High-Pressure Synthesis and Study of NO NO3− and NO2 NO3− Ionic Solids

Hindawi Publishing Corporation Advances in Physical Chemistry Volume 2009, Article ID 180784, 11 pages doi: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 andC.A.Achete1

1 Divisao˜ de Metrologia de Materiais (DIMAT), Instituto Nacional de Metrologia, Normalizac¸ao˜ e Qualidade Industrial, Avenida Nossa Senhora das Grac¸as 50, Xer´em, Duque de Caxias, RJ, CEP 25250-020, Brazil 2 Bayerisches Geoinstitut, Universitat¨ Bayreuth, 95440 Bayreuth, Germany 3 Nikolaev Institute of Inorganic Chemistry SB RAS, Pr. Ac. Lavrentieva 3, 630090 Novosibirsk, Russia 4 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 mixturecompressedtodifferent 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.

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

1. Introduction promotes either neutral or ionic form, which is determined by the electrostatic energy contribution to the total energy of Application of high pressure can convert simple molecular the donor-acceptor molecules. The similar neutral-to-ionic structures of elemental and heteronuclear systems into transformation has been reported in several simple molec- + − nonmolecular solids with extended or “infinite” atomic ular systems as well. The formation of H2 H2 dimmer at lattices [1]. Examples of such extended solids are non- 150 GPa in phase III of solid hydrogen [10]wasproposed molecular nitrogen [2–4], metalic oxygen [5], polymeric to explain the discontinuous changes of the Raman and form of carbon dioxide [6], and carbon monoxide [7]. infrared vibron frequencies [11, 12]. The nitrogen dioxide + − Such pressure-induced molecular-to-nonmolecular phase dimmer, N2O4, transforms to the ionic NO NO3 complex transitions are one of the ways to reduce a total energy of by temperature variation at ambient pressure [13, 14]and the molecular crystal via local (polymerization) or complete by pressure variation after laser heating [15, 16]. Recently, (metallization) delocalization of intramolecular electrons the same ionic solid was synthesized from N2OandN2O4 between adjacent molecules. Another alternative of a more molecular crystals by laser heating at high pressures [17– stable form of chemical bonds perturbed by pressure and 21]. From the latter studies, one can infer a possibility of + − temperature variation is ionization with the onset of charge- a direct synthesis of NO NO3 crystal from oxygen and transfer interactions. The first reported example of the nitrogen reactants. In particular, the primary dissociation neutral-to-ionic solid transformation concerns the organic of N2OonN2 and O2 at high temperatures and pressures charge-transfer compound tetrathiafulvalene-chloranil [8, below 30 GPa was evident from the Raman spectra [17, 18]. 9]. In this material, the pressure or temperature variation On the other hand, X-ray diffraction studies showed that 2 Advances in Physical Chemistry nitrosonium nitrate is denser than other nitrogen-oxygen collected at Bayreuth Geo-Institute using Dilor XY systems assemblages [19]. Direct confirmation of the O2 +N2 → for the Raman measurements with the 5145 AAr-ionlaser˚ + − NO NO3 reaction has been done during high-pressure excitation line and the incident laser power in the range 200– synthesis of CrO2 from Cr and O2,where addition of nitrogen 250 mW. The Dilor XY Raman spectrometer was calibrated into the Cr + O2 reactants environment resulted in the using the Γ25 phonon of diamond-structured Si (Fd-3m)and + − −1 formation of NO NO3 compound [22]. Recent studies provided the data collection in the 50–3000 cm range. The confirmed the transformation of a compressed mixture of peaks were analyzed using the TOPAS-Academic software + − O2 and N2 to NO NO3 induced either by laser light [23] [26] with pseudo-Voight function describing a peak profile. or by X-ray radiation [24]. The latter study also showed We estimate a resolution of 1 cm−1 for Raman peak position. that at low pressures, the ionic form of dinitrogen pentoxide In situ high-pressure X-ray diffraction measurements of + − NO2 NO3 is rather stable than nitrosonium nitrate. O2-N2 mixture were done at ID30 beam line of European Aconsiderablevolumeofexperimentalstudieshas Synchrotron Radiation Facility (ESRF, Grenoble, France) been carried out in order to ascertain the high-pressure before and after heating by Nd:YAG laser. A procedure of + − structure of NO NO3 ionic crystal. Based on the energy- fitting of outgoing thermal radiation (the energy distribution dispersive X-ray diffraction studies, aragonite-type structure of its emitted light) to a Planck radiation formula yielded + − was proposed for the crystal polymorph of NO NO3 the temperature of