Synthesis of FeN4 at 180 GPa and its crystal structure from a submicron-sized grain

The refined crystal structure model of iron tetranitride, FeN4, at 180 GPa is similar to that at 135 GPa but shows improved structural parameters in terms of bond lengths and angles.


Chemical context
Polynitrogen compounds have attracted great interest because of their potential applications as high-energy-density materials. Recently, a variety of nitrogen-rich compounds containing polymeric and oligomeric nitrogen chains, N 5 or N 6 rings, or even more complex networks have been predicted to be stable at high pressures (Steele & Oleynik, 2016, 2017Zhang et al., 2017;Xia et al., 2018). Predicted lithium and caesium pentazolates LiN 5 and CsN 5 were successfully synthesized at high-pressure conditions via the reaction between a metal or metal azide and nitrogen (Laniel et al., 2018;. Recently, Bykov and co-workers synthesized two compounds containing polymeric nitrogen chains, viz. an inclusion compound ReN 8 ÁN 2 (Bykov et al., 2018b) and iron tetranitride, FeN 4 (Bykov et al. 2018a) via the reaction between Fe or Re and nitrogen in a laser-heated diamond anvil cell (DAC). The crystal structures of these compounds were studied at pressures up to 135 GPa by means of single-crystal X-ray diffraction (SCXRD).
The higher the pressures, the more challenging are synthesis and diffraction studies in DACs, even at dedicated highpressure stations at the 3rd generation synchrotron facilities where the X-ray beam can be focused down to 2-3 mm. First of all, at pressures exceeding 150 GPa, the size of the sample is of only about 10 mm or less, and single-crystalline grains of the reaction product(s) are often of submicron size, which results in a drastic worsening of the signal-to-noise ratio in SCXRD. Additionally, the contribution of parasitic diffraction from the gasket material increases with pressure because the sample chamber becomes smaller upon compression. Submicron focusing of the X-ray beam, which is possible on some synchrotron beamlines, can provide suitable conditions to collect SCXRD data at multi-megabar pressures. Here we report the synthesis of FeN 4 from the elements at a pressure of about 180 GPa and provide the structure refinement for FeN 4 against SCXRD data at this pressure. The X-ray beam focusing down to 0.3Â0.3 mm 2 at the synchrotron beamline ID11 (ESRF, Grenoble, France) allowed us to collect SCXRD data from an FeN 4 grain with linear dimensions of about 0.5 mm.

Structural commentary
The crystal structure (Fig. 1a,b) and the unit-cell volume ( Fig. 2) of FeN 4 at 180 GPa are in a good agreement with the structural model for this compound at 135 GPa and its equation of state as reported by Bykov et al. (2018a). Despite the increased pressure, as a result of the application of the submicron beam focusing, the quality of the SCXRD data collected at 180 GPa turned out to be much better. Thus, the quality of the structure refinement of FeN 4 based on the 180 GPa data set is significantly improved in comparison with that for the 135 GPa data set. This is evident from a comparison of such important refinement indicators such as the data-to-parameter ratio (7.1 vs 4.8), Á max /Á min (0.76/À0.56 vs 0.98/À1.09 e Å À3 ) and R 1 [I>2(I)] (0.040 vs 0.064). Furthermore, the precision of the bond lengths and angles is significantly improved ( Table 1).

Figure 1
The The key parameters for the synthesis of polynitrides are pressure-temperature conditions and the choice of metal and/ or nitrogen precursors. High temperatures and pressures are required to overcome the kinetic barrier for breaking the triple N N bond, to increase the chemical potential of nitrogen and to stabilize the reaction products (Sun et al., 2017). It is known that increasing pressure allows compounds with higher nitrogen content to be obtained, e.g. for the Fe-N system Fe x N (x = 2-8) can be synthesized at ambient and low pressures (Ertl et al. 1979), FeN at 12 GPa (Clark et al., 2017), FeN 2 at 60 GPa, and FeN 4 at 106 GPa (Bykov et al., 2018a). Interestingly, at a given pressure, different metals stabilize different types of nitrogen networks. For example, ReN 8 ÁN 2 synthesized at 106 GPa contains polydiazene chains [-N N-] 1 (Bykov et al., 2018b), whereas alkali metals form pentazolate salts at even lower pressures (Laniel et al., 2018;, i.e. the type of metal, the variety of its oxidation states, and its ionic radius play an important role in the chemistry of the nitrogen network. The current study shows that FeN 4 can be synthesized in a broad pressure range from 106 to 180 GPa. Such an extended stability range for this compound may be related to the favourable sixfold coordination of Fe. On one hand, it perfectly matches the 18 e À rule (Bykov et al., 2018a), and on the other hand, for the Fe-N system coordination number 6 is geometrically preferable. Further systematic studies of various metal polynitrides will allow empirical rules for the design of novel materials at different pressure and temperature conditions to be formulated.

Synthesis and crystallization
A piece of iron powder (Sigma Aldrich, 99.99%) was loaded inside a sample chamber of a BX90-type diamond anvil cell equipped with double-bevelled Boehler-Almax type diamonds (culet diameter 40 mm). Nitrogen was used as a pressure-transmitting medium and as a reagent for the synthesis. The sample was compressed up to 180 GPa and laser-heated from both sides up to 2700 (200) K. The pressure was determined using the equation of state (EoS) of hcp-iron. As there are several equations of state of iron in the literature (Table 2), for a given unit-cell volume of iron V Fe = 15.171 (5) Å 3 one can get slightly different pressures in the range 173.5 to 187.5 GPa with an average of 179.8(5.2) GPa. Taking into account this uncertainty in the pressure determination, we accepted the rounded value of 180 GPa.
In order to locate the FeN 4 grain in the sample chamber we used the following strategy: we collected 27 Â 27 = 729 still images with the exposure time of 6 s. Before taking the next image, either the horizontal or vertical motor was moved by 0.5 mm, allowing a 13 Â 13 mm 2 X-ray diffraction map of the sample chamber to be built up (Fig. 3). The images were then analyzed with XDI software (Hrubiak, 2017).
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 ) x y z U iso */U eq