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Hindawi Publishing CorporationAdvances in Physical ChemistryVolume 2009, Article ID 180784, 11 pagesdoi:10.1155/2009/180784

Research Article

High-Pressure Synthesis and Study of NO+NO3− and

NO2+NO3

− Ionic Solids

A. Yu. Kuznetsov,1 L. Dubrovinsky,2 A. Kurnosov,2, 3 M. M. Lucchese,1

W. Crichton,4 and C. A. Achete1

1 Divisao de Metrologia de Materiais (DIMAT), Instituto Nacional de Metrologia, Normalizacao e Qualidade Industrial,Avenida Nossa Senhora das Gracas 50, Xerem, Duque de Caxias, RJ, CEP 25250-020, Brazil

2 Bayerisches Geoinstitut, Universitat Bayreuth, 95440 Bayreuth, Germany3 Nikolaev Institute of Inorganic Chemistry SB RAS, Pr. Ac. Lavrentieva 3, 630090 Novosibirsk, Russia4 European Synchrotron Radiation Facility, BP 220, 38043 Grenoble Cedex 9, France

Correspondence should be addressed to A. Yu. Kuznetsov, [email protected]

Received 2 April 2008; Revised 19 August 2008; Accepted 16 October 2008

Recommended by Lowell D. Kispert

Nitrosonium-nitrate NO+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 revealedpreviously unreported bands in Raman spectra of NO+NO3

− and NO2+NO3

− ionic crystals. Structural properties of both ioniccompounds are analyzed. Obtained experimental results support a hypothesis of a rotational disorder of NO+ complexes inNO+NO3

− and indicate a rotational disorder of ionic complexes in NO2+NO3

− solid.

Copyright © 2009 A. Yu. Kuznetsov et al. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

1. Introduction

Application of high pressure can convert simple molecularstructures of elemental and heteronuclear systems intononmolecular solids with extended or “infinite” atomiclattices [1]. Examples of such extended solids are non-molecular nitrogen [2–4], metalic oxygen [5], polymericform of carbon dioxide [6], and carbon monoxide [7].Such pressure-induced molecular-to-nonmolecular phasetransitions are one of the ways to reduce a total energy ofthe molecular crystal via local (polymerization) or complete(metallization) delocalization of intramolecular electronsbetween adjacent molecules. Another alternative of a morestable form of chemical bonds perturbed by pressure andtemperature variation is ionization with the onset of charge-transfer interactions. The first reported example of theneutral-to-ionic solid transformation concerns the organiccharge-transfer compound tetrathiafulvalene-chloranil [8,9]. In this material, the pressure or temperature variation

promotes either neutral or ionic form, which is determinedby the electrostatic energy contribution to the total energy ofthe donor-acceptor molecules. The similar neutral-to-ionictransformation has been reported in several simple molec-ular systems as well. The formation of H2

+H2− dimmer at

150 GPa in phase III of solid hydrogen [10] was proposedto explain the discontinuous changes of the Raman andinfrared vibron frequencies [11, 12]. The nitrogen dioxidedimmer, N2O4, transforms to the ionic NO+NO3

− complexby temperature variation at ambient pressure [13, 14] andby 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 ofa direct synthesis of NO+NO3

− crystal from oxygen andnitrogen reactants. In particular, the primary dissociationof N2O on N2 and O2 at high temperatures and pressuresbelow 30 GPa was evident from the Raman spectra [17, 18].On the other hand, X-ray diffraction studies showed that

2 Advances in Physical Chemistry

nitrosonium nitrate is denser than other nitrogen-oxygenassemblages [19]. Direct confirmation of the O2 + N2 →NO+NO3

− reaction has been done during high-pressuresynthesis of CrO2 from Cr and O2,where addition of nitrogeninto the Cr + O2 reactants environment resulted in theformation of NO+NO3

− compound [22]. Recent studiesconfirmed the transformation of a compressed mixture ofO2 and N2 to NO+NO3

− induced either by laser light [23]or by X-ray radiation [24]. The latter study also showedthat at low pressures, the ionic form of dinitrogen pentoxideNO2

+NO3− is rather stable than nitrosonium nitrate.

