].
The latter study also showed that at low pressures, the ionic form of
dinitrogen pentoxide NO2+NO3− is
rather stable than nitrosonium nitrate.A considerable volume of experimental studies has been carried out in
order to ascertain the high-pressure structure ofNO+NO3− ionic crystal. Based on the energy-dispersive X-ray diffraction studies,
aragonite-type structure was proposed for the crystal polymorph of NO+NO3− at pressures above 5 GPa and two space groups, P21cn and Pmcn,
equally well accounted for the observed Bragg reflections [17, 19].
Angle-dispersive X-ray diffraction study revealed the monoclinic P21/m structure of NO+NO3− at low pressures [24]. Several other space groups were also
proposed for the low-pressure phase of NO+NO3− [23],
however with the limited X-ray diffraction data quality and a nondecisive
conclusion about the space group. At the same time, no experimental information
is available on crystal structure of NO2+NO3− at high pressures.In this work, we have explored the question of stability of
high-pressure/high-temperature forms of nitrogen oxides obtained by laser
heating of oxygen richO2-N2 mixture. In addition to the
expected NO+NO3− ionic crystal, dinitrogen
pentoxide NO2+NO3− ionic solid is
formed at pressures below 30 GPa. We report the results of high-pressure/high-temperature
Raman and X-ray diffraction studies on the synthesized compounds. The
conversion of NO+NO3− into NO2+NO3− at high temperatures and pressure below 9 GPa is documented. Structural
properties of NO+NO3− and NO2+NO3− ionic compounds as a function of pressure and temperature are discussed


## 2. Experimental Methods

Investigated samples consisted of a mixture of the high-purity (99.999%)
oxygen and nitrogen (99.99%) gases condensed at ambient pressure and liquid
nitrogen temperature. Diamond anvil cells (DACs) were merged and closed in the
liquid phase of studiedO2-N2 assemblages. The small
crystals of ruby in the gasket hole provided a pressure calibration by standard
fluorescence technique. A predominant content of oxygen in condensed O2-N2 mixtures was ensured by adjusting the partial pressures of oxygen and nitrogen
gases during the condensation process. After a compression to a desired
pressure, an O2-N2 mixture was heated by Nd:YLF (1067 nm) laser. Temperature
estimations of the heated area during laser heating relied on observed hot spot
intensities  [25].
Several experiments with O2-N2 mixture covered the range
of pressure before laser heating from 10 GPa to 40 GPa, and the estimated
temperature varied in the range from 1300 to 2000 K. Raman spectra from samples
before and after laser heating were collected at Bayreuth Geo-Institute using
Dilor XY systems for the Raman measurements with the 5145 Å Ar-ion laser
excitation line and the incident laser power in the range 200–250 mW. The Dilor
XY Raman spectrometer was calibrated using the Γ25 phonon of
diamond-structured Si (Fd-3m) and
provided the data collection in the 50–3000 cm−1 range. The peaks were analyzed using the TOPAS-Academic software  [26]
with pseudo-Voight function describing a peak profile. We estimate a resolution
of 1 cm−1 for Raman peak position.In situ high-pressure X-ray diffraction measurements ofO2-N2 mixture were done at ID30 beam line of European Synchrotron Radiation Facility
(ESRF, Grenoble, France) before and after heating by Nd:YAG laser. A procedure
of fitting of outgoing thermal radiation (the energy distribution of its
emitted light) to a Planck
radiation formula yielded the temperature of the heated area of approximately
2000 K. Diffraction patterns were collected using focused monochromatic (λ=0.3738 Å) X-ray radiation with an image
plate detector (MAR345). One-dimensional 2θ dependences of the X-ray diffracted
intensities were obtained by integration of two-dimensional diffraction images
using the ESRF Fit2D software  [27].
Small crystals of ruby provided a pressure calibration of the sample by
fluorescence technique.