the heated area of approximately 2000 K. at pressures above 5 GPa and two space groups, P21cn Diffraction patterns were collected using focused monochro- and Pmcn, equally well accounted for the observed Bragg matic (λ = 0.3738 A)˚ X-ray radiation with an image plate reflections [17, 19]. Angle-dispersive X-ray diffraction study detector (MAR345). One-dimensional 2θ dependences of the + − revealed the monoclinic P21/m structure of NO NO3 at X-ray diffracted intensities were obtained by integration of low pressures [24]. Several other space groups were also two-dimensional diffraction images using the ESRF Fit2D + − proposed for the low-pressure phase of NO NO3 [23], software [27]. Small crystals of ruby provided a pressure however with the limited X-ray diffraction data quality and a calibration of the sample by fluorescence technique. nondecisive conclusion about the space group. At the same time, no experimental information is available on crystal + − 3. Results and Discussion structure of NO2 NO3 at high pressures. + − In this work, we have explored the question of stability 3.1. High-Pressure Synthesis and Phase Relations of NO NO3 . of high-pressure/high-temperature forms of nitrogen oxides The Raman spectra of the samples at high pressures and obtained by laser heating of oxygen rich O2-N2 mixture. In room temperature before laser heating contained the well- + − addition to the expected NO NO3 ionic crystal, dinitrogen known features of the O2-N2 solid mixture. The intense + − −1 pentoxide NO2 NO3 ionic solid is formed at pressures oxygen vibron at about 1575 cm , the nitrogen stretching −1 below 30 GPa. We report the results of high-pressure/high- mode at about 2365 cm , and the lattice modes of O2-N2 temperature Raman and X-ray diffraction studies on the mixture in a low-frequency part of spectra characterized the + − synthesized compounds. The conversion of NO NO3 into Raman spectra collected at high pressure before laser heating + − NO2 NO3 at high temperatures and pressure below 9 GPa (see the selected examples in Figures 1(a) and 1(b)). The + − is documented. Structural properties of NO NO3 and pressure evolution of a characteristic splitting of O2 and N2 + − NO2 NO3 ionic compounds as a function of pressure and stretching modes as well as the pressure dependence of the temperature are discussed relative intensities of the O2 and N2 vibrons were compared with the available studies of O2-N2 mixture [28, 29]. We 2. Experimental Methods estimate the oxygen concentration in all studied O2-N2 samples to be within the range from 70% mol up to almost Investigated samples consisted of a mixture of the high- 100%. We did not control precisely the oxygen content in purity (99.999%) oxygen and nitrogen (99.99%) gases the O2-N2 mixture since the results of the laser heating condensed at ambient pressure and liquid nitrogen tem- experiments were independent on the variation of oxygen perature. Diamond anvil cells (DACs) were merged and concentration. The compression to pressures above 30 GPa closed in the liquid phase of studied O2-N2 assemblages. and subsequent laser heating in the range of temperatures The small crystals of ruby in the gasket hole provided from 1300 K to 2000 K of the O2-N2mixture resulted in the + − a pressure calibration by standard fluorescence technique. formation of nitrosonium-nitrate, NO NO3 ,ascompared + − A predominant content of oxygen in condensed O2-N2 to similar spectra of the earlier studies on NO NO3 mixtures was ensured by adjusting the partial pressures [15–18]. The Raman peaks of this ionic compound can be of oxygen and nitrogen gases during the condensation easily separated from the peaks of nontransformed O-N process. After a compression to a desired pressure, an O2-N2 mixture by comparing the Raman spectra collected from the mixture was heated by Nd:YLF (1067 nm) laser. Temperature nonheated (Figure 1(c)) and heated regions of the sample estimations of the heated area during laser heating relied (Figures 1(d) and 1(e)). In the high-frequency range of + − on observed hot spot intensities [25]. Several experiments the Raman spectrum, NO NO3 ionic crystal exhibits the with O2-N2 mixture covered the range of pressure before peaks associated with the intramolecular vibrations of the − + + laser heating from 10 GPa to 40 GPa, and the estimated NO3 and NO groups. The intense ν(NO ) stretching temperature varied in the range from 1300 to 2000 K. Raman mode of NO+ ion at 2268 cm−1 and the Raman bands of − −1 −1 spectra from samples before and after laser heating were NO3 molecular complex ν1 (1112 cm ), ν2 (825 cm ), Advances in Physical Chemistry 3

2280 νNO+ ν 2260 4 ν O2 ν ν0 NO+ 2240 ν O2 NO+ ν1 ν ν 2 ν 2 2  2220 ν 3 ν NO+ NO+