A considerable volume of experimental studies hasbeen carried out in order to ascertain the high-pressurestructure of NO+NO3

− ionic crystal. Based on the energy-dispersive X-ray diffraction studies, aragonite-type structurewas proposed for the crystal polymorph of NO+NO3

−

at pressures above 5 GPa and two space groups, P21cnand Pmcn, equally well accounted for the observed Braggreflections [17, 19]. Angle-dispersive X-ray diffraction studyrevealed the monoclinic P21/m structure of NO+NO3

− atlow pressures [24]. Several other space groups were alsoproposed for the low-pressure phase of NO+NO3

− [23],however with the limited X-ray diffraction data quality and anondecisive conclusion about the space group. At the sametime, no experimental information is available on crystalstructure of NO2

+NO3− at high pressures.

In this work, we have explored the question of stabilityof high-pressure/high-temperature forms of nitrogen oxidesobtained by laser heating of oxygen rich O2-N2 mixture. Inaddition to the expected NO+NO3

− ionic crystal, dinitrogenpentoxide 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 thesynthesized compounds. The conversion of NO+NO3

− intoNO2

+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 andtemperature are discussed

2. Experimental Methods

Investigated samples consisted of a mixture of the high-purity (99.999%) oxygen and nitrogen (99.99%) gasescondensed at ambient pressure and liquid nitrogen tem-perature. Diamond anvil cells (DACs) were merged andclosed in the liquid phase of studied O2-N2 assemblages.The small crystals of ruby in the gasket hole provideda pressure calibration by standard fluorescence technique.A predominant content of oxygen in condensed O2-N2

mixtures was ensured by adjusting the partial pressuresof oxygen and nitrogen gases during the condensationprocess. After a compression to a desired pressure, an O2-N2

mixture was heated by Nd:YLF (1067 nm) laser. Temperatureestimations of the heated area during laser heating reliedon observed hot spot intensities [25]. Several experimentswith O2-N2 mixture covered the range of pressure beforelaser heating from 10 GPa to 40 GPa, and the estimatedtemperature varied in the range from 1300 to 2000 K. Ramanspectra from samples before and after laser heating were

collected at Bayreuth Geo-Institute using Dilor XY systemsfor the Raman measurements with the 5145 A Ar-ion laserexcitation line and the incident laser power in the range 200–250 mW. The Dilor XY Raman spectrometer was calibratedusing the Γ25 phonon of diamond-structured Si (Fd-3m) andprovided the data collection in the 50–3000 cm−1 range. Thepeaks 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 EuropeanSynchrotron Radiation Facility (ESRF, Grenoble, France)before and after heating by Nd:YAG laser. A procedure offitting of outgoing thermal radiation (the energy distributionof its emitted light) to a Planck radiation formula yieldedthe temperature of the heated area of approximately 2000 K.Diffraction patterns were collected using focused monochro-matic (λ = 0.3738 A) X-ray radiation with an image platedetector (MAR345). One-dimensional 2θ dependences of theX-ray diffracted intensities were obtained by integration oftwo-dimensional diffraction images using the ESRF Fit2Dsoftware [27]. Small crystals of ruby provided a pressurecalibration of the sample by fluorescence technique.

3. Results and Discussion

3.1. High-Pressure Synthesis and Phase Relations of NO+NO3−.

The Raman spectra of the samples at high pressures androom temperature before laser heating contained the well-known features of the O2-N2 solid mixture. The intenseoxygen vibron at about 1575 cm−1, the nitrogen stretchingmode at about 2365 cm−1, and the lattice modes of O2-N2

mixture in a low-frequency part of spectra characterized theRaman spectra collected at high pressure before laser heating(see the selected examples in Figures 1(a) and 1(b)). Thepressure evolution of a characteristic splitting of O2 and N2

stretching modes as well as the pressure dependence of therelative intensities of the O2 and N2 vibrons were comparedwith the available studies of O2-N2 mixture [28, 29]. Weestimate the oxygen concentration in all studied O2-N2

samples to be within the range from 70% mol up to almost100%. We did not control precisely the oxygen content inthe O2-N2 mixture since the results of the laser heatingexperiments were independent on the variation of oxygenconcentration. The compression to pressures above 30 GPaand subsequent laser heating in the range of temperaturesfrom 1300 K to 2000 K of the O2-N2mixture resulted in theformation of nitrosonium-nitrate, NO+NO3