## 3. Results and Discussion

## 3.1. High-Pressure Synthesis and Phase Relations ofNO+NO3−

The Raman spectra of the samples at high pressures and room temperature
before laser heating contained the well-known features of theO2-N2 solid mixture. The intense oxygen vibron at about 1575 cm−1, the
nitrogen stretching mode at about 2365 cm−1, and the
lattice modes of O2-N2 mixture in a low-frequency part of
spectra characterized the Raman spectra collected at high pressure before laser
heating (see the selected examples in Figures 1(a) and 1(b)). The pressure
evolution of a characteristic splitting of O2 and N2 stretching modes as well as the pressure dependence of the relative intensities
of the O2 and N2 vibrons were compared with the available
studies of O2-N2 mixture
[28, 29].
We estimate the oxygen concentration in all studied O2-N2 samples to be within the range from 70% mol up to almost 100%. We did not
control precisely the oxygen content in the O2-N2 mixture
since the results of the laser heating experiments were independent on the
variation of oxygen concentration. The compression to pressures above 30 GPa
and subsequent laser heating in the range of temperatures from 1300 K to 2000 K
of the O2-N2
mixture resulted in the formation of nitrosonium-nitrate, NO+NO3−,
as compared to similar spectra of the earlier studies on NO+NO3− [15–18]. The Raman peaks of this ionic compound can be easily
separated from the peaks of nontransformed O-N mixture by comparing the Raman
spectra collected from the nonheated (Figure 1(c)) and heated regions of the
sample (Figures 1(d) and 1(e)). In the high-frequency range of the Raman
spectrum, NO+NO3− ionic crystal exhibits the
peaks associated with the intramolecular vibrations of the NO3− and NO+ groups. The intense ν(NO+) stretching mode of NO+ ion at 2268 cm−1 and the Raman bands of NO3− molecular complex ν1 (1112cm−1), ν2 (825cm−1), ν3 (1470cm−1), ν4 (750cm−1), and
overtone of ν2  (1650cm−1) are
identified in Figure 1(d). The lower frequency range of the Raman spectra
exhibits the peaks that can be related to the lattice vibrations of the NO3− and  NO+ ions and to libration, that is, rotation, of these ionic
groups. It is interesting to note almost identical resemblance of the lower
part of the Raman spectrum in Figure 1(d)
with the respective part of the Raman spectra reported by Yoo et al.
[18],
Song et al. [20], and Somayazulu
et al. [17].  This similarity indicates an invariance of a crystal
structure of NO+NO3− produced from different
starting forms of N-O system.Figure 1
Vibrational modes assignment forO2-N2 mixture before laser heating at (a) 15 GPa and (b) 39 GPa; (d) NO+NO3− ionic crystal at 32 GPa and (e) its
lattice modes at 30 GPa after laser heating. Raman spectrum of untransformed
O2-N2 mixture after laser heating at 32 GPa (c) is shown for comparison.High-pressure
polymorphism ofNO+NO3− can be inferred from X-ray diffraction studies [17, 19, 24]. This
conclusion corroborates with the Raman spectroscopy data of this work and
previous studies [18, 21], where the pressure dependence of mode
frequencies of NO+NO3− exhibits the change of
a slope at ~5 GPa (Figure 2). This singularity delimits fairly well the pressure
ranges of observation of monoclinic [24] and orthorhombic [17, 19] phases of NO+NO3−. One more change of a slope
can be identified in Figure 2 at ~22 GPa indicating an eventual structural
alteration in NO+NO3− system at higher
pressures.Pressure variation of the vibrational
frequencies obtained on decompression ofNO+NO3− (solid squares) and N2O4 (opened squares) molecular
crystals.