 )  ν ν0 1 2200 NO+ N2 −

 2180

Intensity νN2 O4 1700 5 − νNO3 1650 2 2 Raman shift (cm ν 1600 N2 ν0 N 1550 ν 2 − O2 νNO3 1500 3 1450 νN2 O4 1 1400 0 500 1000 1500 2000 2500 3000 1350 Raman shift (cm−1) 0 5 10 15 20 25 30 35 Pressure (GPa) Figure 1: Vibrational modes assignment for O2-N2 mixture before + − (a) 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 1200 heating. Raman spectrum of untransformed O2-N2 mixture after − νNO3 laser heating at 32 GPa (c) is shown for comparison. 1100 1 1000

900 νN2 O4 2 − νNO3 2 ) − − − 1 800 ν 1 ν 1 ν NO3 − ν 3 (1470 cm ), 4 (750 cm ), and overtone of 2 4 −1 700 (1650 cm )areidentifiedinFigure 1(d). The lower νN2 O4 8 frequency range of the Raman spectra exhibits the peaks that can be related to the lattice vibrations of the NO − and 3 500 νN2 O4 + 6 NO ions and to libration, that is, rotation, of these ionic Lattice mode 6 groups. It is interesting to note almost identical resemblance Raman shift (cm 400 νN2 O4 3 5 of the lower part of the Raman spectrum in Figure 1(d) with 300 4 3 3 the respective part of the Raman spectra reported by Yoo et 200 2 al. [18], Song et al. [20], and Somayazulu et al. [17]. This 2 100 1 similarity indicates an invariance of a crystal structure of + − ff 0 NO NO3 produced from di erent starting forms of N-O 0 5 10 15 20 25 30 35 system. + − Pressure (GPa) High-pressure polymorphism of NO NO3 can be inferred from X-ray diffraction studies [17, 19, 24]. This (b) conclusion corroborates with the Raman spectroscopy data Figure 2: Pressure variation of the vibrational frequencies obtained of this work and previous studies [18, 21], where the pressure on decompression of NO+NO − (solid squares) and N O (opened + − 3 2 4 dependence of mode frequencies of NO NO3 exhibits the squares) molecular crystals. change of a slope at ∼5GPa (Figure 2). This singularity delimits fairly well the pressure ranges of observation of monoclinic [24] and orthorhombic [17, 19] phases of + − ν NO NO3 . One more change of a slope can be identified and NO+ . The obtained frequencies for the latter peaks and ∼ ν ν in Figure 2 at 22 GPa indicating an eventual structural their pressure behavior suggest that NO+ and NO+ modes + − alteration in NO NO3 system at higher pressures. are not the overtones or combination bands. The intensities + − A symmetrization of high-pressure NO NO3 crystal of these peaks are weakly affected by the pressure variation; structure with decreasing pressure has been pointed out as a result, no correlation between pressure behavior of as a possible general phase transformation path [18]. The intensities of these peaks and symmetrization of stretching asymmetry of stretching mode of NO+ ion (∼2270 cm−1) mode of NO+ ion could be seen. A plausible assumption that visibly vanishes on decompression at pressures about about the origin of these peaks could be an orientational 5 GPa (compare respective insets in Figures 1(d) and 3(a)) (dynamic or static) disorder of NO+ ionic groups. The can be a consequence of such a symmetrization. In addition orientational disorder may result in metastable orientations + + + − to the asymmetry of the ν(NO ) stretching mode, we could of NO ions in the structure of NO NO3 with slightly clearly detect two weak peaks in the vicinity that were not different stretching frequencies of NO+ ions. reported in previous studies. These two peaks are identified In order to obtain an additional insight into structural ∼ −1 ν + − in the insets of Figures 1(d) and 3(a) at 2200 cm as NO+ properties of NO NO3 at high pressures, one can confront 4 Advances in Physical Chemistry

Table 1: Correlation between point group, site group, and factor group symmetry species and their Raman and IR activities for the NO+ − 2 + − and NO3 molecular ions in the monoclinic, C2h (P21/m), NO NO3 crystal.