−, as comparedto similar spectra of the earlier studies on NO+NO3

−

[15–18]. The Raman peaks of this ionic compound can beeasily separated from the peaks of nontransformed O-Nmixture by comparing the Raman spectra collected from thenonheated (Figure 1(c)) and heated regions of the sample(Figures 1(d) and 1(e)). In the high-frequency range ofthe Raman spectrum, NO+NO3

− ionic crystal exhibits thepeaks associated with the intramolecular vibrations of theNO3

− and NO+ groups. The intense ν(NO+) stretchingmode of NO+ ion at 2268 cm−1 and the Raman bands ofNO3

− molecular complex ν1 (1112 cm−1), ν2 (825 cm−1),

Advances in Physical Chemistry 3

Inte

nsi

ty

0 500 1000 1500 2000 2500 3000

Raman shift (cm−1)

ν4

ν2

ν1

ν32ν 2

νO2

νO2

ν0O2

ν0N2

ν0N2

νN2

ν ′NO+

ν′′NO+

νNO+

′

′

Figure 1: Vibrational modes assignment for O2-N2 mixture beforelaser heating at (a) 15 GPa and (b) 39 GPa; (d) NO+NO3

− ioniccrystal at 32 GPa and (e) its lattice modes at 30 GPa after laserheating. Raman spectrum of untransformed O2-N2 mixture afterlaser heating at 32 GPa (c) is shown for comparison.

ν3 (1470 cm−1), ν4 (750 cm−1), and overtone of ν2

(1650 cm−1) are identified in Figure 1(d). The lowerfrequency range of the Raman spectra exhibits the peaksthat can be related to the lattice vibrations of the NO3

− andNO+ ions and to libration, that is, rotation, of these ionicgroups. It is interesting to note almost identical resemblanceof the lower part of the Raman spectrum in Figure 1(d) withthe respective part of the Raman spectra reported by Yoo etal. [18], Song et al. [20], and Somayazulu et al. [17]. Thissimilarity indicates an invariance of a crystal structure ofNO+NO3

− produced from different starting forms of N-Osystem.

High-pressure polymorphism of NO+NO3− can be

inferred from X-ray diffraction studies [17, 19, 24]. Thisconclusion corroborates with the Raman spectroscopy dataof this work and previous studies [18, 21], where the pressuredependence of mode frequencies of NO+NO3

− exhibits thechange of a slope at ∼5 GPa (Figure 2). This singularitydelimits fairly well the pressure ranges of observation ofmonoclinic [24] and orthorhombic [17, 19] phases ofNO+NO3

−. One more change of a slope can be identifiedin Figure 2 at ∼22 GPa indicating an eventual structuralalteration in NO+NO3

− system at higher pressures.A symmetrization of high-pressure NO+NO3

− crystalstructure with decreasing pressure has been pointed outas a possible general phase transformation path [18]. Theasymmetry of stretching mode of NO+ ion (∼2270 cm−1)that visibly vanishes on decompression at pressures about5 GPa (compare respective insets in Figures 1(d) and 3(a))can be a consequence of such a symmetrization. In additionto the asymmetry of the ν(NO+) stretching mode, we couldclearly detect two weak peaks in the vicinity that were notreported in previous studies. These two peaks are identifiedin the insets of Figures 1(d) and 3(a) at ∼2200 cm−1 as ν′NO+

13501400145015001550160016501700

2180

2200

2220

2240

2260

2280

Ram

ansh

ift

(cm−1

)

0 5 10 15 20 25 30 35

Pressure (GPa)

νN2 O41

νN2 O45

νNO3−

3

2νNO3−

2

νNO+

ν′NO+

ν′′NO+

(a)

0

100

200

300

400

500

700

800

900

1000

1100

1200

Ram

ansh

ift

(cm−1

)

0 5 10 15 20 25 30 35

Pressure (GPa)

νN2 O43

νN2 O46

νN2 O42

νN2 O48

νNO3−

1

νNO3−

2

νNO3−

4

Lattice mode

1

22′33′45

6

(b)

Figure 2: Pressure variation of the vibrational frequencies obtainedon decompression of NO+NO3

− (solid squares) and N2O4 (openedsquares) molecular crystals.

and ν′′NO+ . The obtained frequencies for the latter peaks andtheir pressure behavior suggest that ν′NO+ and ν′′NO+ modesare not the overtones or combination bands. The intensitiesof these peaks are weakly affected by the pressure variation;as a result, no correlation between pressure behavior ofintensities of these peaks and symmetrization of stretchingmode of NO+ ion could be seen. A plausible assumptionabout the origin of these peaks could be an orientational(dynamic or static) disorder of NO+ ionic groups. Theorientational disorder may result in metastable orientationsof NO+ ions in the structure of NO+NO3

− with slightlydifferent stretching frequencies of NO+ ions.

