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Photochemistry within Compressed Sodium Azide Nicholas Holtgrewe, Sergey S. Lobanov, Mohammad F. Mahmood, and Alexander F Goncharov J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09103 • Publication Date (Web): 21 Nov 2016 Downloaded from http://pubs.acs.org on November 22, 2016

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The Journal of Physical Chemistry

Photochemistry within Compressed Sodium Azide Nicholas Holtgrewe1,2,*, Sergey S. Lobanov1,3, Mohammad F. Mahmood1,2, Alexander F. Goncharov1 1

Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015, USA

2

Department of Mathematics, Howard University, 2400 Sixth Street NW, Washington, DC

20059, USA 3

Sobolev Institute of Geology and Mineralogy Siberian Branch Russian Academy of Sciences, 3

Pr. Ac. Koptyga, Novosibirsk 630090, Russia *Corresponding author email: [email protected], telephone: 314-707-1975 Abstract Synthesis of high nitrogen containing materials has been the subject of research interest for use as alternative clean sources of fuel and explosives. Here we present experimental evidence for the photochemical synthesis of new energetic materials from sodium azide (NaN3) at 4.8-8.1 GPa. We show that excitation into the conduction band generates color centers within the compressed α-NaN3 phase lattice with minimal or no molecular N2 evolution. Photochemical changes to the sample were monitored by X-ray diffraction (XRD), infrared (IR) absorption, and Raman spectroscopy. These high pressure products were found to be stable upon decompression at 300 K down to 1.6 GPa, although it is suspected that the material can be recoverable to ambient pressure with cold decompression.

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Introduction The synthesis of green high energy density materials (HEDMs) over recent years has the potential for applications in new environmentally friendly fuel, explosives, and energy storage devices1. Specifically, high nitrogen containing materials have been good candidates for new fuels and explosives, considering the large energy differences between its single, double, and triple bonded forms (167, 418, and 942 kJ/mol respectively)2,3. An all-nitrogen single bonded structure would be an ideal material, with the only reaction byproducts being stable diatomic nitrogen. However, synthesis of such material, for example cubic gauche nitrogen (cg-N)4,5, has proven to be extremely difficult, requiring pressures up to 120 GPa and temperatures >2000 K. The compound was found to be metastable upon room temperature decompression down to 42 GPa, although it has been theoretically predicted to be stable at ambient conditions 6,7, along with many undiscovered nitrogen compounds 8–10. Attempts have been made to lower the synthesis pressure by addition of stabilization molecules (e.g. N2 + H2 11–14, metals for formation of nitrides 15–17) or by starting from different precursor molecules other than stable N2 (i.e. nontriple bonded forms of nitrogen), such as hydrazine (N2H4)18 ammonium azide19,20 or alkai metal azides (MN3, M+ = alkali metal, N3− = azide)21–24. This work will specifically focus on sodium azide (NaN3), but we suspect the methods shown here may be applied to all alkali metal azides and possibly all metal azides. NaN3 has been a popular choice as a precursor to high nitrogen containing materials due to its thermal and mechanical stability relative to other azide compounds. A previous high pressure study has shown the stability of molecular azide up to 120-160 GPa, transforming into an opaque non-molecular phase, which can also be accessed at lower pressures (56-86 GPa) by shear deformations or intense 514.5 nm light22. Heating to 3300 K transforms this non-molecular phase


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to a polymeric network of single bonded nitrogen, recoverable to < 1 GPa but not ambient conditions. It is clear from this study that metastable compounds exist within this low pressure regime (0-10 GPa) but cannot be accessed until high pressures (>50 GPa) are involved. One major goal of our study is to explore the formation of any metastable structures at significantly lower pressures ( 6.5 GPa37, and experimentally shown to transform from the α-NaN3 phase at 3.33 GPa and 393 K38. UV-Visible OPA fs products: A series of 2 hour irradiation runs were performed with varying wavelengths (400-650 nm) on a single DAC cavity filled with NaN3 compressed to 4.8 GPa. The method employed was 2 photon absorption into the α-NaN3 absorption edge (Fig. 3a, colored arrows)30. The absorption edge in Fig. 3a is taken from Ref.27 due to UV transmission limitations (type I diamonds) in our absorption setup. It is important to point out that this data is for NaN3 at 1 atm and 77 K (same α-NaN3 phase), but higher pressures may shift this absorption edge in energy. Fig. 3b shows the Raman spectra of the products obtained after irradiation runs with 400650 nm light. Below λ = 450 nm (hv = 2.75 eV) photon energy we see two weak, broad peaks at 113 & 366 cm-1 and two strong peaks at 1405 & 1923 cm-1, particularly noticeable in the 500 nm spectrum (Fig. 3b, turquoise). Collectively these modes resemble the η-phase (NaN3 heated/quenched to >800 K at 3.5 GPa) in Crowhurst et. al.39, theoretically predicted to be a