(a)(b)A symmetrization of high-pressureNO+NO3− crystal structure with decreasing pressure has been pointed out as a possible
general phase transformation path [18]. The asymmetry of stretching
mode of NO+ ion (~2270 cm−1) that visibly vanishes on
decompression at pressures about 5 GPa (compare respective insets in Figures 1(d)
and 3(a)) can be a consequence of such a symmetrization. In addition to the
asymmetry of the ν(NO+) stretching mode, we
could clearly detect two weak peaks in the vicinity that were not reported in
previous studies. These two peaks are identified in the insets of Figures 1(d)
and 3(a) at ~2200 cm−1 as νNO+′ and νNO+′′.
The obtained frequencies for the latter peaks and their pressure behavior
suggest that νNO+′ and νNO+′′ modes are not the overtones or combination
bands. The intensities of these peaks are weakly affected by the pressure
variation; as a result, no correlation between pressure behavior of intensities
of these peaks and symmetrization of stretching mode of NO+ ion
could be seen. A plausible assumption about the origin of these peaks could be
an orientational (dynamic or static) disorder of NO+ ionic groups.
The orientational disorder may result in metastable orientations of NO+ ions in the structure of NO+NO3− with slightly
different stretching frequencies of NO+ ions.Figure 3
Raman spectra of (a)NO+NO3− ionic crystal at 2.8 GPa, (b) a mixture of NO+NO3− ionic and
N2O4 molecular crystals at 2 GPa, and (c)
N2O4 molecular crystal at 1.7 GPa, showing the phase transition of
N2O4 molecular compound from ionic to neutral form. The assignment of the internal
vibrational modes in (c) corresponds to symmetrical N2O4 dimer [14].In order to obtain an additional insight into structural properties ofNO+NO3− at high pressures, one can confront the structural information obtained from
X-ray diffraction studies with Raman spectroscopy data. Specifically, in case
of a known or assumed crystal structure, the correlation analysis  [30, 31] allows establishing the number, symmetry, and spectral activity of the
external and internal optics modes. Since no such study has been done so far
for NO+NO3−, we present briefly the main
results of correlation analysis for low-pressure monoclinic phase (P21/m) whose structure was
established from X-ray diffraction measurements [24]. We also discuss
the implications of correlation analysis under assumption of orthorhombic
phases P21cn and Pmcn with aragonite-type structure as a
most plausible structural models of high-pressure phase of NO+NO3− inferred from earlier X-ray diffraction studies [17, 19].Table1 summarizes the result of the correlation analysis for the
monoclinic structure of NO+NO3−. The
correlations between molecular and site group species of NO3− ions result in splitting of ν3 and ν4 modes, all of them being
infrared and Raman active. Indeed, our Raman spectra (Figure 3(a)) as well as
Raman spectra of previous studies [17, 20, 23] clearly show a splitting
of ν4 fundamental. Besides,
somewhat broad and asymmetric ν3 band detected in our study
suggests a poorly resolved doublet. This doublet was clearly resolved in the
study by Sihachakr and Loubeyre  [23]. One can observe, consequently, six
internal optic modes originated by NO3− ions and one
NO+ stretching mode in both Raman and IR spectra of monoclinic NO+NO3− crystal. This prediction is in excellent agreement with our Raman and previous
Raman and IR studies.Table 1
Correlation between point group, site group,
and factor group symmetry species and their Raman and IR activities for theNO+ and NO3− molecular ions in the monoclinic, C2h2 (P21/m), NO+NO3− crystal.Superposition of translational and rotational motions ofNO3− and
NO+ ions originates the external optic modes of NO+NO3−.