In order to obtain an additional insight into structuralproperties of NO+NO3

− at high pressures, one can confront


Table 1: Correlation between point group, site group, and factor group symmetry species and their Raman and IR activities for the NO+

and NO3− molecular ions in the monoclinic, C2

2h (P21/m), NO+NO3− crystal.

Ion Mode description Point group Site group Factor group

Out-of-plane deformation

In-plane deformation

Stretch.

Antisymmetry stretch.

Symmetry stretch. (ν1),

(ν2)

(ν3)

(ν4)

(νs)

D3h (−6m2) Cs (m) C2h (2/m)

NO+

NO3−

C∞ν (∞mm) Cs (m) C2h (2/m)

Σ+ (R, IR)

A′1 (R)

A′′2 (IR)

E′ (R, IR)

E′ (R, IR)

A′ (R, IR)

A′ (R, IR)

A′ (R, IR)

A′′ (R, IR)

A′ (R, IR)

A′′ (R, IR)

A′ (R, IR)

Ag (R)

Bu (R)

Au (R)

Bg (R)

Ag (R)

Bu (R)

750 900

(b)

(a)

(c)

710 720 810 820 8301350 1500 1650 2225 2250

2100 2200 2300 2400700 800In

ten

sity

0 500 1000 1500 2000 2500 3000


ν4

ν2

ν1

ν3

2ν2

ν4

ν1

ν3ν8

ν6ν2

ν1ν5

ν0O2

ν0O2

ν0N2ν′NO+

ν′′NO+

νNO+

νNO+

N2O4 Raman active vibrations

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 phasetransition of N2O4 molecular compound from ionic to neutralform. The assignment of the internal vibrational modes in (c)corresponds to symmetrical N2O4 dimer [14].

the structural information obtained from X-ray diffractionstudies with Raman spectroscopy data. Specifically, in case ofa known or assumed crystal structure, the correlation anal-ysis [30, 31] allows establishing the number, symmetry, andspectral 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 analysisfor low-pressure monoclinic phase (P21/m) whose structure

was established from X-ray diffraction measurements [24].We also discuss the implications of correlation analysisunder assumption of orthorhombic phases P21cn and Pmcnwith aragonite-type structure as a most plausible structuralmodels of high-pressure phase of NO+NO3

− inferred fromearlier X-ray diffraction studies [17, 19].

Table 1 summarizes the result of the correlation analysisfor the monoclinic structure of NO+NO3

−. The correlationsbetween molecular and site group species of NO3

− ions resultin splitting of ν3 and ν4 modes, all of them being infrared andRaman active. Indeed, our Raman spectra (Figure 3(a)) aswell as Raman spectra of previous studies [17, 20, 23] clearlyshow a splitting of ν4 fundamental. Besides, somewhat broadand asymmetric ν3 band detected in our study suggests apoorly resolved doublet. This doublet was clearly resolved inthe 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 IRspectra of monoclinic NO+NO3

− crystal. This prediction isin excellent agreement with our Raman and previous Ramanand IR studies.

Superposition of translational and rotational motions ofNO3

− and NO+ ions originates the external optic modes ofNO+NO3

−. In case of P21/m space group, one can find thefollowing irreducible representations of the lattice vibrationsand libration modes [30, 31]:

ΓLatticeNO+NO3

− = 4Ag(R) + 2Au

(IR) + 2Bg(R) + 4Bu

(IR),

ΓLibrationNO+NO3

− = 2Ag(R) + 3Au

(IR) + 3Bg(R) + 2Bu

(IR),(1)

The spectral activity of each symmetry species is indi-cated by a superscript (R) for Raman and (IR) for infraredactive modes. Table 2(a) summarizes the total irreduciblerepresentation of NO+NO3

− crystal under assumption of


Table 2: Total irreducible representation and spectral activity of the monoclinic and aragonite-type NO+NO3− crystal for (a) C2

2h (P21/m),(b) C9

2ν (Pna21), and (c) D162h (Pnma) space group.