Ion Mode description Point group Site group Factor group − NO3 D3h (−6m2) Cs (m) C2h (2/m)

ν   Symmetry stretch. ( 1), A1 (R) A (R, IR) Ag (R)

ν   Out-of-plane deformation ( 2) A2 (IR) A (R, IR) Bu (R)

  Antisymmetry stretch. (ν3) E (R, IR) A (R, IR) Au (R) A (R, IR)   In-plane deformation (ν4) E (R, IR) A (R, IR) Bg (R) A (R, IR)

+ NO C∞ν (∞mm) Cs (m) C2h (2/m)

+  Stretch. (νs) Σ (R, IR) A (R, IR) Ag (R)

Bu (R)

was established from X-ray diffraction measurements [24]. ν ν 3 ν 4 ν0 NO+ We also discuss the implications of correlation analysis O2 ν 2 2 under assumption of orthorhombic phases P21cn and Pmcn ν2 ν with aragonite-type structure as a most plausible structural 710 720 810 820 830 1 + − 1350 1500 1650 2225 2250 models of high-pressure phase of NO NO3 inferred from earlier X-ray diffraction studies [17, 19].  (a)  ν + ν0 Table 1 summarizes the result of the correlation analysis ν NO N − ν NO+ 2 + 4 for the monoclinic structure of NO NO3 . The correlations ν0 O ν − 2 NO+ 2100 2200 2300 2400 between molecular and site group species of NO ions result Intensity 3 700 800 ν1 in splitting of ν3 and ν4 modes, all of them being infrared and (b) ν8 Raman active. Indeed, our Raman spectra (Figure 3(a)) as ν3 N2O4 Raman active vibrations 750 900 well as Raman spectra of previous studies [17, 20, 23]clearly ν1 ν ν ν2 show a splitting of 4 fundamental. Besides, somewhat broad 6 ν5 and asymmetric ν3 band detected in our study suggests a (c) poorly resolved doublet. This doublet was clearly resolved in 0 500 1000 1500 2000 2500 3000 the study by Sihachakr and Loubeyre [23]. One can observe, Raman shift (cm−1) − consequently, six internal optic modes originated by NO3 + + − ions and one NO stretching mode in both Raman and IR Figure 3: Raman spectra of (a) NO NO3 ionic crystal at 2.8 GPa, + − + − spectra of monoclinic NO NO crystal. This prediction is (b) a mixture of NO NO3 ionic and N2O4 molecular crystals at 3 2GPa,and(c)N2O4 molecular crystal at 1.7 GPa, showing the phase in excellent agreement with our Raman and previous Raman transition of N2O4 molecular compound from ionic to neutral andIRstudies. form. The assignment of the internal vibrational modes in (c) Superposition of translational and rotational motions of − + corresponds to symmetrical N2O4 dimer [14]. NO3 and NO ions originates the external optic modes of + − NO NO3 .IncaseofP21/m space group, one can find the following irreducible representations of the lattice vibrations and libration modes [30, 31]: the structural information obtained from X-ray diffraction Lattice = (R) (IR) (R) (IR) studies with Raman spectroscopy data. Specifically, in case of ΓNO+NO − 4Ag +2Au +2Bg +4Bu , 3 (1) a known or assumed crystal structure, the correlation anal- Libration (R) (IR) (R) (IR) Γ + − = 2Ag +3Au +3Bg +2Bu , ysis [30, 31] allows establishing the number, symmetry, and NO NO3 spectral activity of the external and internal optics modes. The spectral activity of each symmetry species is indi- + − SincenosuchstudyhasbeendonesofarforNONO3 , cated by a superscript (R) for Raman and (IR) for infrared we present briefly the main results of correlation analysis active modes. Table 2(a) summarizes the total irreducible + − for low-pressure monoclinic phase (P21/m) whose structure representation of NO NO3 crystal under assumption of Advances in Physical Chemistry 5

+ − 2 Table 2: Total irreducible representation and spectral activity of the monoclinic and aragonite-type NO NO3 crystal for (a) C2h (P21/m), 9 16 (b) C2ν (Pna21), and (c) D2h (Pnma)spacegroup.

(a) Translations Acoustical Rotation Intramolecular Spectral Crystal symmetry C2h (lattice vibrations) vibrations (libration) vibrations activity C2h

Ag 425R

Au 1132IR

Bg 232R

Bu 2225IR Total vibrations 9 3 10 14

(b) Translations Acoustical Rotation Intramolecular Spectral Crystal symmetry C2ν (lattice vibrations) vibrations (libration) vibrations activity C2ν

A1 5 1 5 7 R, IR

A2 657R

B1 5 1 5 7 R, IR

B2 5 1 5 7 R, IR Total vibrations 21 3 20 28

(c) Translations Acoustical Rotation Intramolecular Spectral Crystal symmetry D2h (lattice vibrations) vibrations (libration) vibrations activity D2h