In case of P21/m space
group, one can find the following irreducible representations of the lattice
vibrations and libration modes [30, 31]:(1)ΓNO+NO3−Lattice=4Ag(R)+2Au(IR)+2Bg(R)+4Bu(IR),ΓNO+NO3−Libration=2Ag(R)+3Au(IR)+3Bg(R)+2Bu(IR),The spectral activity of each symmetry species is indicated by a
superscript (R) for Raman and (IR) for infrared active modes. Table2(a)
summarizes the total irreducible representation of NO+NO3− crystal under assumption of P21/m space group. Full factor group for external vibrations of monoclinic NO+NO3− contains 11 Raman active and 8 IR active translational and libration
modes. Raman spectra obtained in our and in the previous studies [17, 18, 20] exhibit only 8-resolved peaks associated with the external modes at high
pressures and room temperatures; this number reduces to 6 peaks at pressures
below 5 GPa (see Figures 1(d), 1(e), and 3(a)). The discrepancy between the
number of observed and predicted external Raman modes can be attributed to a
thermal overlapping of the peaks. Indeed, all 11 peaks are resolved in the low-frequency
range of low-temperature Raman spectra of NO+NO3− [21].Table 2
Total irreducible representation and
spectral activity of the monoclinic and aragonite-typeNO+NO3− crystal for (a) C2h2 (P21/m), (b) C2ν9 (Pna21), and (c) D2h16  (Pnma) space group.
(a)Crystal   symmetryC2hTranslationsAcousticalRotationIntramolecularSpectral(lattice vibrations)vibrations(libration)vibrationsactivityC2hAg425RAu1132IRBg232RBu2225IRTotal vibrations931014(b)Crystal   symmetryC2νTranslationsAcousticalRotationIntramolecularSpectral(lattice  vibrations)vibrations(libration)vibrationsactivityC2νA15157R, IRA2657RB15157R, IRB25157R, IRTotal vibrations2132028(c)Crystal   symmetryD2hTranslationsAcousticalRotationIntramolecularSpectral(lattice  vibrations)vibrations(libration)vibrationsactivityD2hAg415RB1u3115IRB2u3115IRB3g415RAu242B1g242RB2g242RB3u1142IRTotal vibrations2132028Analogously, a site group correlation analysis under assumption of
orthorhombic structure ofNO+NO3− with P21cn and Pmcn space groups leads to six internal
optic modes originated by NO3− ions and one
NO+ stretching mode, all being active in both Raman and IR spectra. The respective
total irreducible representations are summarized in Tables 2(b) and 2(c). The
factor group symmetry species are labeled using standard axial settings (space
groups Pna21 and Pnma, resp.). The correspondence  between
the crystallographic axis and irreducible representations of factor groups of C2ν9 and D2h16 in different
settings can be found elsewhere  [32].
As the result of the C1 site symmetry of NO3− and
NO+ ions, 41 Raman
active and 30 IR active external vibrations are expected in case of P21cn space group. In case of Pmcn space group of NO+NO3− crystal, either C1 or Cs site symmetry can
accommodate four NO3− and four
NO+ ions. Table 2(c) shows the results of correlation analysis assuming Cs site symmetry for both ions. The choice of
this site symmetry is due to the fact that crystallographic positions occupied
by ions in aragonite-type crystal have the same Cs symmetry  [33].
External vibrations in this case consist of 12 Raman and 7 IR active
translational modes, and of 10 Raman and 10 IR active libration modes.A considerable discrepancy between the number of experimentally observed
and predicted external Raman active vibrations in case of orthorhombic phase ofNO+NO3− can reflect a structural peculiarities
of high-pressure polymorph of NO+NO3−. As it
was shown for aragonite, CaCO3, and isomorphic KNO3  [34],
one can expect a considerable reduction in a number of external modes if a
structural model for a compound is a slightly distorted variant of a structure
with higher symmetry. In such a distorted structure, certain vibrational modes
can be associated with a motion of ions in planes that have a symmetry closely
coinciding with a higher base symmetry. Consequently, these modes should be
determined using the high-symmetry model. In particular, if a higher-symmetry
base structure has a reduced translational symmetry in such a plane, one can
expect to observe only lattice and libration modes originated by ions motion
which is translational invariant with respect to a reduced unit cell [34].  It is worth noting, in this regard, that the established structure of the low-pressure
phase of NO+NO3− has the monoclinic unit cell [24] with two molecular units per unit cell in contrast to four molecular units of
the proposed aragonite-type structure [17]. It might be possible that
this monoclinic structure is a descendent of the higher-symmetry base structure
of the high-pressure phase of NO+NO3− with
reduced unit cell.It is worthwhile to mention, in conclusion of this section, the
pressure-induced transformation of ionicNO+NO3− to neutral N2O4 molecular crystal that occurs at
pressures below 2 GPa (Figures 3(b) and 3(c)). We found that the pressure of
this transformation is highly dependent on a specific pressure-time path
followed by the sample. In some experiments on decompression of NO+NO3− at room temperatures, the ionic form could be retained up to a liquid phase.