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N3•(N3−) complex. Based on this comparison, it’s likely that we could be accessing the most thermodynamically stable product through kinetic pathways. As we reduce the photon energy (λ ≥ 550 nm) the weaker peaks are no longer apparent and the strong peaks remain, but it is apparent the reaction is far from completion, evident by the strong presence of the α-NaN3 Raman active modes. The 650 nm irradiation run yields no reaction products (Fig. 3b, red), but it is unclear if there is definitively no reaction or the reaction rate is too slow to see any detectable product within 2 hours. From a qualitative standpoint it would appear that the 2 photon absorption cross section reduces when using lower excitation energy, and reduces substantially between 600 nm and 650 nm. The small presence of the N2 vibron (2342 cm-1) is apparent in all spectra except for 650 nm, suggesting dissociation of N3 is either an intermediate or unfavorable step in the overall reaction. Above λ = 450 nm photon energies (Fig. 3b, blue & purple) we see new Raman peaks formed in the phonon region (248, 290, 350 cm-1), weak peaks between 400-1200 cm-1, and a new sharp peak at 1688 cm-1 followed by two broader peaks at 2049 & 2149 cm-1. Collectively, these Raman peaks are nearly identical to the observed product from irradiation with X-rays, where the two are compared in Fig. 4. There are still features between 1800-2000 cm-1 that resemble the lower energy (λ > 450 nm) product, suggesting a competing reaction. We believe the difference between these two products to be indirect evidence for crossing the energy for 1 photon electron detachment from the azide anion, which has an electron affinity of 2.68 eV (EEA in Fig. 3a)40,41. Below this excitation energy, it would require 2 photons to detach the electron, consistent with our null observation of a continuous wave (cw) 488 nm (2.54 eV) photolysis run for a few hours. It was discovered that altering the power density on the sample has a profound effect on the reaction products (Fig. 5). Using 450 nm radiation as an example, increasing the average power



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from 20 to 50 µW results in a reduction of all α-NaN3 modes (161, 208, 1271, & 1386 cm-1), as well as all peaks of the product formed from lower power. The low frequency region is now a smooth, broad band out to 300 cm-1 and a new broad structure appears in the 1700-1900 cm-1 range with three decipherable peaks at 1730, 1835, and 1867 cm-1. We suggest that this peak broadening should be due a structural disordering, likely amorphization. The first two broad peaks in this latter series bear striking resemblance to features observed upon compression of NaN3 to 19 GPa 22, which are associated with interactions between azide ions and creation of new bonds. Given this resemblance and the pressure involved in our experiment (4.8 GPa), it is likely that we could be accessing the γ-NaN3 (I4/mcm) phase reported in Refs25,38. The power density at 50 µW could be sufficient to induce slight heating in the sample, able to achieve the α→γ transition within the 2 hour irradiation cycle. Unfortunately no XRD was taken on this particular sample so this cannot be completely confirmed. We conclude that power density of the excitation light is crucial to understanding and altering specific reaction products. No attempts to anneal any of the products were performed. Decompression: Both X-ray (Fig. 2) and 400 nm two photon product (Fig. 4, blue) Raman peak positions were followed as the samples were decompressed at room temperature (Fig. 6). The decompression data for both products look nearly identical, reassuring that the products are of the same nature. All peaks decreased in frequency as the pressure was lowered, albeit some more rapidly than others, perhaps suggesting the presence of combination bands or overtones. When samples were decompressed to pressures between 0.6-1.6 GPa (still within the stability field of α-NaN3 phase) they were found to decompose into another unknown compound. In the decompression of the X-ray product obtained in Fig. 2 this decomposition was found to be correlated with the disappearance of the color centers (Fig. 7a). The color centers red shift upon