(a)

Crystal symmetry C2hTranslations Acoustical Rotation Intramolecular Spectral

(lattice vibrations) vibrations (libration) vibrations activity C2h

Ag 4 2 5 R

Au 1 1 3 2 IR

Bg 2 3 2 R

Bu 2 2 2 5 IR

Total vibrations 9 3 10 14

(b)

Crystal symmetry C2νTranslations Acoustical Rotation Intramolecular Spectral

(lattice vibrations) vibrations (libration) vibrations activity C2ν

A1 5 1 5 7 R, IR

A2 6 5 7 R

B1 5 1 5 7 R, IR

B2 5 1 5 7 R, IR


(c)

Crystal symmetry D2hTranslations Acoustical Rotation Intramolecular Spectral

(lattice vibrations) vibrations (libration) vibrations activity D2h

Ag 4 1 5 R

B1u 3 1 1 5 IR

B2u 3 1 1 5 IR

B3g 4 1 5 R

Au 2 4 2

B1g 2 4 2 R

B2g 2 4 2 R

B3u 1 1 4 2 IR


P21/m space group. Full factor group for external vibrationsof monoclinic NO+NO3

− contains 11 Raman active and 8IR active translational and libration modes. Raman spectraobtained in our and in the previous studies [17, 18, 20]exhibit only 8-resolved peaks associated with the externalmodes at high pressures and room temperatures; thisnumber reduces to 6 peaks at pressures below 5 GPa (seeFigures 1(d), 1(e), and 3(a)). The discrepancy between thenumber of observed and predicted external Raman modescan be attributed to a thermal overlapping of the peaks.Indeed, all 11 peaks are resolved in the low-frequency rangeof low-temperature Raman spectra of NO+NO3

− [21].Analogously, a site group correlation analysis under

assumption of orthorhombic structure of NO+NO3− with

P21cn and Pmcn space groups leads to six internal opticmodes originated by NO3

− ions and one NO+ stretchingmode, all being active in both Raman and IR spectra. Therespective total irreducible representations are summarizedin Tables 2(b) and 2(c). The factor group symmetry speciesare labeled using standard axial settings (space groups Pna21

and Pnma, resp.). The correspondence between the crystallo-graphic axis and irreducible representations of factor groupsof C2ν9 and D16

2h 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 areexpected in case of P21cn space group. In case of Pmcn spacegroup of NO+NO3

− crystal, either C1 or Cs site symmetrycan accommodate four NO3

− and four NO+ ions. Table 2(c)shows the results of correlation analysis assuming Cs sitesymmetry for both ions. The choice of this site symmetry isdue to the fact that crystallographic positions occupied byions in aragonite-type crystal have the same Cs symmetry[33]. External vibrations in this case consist of 12 Raman and7 IR active translational modes, and of 10 Raman and 10 IRactive libration modes.

A considerable discrepancy between the number ofexperimentally observed and predicted external Ramanactive 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


Inte

nsi

ty

0 400 800 1200 1500 2000 2500


NO2+NO3

−

NO2+NO3

−

NO2+NO3

− + NO+NO3−

O2 + N2νN2νO2

νNO+

νNO2

+

2

νNO2

+

1 νNO2

+

3

νNO2

+

2

νNO2

+

1 νNO2

+

3

2vNO2

+

2

νNO3

−4

νNO3

−1

νNO3

−1

νNO3

−2ν

NO3−

4

Figure 4: Raman spectra and vibrational modes assignment for (a)O2-N2 mixture before laser heating, (b) mixture of NO+NO3

− andNO2

+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.

aragonite, CaCO3, and isomorphic KNO3 [34], one canexpect a considerable reduction in a number of externalmodes if a structural model for a compound is a slightlydistorted variant of a structure with higher symmetry. Insuch a distorted structure, certain vibrational modes canbe associated with a motion of ions in planes that have asymmetry closely coinciding with a higher base symmetry.Consequently, these modes should be determined using thehigh-symmetry model. In particular, if a higher-symmetrybase structure has a reduced translational symmetry in sucha plane, one can expect to observe only lattice and librationmodes originated by ions motion which is translationalinvariant with respect to a reduced unit cell [34]. It isworth noting, in this regard, that the established structureof the low-pressure phase of NO+NO3

− has the monoclinicunit cell [24] with two molecular units per unit cell incontrast to four molecular units of the proposed aragonite-type structure [17]. It might be possible that this monoclinicstructure is a descendent of the higher-symmetry basestructure of the high-pressure phase of NO+NO3

− withreduced unit cell.