## 3.2. High-Pressure Synthesis and Structural Properties ofNO2+NO3−

Laser heating of oxygen richO2-N2 mixture at
pressures below 30 GPa produced a mixture of two ionic crystals: NO+NO3− and additional phase identified as ionic form of dinitrogen pentoxide, NO2+NO3− (see selected examples in Figures 4(a) and 4(b)). In order to establish
precisely the nature of the additional phase, we carried out a series of Raman
and diffraction experiments at high pressures and temperatures. The
identification of NO2+NO3− was
facilitated by the fact of a complete conversion of NO+NO3− ionic crystal to NO2+NO3− at
pressures below 9 GPa and temperatures above ambient temperature. This
transformation was confirmed by both Raman and X-ray diffraction measurements
(Figures 4(c) and 5(a)). The determined earlier hexagonal (P63/mmc, two formula units per unit cell) crystal structure
of NO2+NO3−  [35]
perfectly accounted for diffraction patterns obtained at pressures below 9 GPa
(Figure 5(b)). The obtained Raman spectra of NO2+NO3− also showed good agreement with the previous studies  [36, 37].
The Raman bands of NO3− and
NO2+ molecular complexes are indicated in Figure 4(c). Under assumption of hexagonal
crystal structure, the correlation analysis accounts for the presence in the
Raman spectrum of ν1 (1070 cm−1) and ν4 (736 cm−1) bands
of NO3− molecular complex, and ν1 (1403 cm−1) and
ν2 (505 cm−1) bands
of NO2+ ion (see Table 3(a)). A very weak and broad band
around 1370 cm−1 (not shown in Figure 4(c)) was detected in most of collected
Raman spectra of NO2+NO3−. We
assigned this feature to the antisymmetric stretching mode ν3 of NO3−.
Extrapolation to zero pressure of the ν3 mode frequency (Figure 6)
gives a similar value (~1350 cm−1) as the respective band frequency
observed at ambient pressure in the previous studies [36, 37]. The
multiple bands indicated in Figure 4(c) as 2ν2 have been explained by
Wilson and Christe [36] as a split overtone of the
NO2+ deformation mode. The pressure behavior of these bands is a conclusive evidence
of Wilson’s assignment (see Figure 6).Table 3
Correlation between point group, site group,
and factor group symmetry species and their Raman and IR activities for theNO2+ and NO3− molecular ions in the NO2+NO3− crystal assuming (a) D6h4 (P63/mmc) and (b) D2h17 (Cmcm) space group.Figure 4
Raman spectra and vibrational modes
assignment for (a)O2-N2 mixture before laser heating, (b) mixture of NO+NO3− and NO2+NO3− ionic solids after
laser heating (vibrational modes of NO+NO3− are identified), (c) NO2+NO3− at
3.7 GPa and 500 K, (d) NO2+NO3− at
25 GPa and room temperature.(a) Evolution of the diffraction patterns
ofNO+NO3− and NO2+NO3− ionic solids on decompression at 500 K. The transformation of the phase mixture
to pure NO2+NO3− is confirmed by
the Rietveld refinement (b) of the hexagonal structural model (P63/mmc) of NO2+NO3− at 8.2 GPa and 500 K. The refined lattice parameters
are a=5.1377(1); c=5.8930(1). The x, y, and z fractional atomic coordinates for two nitrogen and
two oxygen atoms in asymmetric unit are N(1)=0,0,1/4; N(2)=1/3,2/3,3/4; O(1)=0.1792,0.3583,1/4; O(2)=1/3,2/3,0.5631.
