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 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, , transforms to the ionic 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 and molecular crystals by laser heating at high pressures [17–21]. From the latter studies, one can infer a possibility of a direct synthesis of crystal from oxygen and nitrogen reactants. In particular, the primary dissociation of on and 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 reaction has been done during high-pressure synthesis of from Cr and , where addition of nitrogen into the reactants environment resulted in the formation of compound [22].  Recent studies confirmed the transformation of a compressed mixture of and to 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 is rather stable than nitrosonium nitrate.

A considerable volume of experimental studies has been carried out in order to ascertain the high-pressure structure of ionic crystal. Based on the energy-dispersive X-ray diffraction studies, aragonite-type structure was proposed for the crystal polymorph of at pressures above 5 GPa and two space groups, and , equally well accounted for the observed Bragg reflections [17, 19]. Angle-dispersive X-ray diffraction study revealed the monoclinic structure of at low pressures [24]. Several other space groups were also proposed for the low-pressure phase of [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 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 rich mixture. In addition to the expected ionic crystal, dinitrogen pentoxide 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 into at high temperatures and pressure below 9 GPa is documented. Structural properties of and 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 studied 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 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 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 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 phonon of diamond-structured Si () and provided the data collection in the 50–3000  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  for Raman peak position.

In situ high-pressure X-ray diffraction measurements of 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 ( Å) X-ray radiation with an image plate detector (MAR345). One-dimensional 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 of

The Raman spectra of the samples at high pressures and room temperature before laser heating contained the well-known features of the solid mixture. The intense oxygen vibron at about 1575 , the nitrogen stretching mode at about 2365 , and the lattice modes of 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 and stretching modes as well as the pressure dependence of the relative intensities of the and vibrons were compared with the available studies of mixture [28, 29]. We estimate the oxygen concentration in all studied samples to be within the range from 70% mol up to almost 100%. We did not control precisely the oxygen content in the 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 mixture resulted in the formation of nitrosonium-nitrate, , as compared to similar spectra of the earlier studies on [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, ionic crystal exhibits the peaks associated with the intramolecular vibrations of the and groups. The intense stretching mode of ion at 2268  and the Raman bands of molecular complex (), (), (), (), and overtone of () 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 and 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 produced from different starting forms of N-O system.

Figure 1: Vibrational modes assignment for mixture before laser heating at (a) 15 GPa and (b) 39 GPa; (d) ionic crystal at 32 GPa  and (e) its lattice modes at 30 GPa after laser heating. Raman spectrum of untransformed mixture after laser heating at 32 GPa (c) is shown for comparison.

High-pressure polymorphism of 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 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 . One more change of a slope can be identified in Figure 2 at ~22 GPa indicating an eventual structural alteration in system at higher pressures.

Figure 2: Pressure variation of the vibrational frequencies obtained on decompression of (solid squares) and (opened squares) molecular crystals.

A symmetrization of high-pressure crystal structure with decreasing pressure has been pointed out as a possible general phase transformation path [18]. The asymmetry of stretching mode of ion (~2270 ) 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 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  as and . The obtained frequencies for the latter peaks and their pressure behavior suggest that and 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 ion could be seen. A plausible assumption about the origin of these peaks could be an orientational (dynamic or static) disorder of ionic groups. The orientational disorder may result in metastable orientations of ions in the structure of with slightly different stretching frequencies of ions.

Figure 3: Raman spectra of (a) ionic crystal at 2.8 GPa, (b) a mixture of ionic and molecular crystals at 2 GPa, and (c) molecular crystal at 1.7 GPa, showing the phase transition of molecular compound from ionic to neutral form. The assignment of the internal vibrational modes in (c) corresponds to symmetrical dimer [14].

In order to obtain an additional insight into structural properties of 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 , we present briefly the main results of correlation analysis for low-pressure monoclinic phase () whose structure was established from X-ray diffraction measurements [24]. We also discuss the implications of correlation analysis under assumption of orthorhombic phases and with aragonite-type structure as a most plausible structural models of high-pressure phase of inferred from earlier X-ray diffraction studies [17, 19].

Table 1 summarizes the result of the correlation analysis for the monoclinic structure of . The correlations between molecular and site group species of ions result in splitting of and 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 fundamental. Besides, somewhat broad and asymmetric 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 ions and one stretching mode in both Raman and IR spectra of monoclinic 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 the and molecular ions in the monoclinic, (), crystal.

Superposition of translational and rotational motions of and ions originates the external optic modes of . In case of space group, one can find the following irreducible representations of the lattice vibrations and libration modes [30, 31]:

The spectral activity of each symmetry species is indicated by a superscript (R) for Raman and (IR) for infrared active modes. Table 2(a) summarizes the total irreducible representation of crystal under assumption of space group. Full factor group for external vibrations of monoclinic 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 [21].

Table 2: Total irreducible representation and spectral activity of the monoclinic and aragonite-type crystal for (a) (), (b) (), and (c) () space group.

Analogously, a site group correlation analysis under assumption of orthorhombic structure of with and space groups leads to six internal optic modes originated by ions and one 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 and , resp.). The correspondence between the crystallographic axis and irreducible representations of factor groups of and in different settings can be found elsewhere [32]. As the result of the site symmetry of and ions, 41 Raman active and 30 IR active external vibrations are expected in case of space group. In case of space group of crystal, either or site symmetry can accommodate four and four ions. Table 2(c) shows the results of correlation analysis assuming 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 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 of can reflect a structural peculiarities of high-pressure polymorph of . As it was shown for aragonite, , and isomorphic [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 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 with reduced unit cell.