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decompression from 8.1 to 1.6 GPa from about 550 to 590 nm, respectively (Fig. 7b), and finally disappear between 1.6 and 0.6 GPa. The Raman spectrum for the decompressed X-ray product at 0.6 GPa is shown in Fig. 8. Further decompression to ambient pressure shows that the sample reverts back to the β-NaN3 phase with a small presence of additional unknown Raman peaks. The sample becomes reactive to the incident 488 nm Raman laser (apparent from the formation of visually observed dark spots), but only in areas previously exposed to irradiation at higher pressure. Even using 660 nm Raman excitation, the product was still found to decompose into N2 and what resembles the low pressure γ-phase from Ref.39 with an additional peak at 1826 cm-1. The Raman peaks for this product are listed in Table 1. These low pressure products were not investigated further. Discussion Two photon absorption: As mentioned in the results, the mechanism behind the various reaction products in the UV-visible photolysis runs is thought to be initiated by two photon excitation into the absorption edge (Fig. 3a). With the observation of products formed using 600 nm light (2hv = 4.13 eV) and the null result using 650 nm light (2hv = 3.81 eV), we can narrow the absorption edge to lie somewhere between the two energies. Although the ambient pressure absorption edge shown in Fig. 2a is higher in energy than the lowest doubled excitation frequency that produces reaction products (i.e. 600 nm), one would expect the absorption edge to red shift with increasing pressure. We discuss here the changes in Raman spectra of reaction products created after irradiation between 500 nm and 450 nm in Fig. 3b. In addition to color center formation at both irradiation wavelengths, we associate the different products due to the ability for photons λ ≤ 450 nm (hv ≥



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2.76 eV) having energies capable of 1 photon electron detachment from N3− to produce azide radicals and free electrons (Eq. 1). (1) + ℎ> 2.68 → ● + (a) ● + → ? (b) + → In gas phase conditions the energy required to remove the electron and form the open shell species is 2.68 eV (λ = 462 nm, EEA in Fig. 3a) 40,41, and above this energy the formation of azide radicals could lead to additional reactions between the radical and neighboring N3− ions (Eq. 1a). The newly detached electron is free to wander the lattice until it occupies single or double anion vacancies (i.e. F and F2+ color centers, Figs. 2b, 7, Eq. 1b), with higher photon energies corresponding to higher electron kinetic energies. It is apparent from the spectra in Fig. 3b that no reaction occurs in irradiation with 650 nm light even though 2 photons of λ = 650 nm (2hv = 3.81 eV) have sufficient energy to detach an electron from N3− (EEA = 2.68 eV). A few reasons for the lack of products could be that detaching an electron from N3− by 2 photon absorption (a) does not induce a reaction due to recombination with the N3 radical, (b) does not induce a reaction because 2 photon absorption is favored into the absorption edge (valid for irradiation with λ ≤ 600 nm), (c) does not detach the electron due to 2 photon selection rules, or (d) is not detectable within a 2 hour irradiation cycle. The exact mechanism is outside the scope of this work, but nevertheless, we demonstrate it is still possible to generate the same product as X-rays using 2 photon absorption with significantly lower photon energies (Fig. 4). Color center formation and relation to new nitrogen compounds: Color center formation in irradiated NaN3 has been known for quite some time 27,28. Absorption of X-rays or UV light (λ =