Ag 415R

B1u 3115IR

B2u 3115IR

B3g 415R

Au 242

B1g 242R

B2g 242R

B3u 1142IR Total vibrations 21 3 20 28

P21/m space group. Full factor group for external vibrations and Pnma, resp.). The correspondence between the crystallo- + − of monoclinic NO NO3 contains 11 Raman active and 8 graphic axis and irreducible representations of factor groups ν9 16 ff IR active translational and libration modes. Raman spectra of C2 and D2h in di erent settings can be found elsewhere − + obtained in our and in the previous studies [17, 18, 20] [32]. As the result of the C1 site symmetry of NO3 and NO exhibit only 8-resolved peaks associated with the external ions, 41 Raman active and 30 IR active external vibrations are modes at high pressures and room temperatures; this expected in case of P21cn spacegroup.IncaseofPmcn space + − number reduces to 6 peaks at pressures below 5 GPa (see group of NO NO3 crystal, either C1 or Cs site symmetry − + Figures 1(d), 1(e), and 3(a)). The discrepancy between the can accommodate four NO3 and four NO ions. Table 2(c) number of observed and predicted external Raman modes shows the results of correlation analysis assuming Cs site can be attributed to a thermal overlapping of the peaks. symmetry for both ions. The choice of this site symmetry is Indeed, all 11 peaks are resolved in the low-frequency range due to the fact that crystallographic positions occupied by + − of low-temperature Raman spectra of NO NO3 [21]. ions in aragonite-type crystal have the same Cs symmetry Analogously, a site group correlation analysis under [33]. External vibrations in this case consist of 12 Raman and + − assumption of orthorhombic structure of NO NO3 with 7 IR active translational modes, and of 10 Raman and 10 IR P21cn and Pmcn space groups leads to six internal optic active libration modes. − + modes originated by NO3 ions and one NO stretching A considerable discrepancy between the number of mode, all being active in both Raman and IR spectra. The experimentally observed and predicted external Raman respective total irreducible representations are summarized active vibrations in case of orthorhombic phase of + − in Tables 2(b) and 2(c). The factor group symmetry species NO NO3 can reflect a structural peculiarities of high- + − are labeled using standard axial settings (space groups Pna21 pressure polymorph of NO NO3 . As it was shown for 6 Advances in Physical Chemistry