It is worthwhile to mention, in conclusion of this section, the pressure-induced transformation of ionic to neutral 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 at room temperatures, the ionic form could be retained up to a liquid phase.

3.2. High-Pressure Synthesis and Structural Properties of

Laser heating of oxygen rich mixture at pressures below 30 GPa produced a mixture of two ionic crystals: and additional phase identified as ionic form of dinitrogen pentoxide, (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 was facilitated by the fact of a complete conversion of ionic crystal to 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 (, two formula units per unit cell) crystal structure of [35] perfectly accounted for diffraction patterns obtained at pressures below 9 GPa (Figure 5(b)). The obtained Raman spectra of also showed good agreement with the previous studies [36, 37]. The Raman bands of and 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 (1070 ) and (736 ) bands of molecular complex, and (1403 ) and (505 ) bands of ion (see Table 3(a)). A very weak and broad band around 1370  (not shown in Figure 4(c)) was detected in most of collected Raman spectra of . We assigned this feature to the antisymmetric stretching mode of . Extrapolation to zero pressure of the mode frequency (Figure 6) gives a similar value (~1350 ) as the respective band frequency observed at ambient pressure in the previous studies [36, 37]. The multiple bands indicated in Figure 4(c) as have been explained by Wilson and Christe [36] as a split overtone of the 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 the and molecular ions in the crystal assuming (a) () and (b) () space group.

Figure 4: Raman spectra and vibrational modes assignment for (a) mixture before laser heating,  (b) mixture of and ionic solids after laser heating (vibrational modes of are identified), (c) at 3.7 GPa and 500 K, (d) at 25 GPa and room temperature.

Figure 5: (a) Evolution of the diffraction patterns of and ionic solids on decompression at 500 K. The transformation of the phase mixture to pure is confirmed by the Rietveld refinement (b) of the hexagonal structural model () of at 8.2 GPa and 500 K. The refined lattice parameters are ; . The , , and fractional atomic coordinates for two nitrogen and two oxygen atoms in asymmetric unit are ; ; ; .

Figure 6: Raman shifts as a function of pressure for 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 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).

The important different feature in Raman spectra of this study, however, was the presence of antisymmetric stretching mode (2260 ) of complex. Correlation analysis cannot explain this Raman band, which is silent under assumption of hexagonal structure of (see Table 3(a)). A plausible explanation of the Raman activity of the band can be based on a bent structure of rotating ion. A reduction of ion’s symmetry from a linear to a bent one implies a reduction of symmetry of 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 ( space group, four formula units per unit cell) for crystal and a bent configuration of ion, one can deduce the Raman activity of all three normal vibrational modes of the cation observed in our Raman spectra (see Table 3(b)). As for ions vibrational modes, the symmetry reduction of the unit cell has to produce a splitting of the antisymmetric stretching vibration and in plane deformation mode, analogously to the case of nitrosonium-nitrate . Indeed, at pressures above ~20 GPa, we observed a splitting of 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 and evidence an increase of an orthorhombic distortion with increasing pressure.

Another strong evidence of a bent structure of ion is a soft behavior of deformation mode 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.

Figure 7: Pressure evolution of the Raman spectra of at 600 K (a) and Raman spectra of region at 11.3 GPa, 8.4 and 5.6 GPa at ambient temperature (b). A shoulder in the vicinity of vibration mode indicates an orientational and/or site mobility of ionic complexes in ionic crystal.

The temperature effect on the internal vibrational modes of is detailed for selected modes in Figure 6(b). A shift of 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 mode. Such a behavior may indicate a less bent geometry of ion in the expanded lattice of . It should be pointed out that the temperature increase results in downshifted Raman peak associated with antisymmetric stretching mode of ion as well. At the same time, all other vibrational modes of are shifted toward a higher frequency with increasing temperature.

It is worth noting that a transformation to a bent configuration of ion can be viewed as a removal of degeneration in doubly degenerated bending mode of a linear ion. One of the components of the mode reduces symmetry to , and other component becomes the rotation of the ion. Consequently, a rotation of ion in 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 ion detected by X-ray diffraction technique.

An orientational disorder of ionic groups in crystal can be inferred from our Raman data as well. Numerous studies on nitrate salts showed that the totally symmetric vibration 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 exhibit a shoulder next to the symmetric vibration (see Figure 7), and analogously to nitrate salts this shoulder can be attributed to a rotational or site disorder of ions. It is interesting to note that Wilson and Christe [36] also observed an unexplained peak near the stretching mode of groups in Raman spectra of 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, and . Raman and X-ray diffraction data show unambiguously the instability of 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 ionic crystal mapped from our Raman and X-ray diffraction experiments.

Figure 8: Phase diagram showing the domain of stability of crystalline on decompression of a mixture of and . A textured area indicates the eventual extension of the region of (meta)stability of solid to low pressures.

On the other hand, a stability of 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 to at sufficiently high pressures. A big pressure region of coexistence of and phases observed in our study can be attributed to kinetics of the to transformation. In particular, high-kinetic barrier of to conversion can explain a stability of at pressures below 9 GPa and ambient temperature. The high-pressure IR spectra of [21] showed that a degree of ionicity of decreases with decreasing pressures. It follows from our results that a reduction of ionicity of toward a neutral form of nitrogen dioxide is intermediated by another stable ionic form of nitrogen oxide, . 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, and , directly from oxygen-rich 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 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 phase is characterized by increased orientational and/or site disorder of ionic groups.