(a)(b)Raman shifts as a function of pressure forNO2+NO3− ionic crystal (a). Opened
squares refer to ambient temperature data obtained on compression and solid
squares represent the data obtained at 600 K on decompression. Green and red
lines correspond to ν3 mode pressure evolution at room temperature and at
600 K, respectively. The influence of temperature on the vibrational
frequencies can be seen at a zoomed scale (b).
(a)(b)The important different feature inNO2+NO3− Raman spectra of this study, however, was the presence of antisymmetric
stretching mode ν3 (2260 cm−1) of
NO2+ complex. Correlation analysis cannot explain this
Raman band, which is silent under assumption of hexagonal structure of NO2+NO3− (see Table 3(a)). A plausible explanation of the Raman activity of the ν3 band can be based on a bent
structure of rotating NO2+ ion. A reduction of
NO2+ ion’s symmetry from a linear to a bent one implies a reduction of symmetry of
NO2+NO3− crystal structure. As it
was pointed out by Simon et al.  [38],
a very small deviation from hexagonal unit cell toward an orthorhombic one is
needed in order to explain the low-temperature single-crystal diffraction data.
Assuming the orthorhombic (Cmcm space
group, four formula units per unit cell) for NO2+NO3− crystal and a bent configuration
of NO2+ ion, one
can deduce the Raman activity of all three normal vibrational modes of the
NO2+ cation observed in our Raman spectra (see Table 3(b)). As for NO3− ions vibrational modes, the symmetry reduction of the unit cell has to produce
a splitting of the ν3 antisymmetric stretching
vibration and ν4 in plane deformation mode,
analogously to the case of nitrosonium-nitrate NO+NO3−.
Indeed, at pressures above ~20 GPa, we observed a splitting of ν4 mode (compare the
respective insets of Figures 4(c) and 4(d)). In addition, the low frequency
part of the collected Raman spectra suffered strong alterations at high
pressures, indicating a considerable structural changes induced by pressure.
These results support the suggestion of Simon et al. [38]
about
orthorhombic nearly hexagonal structure
of NO2+NO3− and evidence an
increase of an orthorhombic distortion with increasing pressure.Another strong evidence of a bent structure ofNO2+ ion is a soft behavior of deformation mode ν2 as a function of pressure
(Figures 6 and 7). Such behavior would reflect the reduction of the bending
force constant as the bent O–N–O structure deforms toward a linear configuration
with increasing pressure.Pressure evolution of the Raman spectra ofNO2+NO3− at 600 K (a) and Raman
spectra of ν1(NO3−) region at 11.3 GPa, 8.4
and 5.6 GPa at ambient temperature (b). A shoulder in the vicinity of ν1(NO3−) vibration mode
indicates an orientational and/or site mobility of NO3− ionic complexes in NO2+NO3− ionic
crystal.
(a)(b)The temperature effect on the internal vibrational modes ofNO2+NO3− is detailed for selected modes in Figure 6(b). A shift of
ν2(NO2+)
Raman peak toward a lower frequency with increasing temperature was observed in
high-pressure/high-temperature Raman experiments. This trend can be seen more
pronouncedly for overtone bands of ν2 mode. Such a behavior may
indicate a less bent geometry of
NO2+ ion in the expanded
lattice of NO2+NO3−. It should be
pointed out that the temperature increase results in downshifted Raman peak
associated with antisymmetric stretching mode ν3 of
NO2+ ion as well. At the same time, all other vibrational modes of NO2+NO3− are shifted toward a higher frequency with increasing temperature.It is worth noting that a transformation to a bent configuration ofNO2+ ion can be viewed as a removal of degeneration in doubly degenerated Πu bending mode of a linear
ion. One of the components of the Πu mode  reduces D∞h symmetry to C2ν, and other component
becomes the rotation of the ion. Consequently, a rotation of
NO2+ ion in NO2+NO3− crystal can  be
expected just from symmetry considerations. As was already discussed by Wilson
and Christe [36], namely, this rotation may result in an average linear
structure of NO2+ ion detected by X-ray diffraction
technique.An orientational disorder ofNO3− ionic groups in NO2+NO3− crystal can be inferred from our Raman data as well. Numerous studies on
nitrate salts showed that the totally symmetric vibration ν1 of nitrate ions may have a
complex structure; in particular, an anomalous second component is present near
the main Raman peak at slightly lower frequencies  [39–41].