+ selected examples in Figures 4(a) and 4(b)). In order to νNO2 + + 1 νNO2 NO2 establish precisely the nature of the additional phase, we + − ν 3 NO2 NO3 2 carried out a series of Raman and diffraction experiments at high pressures and temperatures. The identification of + − NO2 + − + ν + NO2 NO3 was facilitated by the fact of a complete conver- + NO2 − 1 νNO2 NO2 NO3 2v νNO3 3 + − + − 2 1 + sion of NO NO3 ionic crystal to NO2 NO3 at pressures νNO2 2 − νNO3 below 9 GPa and temperatures above ambient temperature. 4 This transformation was confirmed by both Raman and X- − νNO+ νNO3 ff Intensity 1 ray di raction measurements (Figures 4(c) and 5(a)). The + − + − NO2 NO3 +NO NO3 − − determined earlier hexagonal (P63/mmc, two formula units NO3 νNO3 ν 2 + − 4 per unit cell) crystal structure of NO2 NO3 [35]perfectly ff ν accounted for di raction patterns obtained at pressures ν N2 O2 +N2 O2 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 0 400 800 1200 1500 2000 2500 molecular complexes are indicated in Figure 4(c). Under −1 Raman shift (cm ) assumption of hexagonal crystal structure, the correlation analysis accounts for the presence in the Raman spectrum of Figure 4: Raman spectra and vibrational modes assignment for (a) − − − + − ν 1 ν 1 O2-N2 mixture before laser heating, (b) mixture of NO NO3 and 1 (1070 cm )and 4 (736 cm ) bands of NO3 molecular + − ν −1 ν −1 NO2 NO3 ionic solids after laser heating (vibrational modes of complex, and 1 (1403 cm )and 2 (505 cm ) bands of + − + − + NO NO3 are identified), (c) NO2 NO3 at 3.7 GPa and 500 K, (d) NO2 ion (see Table 3(a)). A very weak and broad band + − −1 NO2 NO3 at 25 GPa and room temperature. around 1370 cm (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 ν − ν aragonite, CaCO3, and isomorphic KNO3 [34], one can 3 of NO3 . Extrapolation to zero pressure of the 3 mode expectaconsiderablereductioninanumberofexternal frequency (Figure 6) gives a similar value (∼1350 cm−1)as modes if a structural model for a compound is a slightly the respective band frequency observed at ambient pressure distorted variant of a structure with higher symmetry. In in the previous studies [36, 37]. The multiple bands indicated such a distorted structure, certain vibrational modes can in Figure 4(c) as 2ν2 have been explained by Wilson and + be associated with a motion of ions in planes that have a Christe [36] as a split overtone of the NO2 deformation symmetry closely coinciding with a higher base symmetry. mode. The pressure behavior of these bands is a conclusive Consequently, these modes should be determined using the evidence of Wilson’s assignment (see Figure 6). + − high-symmetry model. In particular, if a higher-symmetry The important different feature in NO2 NO3 Raman base structure has a reduced translational symmetry in such spectra of this study, however, was the presence of anti- −1 + a plane, one can expect to observe only lattice and libration symmetric stretching mode ν3 (2260 cm )ofNO2 com- modes originated by ions motion which is translational plex. Correlation analysis cannot explain this Raman band, invariant with respect to a reduced unit cell [34]. It is which is silent under assumption of hexagonal structure + − worth noting, in this regard, that the established structure of NO2 NO3 (see Table 3(a)). A plausible explanation of + − ν of the low-pressure phase of NO NO3 has the monoclinic the Raman activity of the 3 band can be based on a bent + + unit cell [24] with two molecular units per unit cell in structure of rotating NO2 ion. A reduction of NO2 ion’s contrast to four molecular units of the proposed aragonite- symmetry from a linear to a bent one implies a reduction + − type structure [17]. It might be possible that this monoclinic of symmetry of NO2 NO3 crystal structure. As it was structure is a descendent of the higher-symmetry base pointed out by Simon et al. [38], a very small deviation + − structure of the high-pressure phase of NO NO3 with from hexagonal unit cell toward an orthorhombic one is reduced unit cell. needed in order to explain the low-temperature single-crystal It is worthwhile to mention, in conclusion of this section, diffraction data. Assuming the orthorhombic (Cmcm space + − + − the pressure-induced transformation of ionic NO NO3 group, four formula units per unit cell) for NO2 NO3 + to neutral N2O4 molecular crystal that occurs at pressures crystal and a bent configuration of NO2 ion, one can below 2 GPa (Figures 3(b) and 3(c)). We found that the pres- deduce the Raman activity of all three normal vibrational + sure of this transformation is highly dependent on a specific modes of the NO2 cation observed in our Raman spectra − pressure-time path followed by the sample. In some exper- (see Table 3(b)). As for NO3 ions vibrational modes, the + − iments on decompression of NO NO3 at room tempera- symmetry reduction of the unit cell has to produce a tures, the ionic form could be retained up to a liquid phase. splitting of the ν3 antisymmetric stretching vibration and ν4 in plane deformation mode, analogously to the case of + − 3.2. High-Pressure Synthesis and Structural Properties of nitrosonium-nitrate NO NO3 . Indeed, at pressures above + − NO2 NO3 . Laser heating of oxygen rich O2-N2 mixture ∼20 GPa, we observed a splitting of ν4 mode (compare at pressures below 30 GPa produced a mixture of two the respective insets of Figures 4(c) and 4(d)). In addition, + − ionic crystals: NO NO3 and additional phase identified the low frequency part of the collected Raman spectra + − as ionic form of dinitrogen pentoxide, NO2 NO3 (see suffered strong alterations at high pressures, indicating a Advances in Physical Chemistry 7

+ Table 3: Correlation between point group, site group, and factor group symmetry species and their Raman and IR activities for the NO2 − + − 4 17 and NO3 molecular ions in the NO2 NO3 crystal assuming (a) D6h (P63/mmc) and (b) D2h (Cmcm)spacegroup. (a) Ion Mode description Point group Site group Factor group − −  − NO3 D3h ( 6m2) D3h(C2 )( 62m) D6h (6/mmm)

  Symmetry stretch. (ν1) A1 (R) A1 (R, IR) A1g (R)

B2u

  Out-of-plane deformation (ν2) A2 (IR) A2 (IR) A2u (IR)

B1g   Antisymmetry stretch. (ν3) E (R, IR) E (R, IR) E1u (IR)

E2g (R)   In-plane deformation (ν4) E (R, IR) E (R, IR) E1u (IR)

E2g (R)

+ ∞  − NO2 D∞h ( mm) D3h(C2 )( 62m) D6h (6/mmm)