It is worthwhile to mention, in conclusion of this section,the pressure-induced transformation of ionic NO+NO3

−

to neutral N2O4 molecular crystal that occurs at pressuresbelow 2 GPa (Figures 3(b) and 3(c)). We found that the pres-sure of this transformation is highly dependent on a specificpressure-time path followed by the sample. In some exper-iments on decompression of NO+NO3

− at room tempera-tures, 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 rich O2-N2 mixture

at pressures below 30 GPa produced a mixture of twoionic crystals: NO+NO3

− and additional phase identifiedas ionic form of dinitrogen pentoxide, NO2

+NO3− (see

selected examples in Figures 4(a) and 4(b)). In order toestablish precisely the nature of the additional phase, wecarried out a series of Raman and diffraction experimentsat high pressures and temperatures. The identification ofNO2

+NO3− was facilitated by the fact of a complete conver-

sion 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)). Thedetermined earlier hexagonal (P63/mmc, two formula unitsper unit cell) crystal structure of NO2

+NO3− [35] perfectly

accounted for diffraction patterns obtained at pressuresbelow 9 GPa (Figure 5(b)). The obtained Raman spectra ofNO2

+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). Underassumption of hexagonal crystal structure, the correlationanalysis accounts for the presence in the Raman spectrum ofν1 (1070 cm−1) and ν4 (736 cm−1) bands of NO3

− molecularcomplex, and ν1 (1403 cm−1) and ν2 (505 cm−1) bands ofNO2

+ ion (see Table 3(a)). A very weak and broad bandaround 1370 cm−1 (not shown in Figure 4(c)) was detectedin most of collected Raman spectra of NO2

+NO3−. We

assigned this feature to the antisymmetric stretching modeν3 of NO3

−. Extrapolation to zero pressure of the ν3 modefrequency (Figure 6) gives a similar value (∼1350 cm−1) asthe respective band frequency observed at ambient pressurein the previous studies [36, 37]. The multiple bands indicatedin Figure 4(c) as 2ν2 have been explained by Wilson andChriste [36] as a split overtone of the NO2

+ deformationmode. The pressure behavior of these bands is a conclusiveevidence of Wilson’s assignment (see Figure 6).

The important different feature in NO2+NO3

− Ramanspectra of this study, however, was the presence of anti-symmetric stretching mode ν3 (2260 cm−1) of NO2

+ com-plex. Correlation analysis cannot explain this Raman band,which is silent under assumption of hexagonal structureof NO2

+NO3− (see Table 3(a)). A plausible explanation of

the Raman activity of the ν3 band can be based on a bentstructure of rotating NO2

+ ion. A reduction of NO2+ ion’s

symmetry from a linear to a bent one implies a reductionof symmetry of NO2

+NO3− crystal structure. As it was

pointed out by Simon et al. [38], a very small deviationfrom hexagonal unit cell toward an orthorhombic one isneeded in order to explain the low-temperature single-crystaldiffraction data. Assuming the orthorhombic (Cmcm spacegroup, 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 vibrationalmodes of the NO2

+ cation observed in our Raman spectra(see Table 3(b)). As for NO3

− ions vibrational modes, thesymmetry reduction of the unit cell has to produce asplitting of the ν3 antisymmetric stretching vibration andν4 in plane deformation mode, analogously to the case ofnitrosonium-nitrate NO+NO3

−. Indeed, at pressures above∼20 GPa, we observed a splitting of ν4 mode (comparethe respective insets of Figures 4(c) and 4(d)). In addition,the low frequency part of the collected Raman spectrasuffered strong alterations at high pressures, indicating a


Table 3: Correlation between point group, site group, and factor group symmetry species and their Raman and IR activities for the NO2+

and NO3− molecular ions in the NO2

+NO3− crystal assuming (a) D4

6h (P63/mmc) and (b) D172h (Cmcm) space group.

(a)


Deformation

(b)


Bending




Symmetry stretch.

Symmetry stretch.





Symmetry stretch.

Symmetry stretch.