The appearance of the second component is generally explained by a disorder in
nitrate ions position or orientations in the crystal lattice of nitrates [39].  The obtained Raman spectra of NO2+NO3− exhibit a shoulder next to the symmetric vibration ν1 (see Figure 7), and
analogously to nitrate salts this shoulder can be attributed to a rotational or
site disorder of NO3− ions. It is interesting to note
that Wilson and Christe [36] also observed an unexplained peak near the ν1 stretching mode of NO3− groups in Raman spectra of NO2+NO3− collected at low temperatures and ambient pressures. It is possible that the
origin of this peak is connected to a disorder of the nitrate ions.It is worthwhile, in conclusion, to discuss briefly the question of
relative stability of two ionic phases of nitrogen oxides,NO+NO3− and NO2+NO3−. Raman and X-ray
diffraction data show unambiguously the instability of NO+NO3− ionic solid at pressures below 9 GPa and temperatures above 450 K
(perhaps, even at lower temperatures). Figure 8 stresses this point showing the
relations between ionic forms of nitrogen oxides and includes the melting line
of NO2+NO3− ionic crystal mapped
from our Raman and X-ray diffraction experiments.Figure 8
Phase diagram showing the domain of
stability of crystallineNO2+NO3− on decompression of a mixture of NO2+NO3− and NO+NO3−. A textured area indicates the
eventual extension of the region of (meta)stability of NO+NO3− solid to low pressures.On the other hand, a stability ofNO+NO3− ionic crystal at pressures above 30 GPa and high temperatures was confirmed by
numerous laser-heating experiments of our and previous studies. One can expect,
consequently, a conversion of NO2+NO3− to NO+NO3− at sufficiently high pressures. A
big pressure region of coexistence of NO+NO3− and NO2+NO3− phases observed in our
study can be attributed to kinetics of the NO+NO3−
to
NO2+NO3− transformation. In
particular, high-kinetic barrier of NO+NO3− to
NO2+NO3− conversion can explain a
stability of NO+NO3− at pressures below 9 GPa
and ambient temperature. The high-pressure IR spectra of NO+NO3−
[21] showed that a degree of ionicity of NO+NO3− decreases with decreasing pressures. It follows from our results that a
reduction of ionicity of NO+NO3− toward a
neutral form of nitrogen dioxide is intermediated by another stable ionic form
of nitrogen oxide, NO2+NO3−. Most
likely, interplay between electrostatic and intraionic covalent bonding energy
contributions to a total energy defines a stable ionic form of nitrogen oxide
at high pressures.


## 4. Conclusions

In summary, we have synthesized the ionic forms of nitrogen oxides,NO+NO3− and NO2+NO3−, directly from oxygen-rich
O2-N2 mixture. A direct high-pressure synthesis of ionic
forms of nitrogen oxide provides a convenient way of its study at high
pressures and high temperatures. The Raman and X-ray diffraction studies showed
that a sequence NO+NO3−-NO2+NO3−-N2O4 of transformations
can be realized with pressure-temperature variation underlying the transition
from highly ionic to neutral form of nitrogen oxides.  Our results evidence that ionic NO2+NO3− phase is characterized by increased orientational and/or site disorder of ionic
groups.



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*Source: 180784-2009-01-04.xml*