+  Symmetry stretch. Σg (R) A1 (R) A1g (R) (ν1) B2u +  Antisymmetry stretch. Σu (IR) A1 A1u (ν3) B2g  Deformation Πu (IR) E (R, IR) E1u (IR) (ν2)

E2g (R)

(b) Ion Mode description Point group Site group Factor group

− NO3 D3h (−6m2) C2ν (y)(m2m) D2h (mmm)

 A (R, IR) A (R) Symmetry stretch. (ν1) A1 (R) 1 g B2u (IR)

 Out-of-plane deformation (ν2) A2 (IR) B1 (R,IR) B1u (IR)

B3g (R)  Antisymmetry stretch. (ν3) E (R, IR) A1 (R, IR) Ag (R)

B2u (IR)

B2 (R, IR) B1g (R)

B3u (IR)  In-plane deformation (ν4) E (R, IR) A1 (R, IR) Ag (R)

B2u (IR)

B2 (R, IR) B1g (R)

B3u (IR)

+ NO2 C2ν (mm2) C2ν (y)(m2m) D2h (mmm)

Symmetry stretch.(ν1) A1 (R, IR) A1 (R, IR) Ag (R)

B2u (IR)

Antisymmetry stretch. (ν3) B2 (R, IR) B2 (R, IR) B1g (R)

B3u (IR)

Bending (ν2) A1 (R, IR) A1 (R, IR) Ag (R)

B2u (IR) 8 Advances in Physical Chemistry

×10−2 25 ν + 22.5 3(NO2 )

) + 1 ν 15 1(NO2 ) 8.2 GPa, 500 K − ν − 12.5 3(NO3 ) − ν1(NO3 ) 10 } ν + 9.2 GPa, 500 K 2 2(NO2 ) ν − 7.5 4(NO3 )

+ Intensity (a.u.) ν 12.5 GPa, 500 K Raman shift (cm 5 2(NO2 ) Lattice 15.5 GPa, 500 K 2.5 } modes 0 18 GPa, 295 K 234567891011 Pressure (GPa) 0 2 4 6 8 10121416182022 (a) 2θ (deg) ×10−2 (a) 11 − ν1(NO3 ) )

+ − 1

NO NO − 4 2 3 10.5 E 1 5 10 .  × 10 + 2ν2(NO2 ) c b a 9.5

1 Raman shift (cm − 7.5 ν4(NO3 )

7 5678910 Pressure (GPa) 0.5

Counts 4 6 8 10 12 14 16 (b)

2θ (deg) + − Figure 6: Raman shifts as a function of pressure for NO2 NO3 (b) ionic crystal (a). Opened squares refer to ambient temperature

+ − data obtained on compression and solid squares represent the Figure 5: (a) Evolution of the diffraction patterns of NO NO3 + − data obtained at 600 K on decompression. Green and red lines and NO2 NO3 ionic solids on decompression at 500 K. The trans- ν + − correspond to 3 mode pressure evolution at room temperature formation of the phase mixture to pure NO2 NO3 is confirmed and at 600 K, respectively. The influence of temperature on the by the Rietveld refinement (b) of the hexagonal structural model + − vibrational frequencies can be seen at a zoomed scale (b). (P63/mmc)ofNO2 NO3 at 8.2 GPa and 500 K. The refined lattice parameters are a = 5.1377(1); c = 5.8930(1). The x, y,andz fractional atomic coordinates for two nitrogen and two oxygen more pronouncedly for overtone bands of ν2 mode. Such a = = + atoms in asymmetric unit are N(1) 0, 0, 1/4; N(2) 1/3, 2/3, 3/4; behavior may indicate a less bent geometry of NO2 ion in O = 0 1792, 0 3583, 1 4; O = 1 3, 2 3, 0 5631. + − (1) . . / (2) / / . 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 + considerable structural changes induced by pressure. These NO2 ion as well. At the same time, all other vibrational + − results support the suggestion of Simon et al. [38]about modes of NO2 NO3 are shifted toward a higher frequency + − orthorhombic nearly hexagonal structure of NO2 NO3 and with increasing temperature. evidence an increase of an orthorhombic distortion with It is worth noting that a transformation to a bent + increasing pressure. configuration of NO2 ion can be viewed as a removal of + Another strong evidence of a bent structure of NO2 degeneration in doubly degenerated Πu bending mode of a ion is a soft behavior of deformation mode ν2 as a function linear ion. One of the components of the Πu mode reduces of pressure (Figures 6 and 7). Such behavior would reflect D∞h symmetry to C2ν, and other component becomes the + the reduction of the bending force constant as the bent O– rotation of the ion. Consequently, a rotation of NO2 ion + − N–O structure deforms toward a linear configuration with in NO2 NO3 crystal can be expected just from symmetry increasing pressure. considerations. As was already discussed by Wilson and The temperature effect on the internal vibrational modes Christe [36], namely, this rotation may result in an average + − + of NO2 NO3 is detailed for selected modes in Figure 6(b). A linear structure of NO2 ion detected by X-ray diffraction + shift of ν2(NO2 ) Raman peak toward a lower frequency with technique. − increasing temperature was observed in high-pressure/high- An orientational disorder of NO3 ionic groups in + − temperature Raman experiments. This trend can be seen NO2 NO3 crystalcanbeinferredfromourRamandata Advances in Physical Chemistry 9