(ν1)

(ν2)

(ν3)

(ν4)

(ν1)

(ν2)

(ν3)

D3h (−6m2) C2ν (y) (m2m) D2h (mmm)

NO2+

NO3−

NO2+

NO3−

C2ν (mm2) C2ν (y) (m2m) D2h (mmm)

A1 (R, IR)

B2 (R, IR)

A1 (R, IR)

A1 (R, IR)

B2 (R, IR)

A1 (R, IR)

A′1 (R)

A′′2 (IR)

E′ (R, IR)

E′ (R, IR)

A1 (R, IR)

B1 (R,IR)

A1 (R, IR)

A1 (R, IR)

B2 (R, IR)

B2 (R, IR)

Ag (R)

B2u (IR)

B1u (IR)

B3g (R)

Ag (R)

B2u (IR)

B1g (R)

B3u (IR)

Ag (R)

B2u (IR)

B1g (R)

B3u (IR)

Ag (R)

B2u (IR)

B1g (R)

B3u (IR)

Ag (R)

B2u (IR)

(ν1)

(ν2)

(ν3)

(ν4)

(ν1)

(ν2)

(ν3)

D3h (−6m2) D3h(C′′2 ) (−62m) D6h (6/mmm)

D∞h (∞mm) D3h(C′′2 ) (−62m) D6h (6/mmm)

A′1 (R)

A′′2 (IR)

E′ (R, IR)

E′ (R, IR)

Σ+g (R)

Σ+u (IR)

Πu (IR)

A′1 (R, IR)

A′′2 (IR)

E′ (R, IR)

E′ (R, IR)

A′1 (R)

A′′1

E′ (R, IR)

A1g (R)

B2u

A2u (IR)

B1g

E1u (IR)

E2g (R)

E1u (IR)

E2g (R)

A1g (R)

B2u

A1u

B2g

E1u (IR)

E2g (R)


15.5 GPa, 500 K

12.5 GPa, 500 K

18 GPa, 295 K

9.2 GPa, 500 K

8.2 GPa, 500 K

Inte

nsi

ty(a

.u.)

0 2 4 6 8 10 12 14 16 18 20 22

2θ (deg)

(a)

0.5

1

1.5

×10E

4C

oun

ts

4 6 8 10 12 14 16

2θ (deg)

NO2+NO3

−

ab

c

(b)

Figure 5: (a) Evolution of the diffraction patterns of NO+NO3−

and NO2+NO3

− ionic solids on decompression at 500 K. The trans-formation 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 zfractional atomic coordinates for two nitrogen and two oxygenatoms 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.

considerable structural changes induced by pressure. Theseresults support the suggestion of Simon et al. [38] aboutorthorhombic nearly hexagonal structure of NO2

+NO3− and

evidence an increase of an orthorhombic distortion withincreasing pressure.

Another strong evidence of a bent structure of NO2+

ion is a soft behavior of deformation mode ν2 as a functionof pressure (Figures 6 and 7). Such behavior would reflectthe reduction of the bending force constant as the bent O–N–O structure deforms toward a linear configuration withincreasing pressure.

The temperature effect on the internal vibrational modesof NO2

+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

0

2.5

5

7.5

10

12.5

15

22.5

25×10−2

Ram

ansh

ift

(cm−1

)

2 3 4 5 6 7 8 9 10 11

Pressure (GPa)

ν2(NO2+)

ν4(NO3−)

2ν2(NO2+)

ν1(NO3−)

ν3(NO3−)

ν1(NO2+)

ν3(NO2+)

}

}

Latticemodes

(a)

7

7.5

9.5

10

10.5

11

×10−2

Ram

ansh

ift

(cm−1

)

5 6 7 8 9 10

Pressure (GPa)

ν4(NO3−)

2ν2(NO2+)

ν1(NO3−)

}

(b)

Figure 6: Raman shifts as a function of pressure for NO2+NO3

−

ionic crystal (a). Opened squares refer to ambient temperaturedata obtained on compression and solid squares represent thedata obtained at 600 K on decompression. Green and red linescorrespond to ν3 mode pressure evolution at room temperatureand at 600 K, respectively. The influence of temperature on thevibrational frequencies can be seen at a zoomed scale (b).

more pronouncedly for overtone bands of ν2 mode. Such abehavior may indicate a less bent geometry of NO2

+ ion inthe expanded lattice of NO2

+NO3−. It should be pointed out

that the temperature increase results in downshifted Ramanpeak associated with antisymmetric stretching mode ν3 ofNO2

+ ion as well. At the same time, all other vibrationalmodes of NO2

+NO3− are shifted toward a higher frequency

with increasing temperature.It is worth noting that a transformation to a bent

configuration of NO2+ ion can be viewed as a removal of

degeneration in doubly degenerated Πu bending mode of alinear ion. One of the components of the Πu mode reducesD∞h symmetry to C2ν, and other component becomes therotation of the ion. Consequently, a rotation of NO2

+ ionin NO2

+NO3− crystal can be expected just from symmetry

considerations. As was already discussed by Wilson andChriste [36], namely, this rotation may result in an averagelinear structure of NO2

+ ion detected by X-ray diffractiontechnique.