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253.7 nm) into the absorption edge was the determined mechanism by which F and F2+ centers, 1 electron in single and double anion vacancies respectively, are generated and absorb in the visible region (Figs. 2b, 7). N2 gas was reported as the main reaction product from dissociation of the N3− ion. Although these past experiments were done at 1 atm and 77 K, NaN3 has the same monoclinic cell at higher pressure (> 0.3 GPa). Unlike previous irradiation experiments, the NaN3 in our experiments is confined to the cavity of the DAC. If N2 were produced, similar to ambient pressure color center formation, one would expect a much larger Raman signature than what is observed in Figs. 2c, 3b. Hence we come to the conclusion that N2 is either produced as an intermediate product, further reacting with other neighboring molecules, or is not the main reaction pathway. We find the former to be unlikely as neutral N2 is a highly stable species that typically requires significantly higher pressures to undergo pressure induced reactions42. When combined with observations of lattice vacancies (Figs. 2b, 7) (i.e. unaccounted for missing N3− molecules) and no observable alteration to the monoclinic unit cell in XRD, we can conclude a new Nx or higher ratio NaNx (x > 3) compound must be formed upon either X-ray irradiation or 2 photon absorption into the absorption edge. Based on no evidence for new patterns in the XRD images, it would appear that whatever new species the Raman/IR is detecting, the substance is either amorphous or composed of nanoclusters. Comparison with past studies: There have been very few experimental photochemistry studies on compressed NaN3 to date43, most of which were noted reactions with the incident Raman laser22,39. Crowhurst et. al.39 reported products, which they refer to as ε- or δ-phase, from reacting NaN3 with 488 nm cw light at 1.2 GPa. We see similar Raman spectral characteristics upon decompression of reacted cells below 1.4 GPa and also report reaction with our incident Raman 488 nm beam. However, in addition to their low pressure spectra we report more peaks in the



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low frequency and > 2500 cm-1 region, the latter most likely composed of vibrational overtones/combination bands. Eremets et. al.22 found that above 56 GPa the sample darkens, which they linked to formation of an amorphous state. This darkening was found to accelerate by increasing the 514.5 nm cw laser power, completely eliminating the presence of the azide N3− symmetric stretch mode. In relation to our observations, it is possible that at these high pressures the band gap starts to close and 514.5 nm (hv = 2.41 eV) photons have sufficient energy to be absorbed and generate vacancies (i.e. darkening the sample). While we see similar results (darkening of areas and decreasing αNaN3 phase Raman intensities), it is difficult to compare the Raman spectra of the products due to the large difference in pressures. It is interesting to note that when the sample of Ref. 22 was decompressed from 120 GPa two of their Raman spectra at < 1 GPa bare slight resemblance to our photoinduced products both at high pressure (4.8-8.1 GPa) and decompressed to 0.6 GPa (Fig. 2c, 3b, & 8). Photolysis of compressed NaN3 (1-4 GPa) using 7-8 µs 514 nm pulses was accomplished by Peiris et. al.43. They provide evidence for temporary N3 radical and N6− formation from changes in the absorption spectrum as a function of time after the initial laser pulse. Perhaps most important to our results, they report a Raman spectrum of a new product generated after the experiments and describe it as a “red liquid”, although the “liquid” has no strong absorbance between 550-850 nm. We suspect this null absorbance reading is because of the thin sample size used. It is tempting to claim based on the “red” description that what they observed were actually F-type vacancies within the lattice, but unfortunately the added complexity of NaCl as a pressure medium complicates matters due to possible reactions with Cl radicals. Nevertheless, the Raman spectrum of the “red liquid” contains certain peaks that could be related to our generated product


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(Fig. 4, blue), but mostly resembles the material observed in Crowhurst et.al39. They report the “red liquid” was not maintained when decompressed back to ambient pressure, consistent with our observed results (Fig. 7). Products: Although we do not definitively claim to know the newly synthesized material, we have inferred based on spectral analyses (Raman/IR/XRD) that either an all nitrogen Nx or NaNx (x > 3) compound must be formed in the photolysis. We are most likely generating multiple Nx or NaNx compounds, which makes the spectra difficult to interpret, but nevertheless we can take a brief look at what facts we know about the synthesis and what we can speculate based on the experimental spectra. The Raman peaks for the three unknown products from photolysis and decompression are summarized in Table 1, which can be categorized by their pressure ranges at room temperature, > 1.6 GPa (Figs. 2c, 3b), 0.3-1.6 GPa (Fig. 8), and

Photochemistry within Compressed Sodium Azide - The Journal of

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