− NO + ν 3 600 νNO2 1 NO + − 2 ν 2 νNO3 { 2 2 4 10 550 Melt 500 + − 8.3 NO2 NO3 Mixture: 450 + − NO2 NO3 6.8 400 and NO+NO − Intensity (GPa) 3 Temperature (K) 350 4.5 300 2.4 250 500 600 700 800 900 1000 1100 0246810121416 Raman shift (cm−1) Pressure (GPa)

(a) 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 solidtolowpressures.

11.3 GPa 8.4 GPa 5.6 GPa ionic forms of nitrogen oxides and includes the melting line + − of NO2 NO3 ionic crystal mapped from our Raman and 1080 1090 1100 1060 1070 1080 1090 1050 1060 1070 1080 X-ray diffraction experiments. + − v(cm−1) On the other hand, a stability of NO NO3 ionic crystal at pressures above 30 GPa and high temperatures (b) was confirmed by numerous laser-heating experiments of + − Figure 7: Pressure evolution of the Raman spectra of NO2 NO3 our and previous studies. One can expect, consequently, − + − + − at 600 K (a) and Raman spectra of ν1(NO3 ) region at 11.3 GPa, 8.4 aconversionofNO2 NO3 to NO NO3 at sufficiently and 5.6 GPa at ambient temperature (b). A shoulder in the vicinity high pressures. A big pressure region of coexistence ν − + − + − of 1(NO3 ) vibration mode indicates an orientational and/or site of NO NO3 and NO2 NO3 phases observed in our mobility of NO − ionic complexes in NO +NO − ionic crystal. + − 3 2 3 study can be attributed to kinetics of the NO NO3 to + − NO2 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 as well. Numerous studies on nitrate salts showed that the + − temperature. The high-pressure IR spectra of NO NO3 totally symmetric vibration ν of nitrate ions may have + − 1 [21] showed that a degree of ionicity of NO NO3 decreases a complex structure; in particular, an anomalous second with decreasing pressures. It follows from our results that a component is present near the main Raman peak at slightly + − reduction of ionicity of NO NO3 toward a neutral form lower frequencies [39–41]. The appearance of the second of nitrogen dioxide is intermediated by another stable ionic component is generally explained by a disorder in nitrate + − form of nitrogen oxide, NO2 NO3 . Most likely, interplay ions position or orientations in the crystal lattice of nitrates between electrostatic and intraionic covalent bonding energy + − [39]. The obtained Raman spectra of NO2 NO3 exhibit a contributions to a total energy defines a stable ionic form of ν shoulder next to the symmetric vibration 1 (see Figure 7), nitrogen oxide at high pressures. 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 4. Conclusions − an unexplained peak near the ν1 stretching mode of NO3 + − groups in Raman spectra of NO2 NO3 collected at low tem- In summary, we have synthesized the ionic forms of nitrogen + − + − peratures and ambient pressures. It is possible that the origin oxides, NO NO3 and NO2 NO3 , directly from oxygen- of this peak is connected to a disorder of the nitrate ions. rich O2-N2 mixture. A direct high-pressure synthesis of It is worthwhile, in conclusion, to discuss briefly the ionic forms of nitrogen oxide provides a convenient way question of relative stability of two ionic phases of nitro- of its study at high pressures and high temperatures. The + − + − gen oxides, NO NO3 and NO2 NO3 . Raman and X- Raman and X-ray diffraction studies showed that a sequence + − + − ray diffraction data show unambiguously the instability of NO NO3 -NO2 NO3 -N2O4 of transformations can be + − NO NO3 ionic solid at pressures below 9 GPa and temper- realized with pressure-temperature variation underlying the atures above 450 K (perhaps, even at lower temperatures). transition from highly ionic to neutral form of nitrogen + − Figure 8 stresses this point showing the relations between oxides. 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