An orientational disorder of NO3− ionic groups in

NO2+NO3

− crystal can be inferred from our Raman data


{

2.4

4.5

6.8

8.3

10

Inte

nsi

ty(G

Pa)

500 600 700 800 900 1000 1100


νNO2

+

2 νNO3

−4

2νNO2

+

2

νNO3

−1

(a)

5.6 GPa8.4 GPa11.3 GPa

1080 1090 1100 1060 1070 1080 1090 1050 1060 1070 1080

v(cm−1)

(b)

Figure 7: Pressure evolution of the Raman spectra of NO2+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 vicinityof ν1(NO3

−) vibration mode indicates an orientational and/or sitemobility of NO3

− ionic complexes in NO2+NO3

− ionic crystal.

as well. Numerous studies on nitrate salts showed that thetotally symmetric vibration ν1 of nitrate ions may havea complex structure; in particular, an anomalous secondcomponent is present near the main Raman peak at slightlylower frequencies [39–41]. The appearance of the secondcomponent is generally explained by a disorder in nitrateions 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 beattributed to a rotational or site disorder of NO3

− ions. It isinteresting to note that Wilson and Christe [36] also observedan unexplained peak near the ν1 stretching mode of NO3

−

groups in Raman spectra of NO2+NO3

− collected at low tem-peratures and ambient pressures. It is possible that the originof this peak is connected to a disorder of the nitrate ions.

It is worthwhile, in conclusion, to discuss briefly thequestion of relative stability of two ionic phases of nitro-gen oxides, NO+NO3

− and NO2+NO3

−. Raman and X-ray diffraction data show unambiguously the instability ofNO+NO3

− ionic solid at pressures below 9 GPa and temper-atures above 450 K (perhaps, even at lower temperatures).Figure 8 stresses this point showing the relations between

250

300

350

400

450

500

550

600

Tem

per

atu

re(K

)

0 2 4 6 8 10 12 14 16

Pressure (GPa)

Melt

NO2+NO3

−Mixture:

NO2+NO3

−

andNO+NO3

−

Figure 8: Phase diagram showing the domain of stability of crys-talline NO2

+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.

ionic forms of nitrogen oxides and includes the melting lineof NO2

+NO3− ionic crystal mapped from our Raman and

X-ray diffraction experiments.On the other hand, a stability of NO+NO3

− ioniccrystal at pressures above 30 GPa and high temperatureswas confirmed by numerous laser-heating experiments ofour and previous studies. One can expect, consequently,a conversion of NO2

+NO3− to NO+NO3

− at sufficientlyhigh pressures. A big pressure region of coexistenceof NO+NO3

− and NO2+NO3

− phases observed in ourstudy can be attributed to kinetics of the NO+NO3

−toNO2

+NO3−transformation. In particular, high-kinetic bar-

rier 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 areduction of ionicity of NO+NO3

− toward a neutral formof nitrogen dioxide is intermediated by another stable ionicform of nitrogen oxide, NO2

+NO3−. Most likely, interplay

between electrostatic and intraionic covalent bonding energycontributions to a total energy defines a stable ionic form ofnitrogen oxide at high pressures.

4. Conclusions

In summary, we have synthesized the ionic forms of nitrogenoxides, NO+NO3

− and NO2+NO3

−, directly from oxygen-rich O2-N2 mixture. A direct high-pressure synthesis ofionic forms of nitrogen oxide provides a convenient wayof its study at high pressures and high temperatures. TheRaman and X-ray diffraction studies showed that a sequenceNO+NO3

−-NO2+NO3

−-N2O4 of transformations can berealized with pressure-temperature variation underlying thetransition from highly ionic to neutral form of nitrogenoxides. Our results evidence that ionic NO2

+NO3− phase is


characterized by increased orientational and/or site disorderof ionic groups.

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