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

Iron tetra-nitride, FeN4, was synthesized from the elements in a laser-heated diamond anvil cell at 180 (5) GPa and 2700 (200) K. Its crystal structure was determined based on single-crystal X-ray diffraction data collected from a submicron-sized grain at the synchrotron beamline ID11 of ESRF. The compound crystallizes in the triclinic space group P . In the asymmetric unit, the Fe atom occupies an inversion centre (Wyckoff position 1d), while two N atoms occupy general positions (2i). The structure is made up from edge-sharing [FeN6] octa-hedra forming chains along [100] and being inter-connected through N-N bridges. N atoms form catena-poly[tetraz-1-ene-1,4-di-yl] anions [-N=N-N-N-]∞ 2- running along [001]. In comparison with the previously reported structure of FeN4 at 135 GPa [Bykov et al. (2018). Nat. Commun. 9, 2756], the crystal structure of FeN4 at 180 GPa is similar but the structural model is significantly improved in terms of the precision of the 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, N5 or N6 rings, or even more complex networks have been predicted to be stable at high pressures (Steele & Oleynik, 2016 ▸, 2017 ▸; Zhang et al., 2017 ▸; Xia et al., 2018 ▸). Predicted lithium and caesium penta­zolates LiN5 and CsN5 were successfully synthesized at high-pressure conditions via the reaction between a metal or metal azide and nitro­gen (Laniel et al., 2018 ▸; Steele et al., 2017 ▸). Recently, Bykov and co-workers synthesized two compounds containing polymeric nitro­gen chains, viz. an inclusion compound ReN8·N2 (Bykov et al., 2018b ▸) and iron tetra­nitride, FeN4 (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 high-pressure stations at the 3rd generation synchrotron facilities where the X-ray beam can be focused down to 2–3 µm. First of all, at pressures exceeding 150 GPa, the size of the sample is of only about 10 µm 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 FeN4 from the elements at a pressure of about 180 GPa and provide the structure refinement for FeN4 against SCXRD data at this pressure. The X-ray beam focusing down to 0.3×0.3 µm2 at the synchrotron beamline ID11 (ESRF, Grenoble, France) allowed us to collect SCXRD data from an FeN4 grain with linear dimensions of about 0.5 µm.

Structural commentary

The crystal structure (Fig. 1 ▸ a,b) and the unit-cell volume (Fig. 2 ▸) of FeN4 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 FeN4 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), Δρmax/Δρmin (0.76/−0.56 vs 0.98/−1.09 e Å−3) and R 1[I>2σ(I)] (0.040 vs 0.064). Furthermore, the precision of the bond lengths and angles is significantly improved (Table 1 ▸).

Figure 1

The crystal structure of FeN4 at 180 GPa. (a) and (b) Projections of the crystal structure along [100] and [001], respectively. (c) A fragment of the crystal structure showing the coordination of Fe atoms. [Symmetry codes: (i) x, y, 1 + z; (ii) 1 − x, −y, −z; (iii) −x, 1 − y, −z; (iv) 1 + x, 1 + y, 2 + z; (v) −x, −1 − y, −1 − z; (vi) 1 + x, 1 + y, 1 + z; (vii) 1 − x, −y, 1 − z; (viii) 1 + x, 1 + y, z.] (d) The crystal structure of FeN4 in polyhedral representation.

Figure 2

Pressure-dependence of the unit-cell volume of FeN4. Blue points and the equation of state (blue line) are taken without modification from Bykov et al. (2018a ▸) [V 94.5 = 29.94 (4) Å3, K 94.5 = 603 (22) GPa, K′94.5 = 4.0 (fixed)]. Red point – current study.

Table 1

Selected bond lengths for FeN4 at 135 GPa (Bykov et al., 2018a ▸) and at 180 GPa (this study)

	135 GPa	180 GPa
Fe—N1	1.73 (2)	1.707 (10)
Fe—N1i	1.73 (2)	1.707 (10)
Fe—N2ii	1.81 (3)	1.783 (14)
Fe—N2iii	1.81 (3)	1.783 (14)
Fe—N2iv	1.78 (3)	1.763 (6)
Fe—N2v	1.78 (3)	1.763 (6)
N1—N1vi	1.29 (5)	1.277 (14)
N1—N2	1.30 (3)	1.298 (8)
N2—N2vii	1.43 (4)	1.37 (3)

Symmetry codes: (i) −x + 1, −y, −z; (ii) x + 1, y, z + 1; (iii) −x, −y, −z − 1; (iv) −x + 1, −y, −z − 1; (v) x, y, z + 1; (vi) −x, −y − 1, −z − 1; (vii) −x, −y − 1, −z − 2.

The Fe1 atom occupies an inversion centre of space-group type P (Wyckoff position 1d), while the two nitro­gen atoms N1 and N2 occupy general positions (2i). The iron atom is coordinated by six nitro­gen atoms, forming a distorted octa­hedron. [FeN6] octa­hedra share opposite edges, thus forming infinite chains along [100]. These chains are inter­connected through N—N bridges as shown in Fig. 1 ▸ d. The covalently bonded nitro­gen atoms form infinite zigzag chains running along [001] (Fig. 1 ▸ a–c). The N1 atom has a trigonal–planar coordination, while N2 is tetra­hedrally coordinated, suggesting sp 2 and sp 3 hybridization, respectively. In agreement with the study of Bykov et al. (2018a ▸), the N—N distances increase in the following order d(N1—N1) < d(N1—N2) < d(N2—N2) (Table 1 ▸), supporting the conclusion that the N1—N1 bond is a double-bond, while N1—N2 and N2—N2 bonds are single bonds. Therefore, the nitro­gen atoms form catena-poly[tetraz-1-ene-1,4-di­yl] anions [–N=N—N—N–]∞ 2–.

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 FeN (x = 2–8) can be synthesized at ambient and low pressures (Ertl et al. 1979 ▸), FeN at 12 GPa (Clark et al., 2017 ▸), FeN2 at 60 GPa, and FeN4 at 106 GPa (Bykov et al., 2018a ▸). Inter­estingly, at a given pressure, different metals stabilize different types of nitro­gen networks. For example, ReN8·N2 synthesized at 106 GPa contains polydiazene chains [–N=N–]∞ (Bykov et al., 2018b ▸), whereas alkali metals form penta­zolate salts at even lower pressures (Laniel et al., 2018 ▸; Steele et al., 2017 ▸), 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 FeN4 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 − 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 µm). 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) Å3 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.

Table 2

Pressures for FeN4 synthesis based on different 3rd order Birch–Murnaghan EoS’s of hcp-Fe reported in the literature [V Fe = 15.171 (5) Å3/unit cell]

Reference	V 0 (Å3)	K (GPa)	K′	Pressure (GPa)
Dewaele et al. (2006 ▸)	22.468 (24)	165 (fixed)	4.97 (4)	173.5(2.2)
Fei et al. (2016 ▸)	22.428 (fixed)	172.7(1.4)	4.79 (5)	174.1(1.4)
Sakai et al. (2014 ▸)	22.18 (20)	179.6(2.2)	4.91 (12)	174.9(2.1)
Mao et al. (1990 ▸)	22.35 (3)	164.8(3.6)	5.33 (9)	179.8(4.3)
Yamazaki et al. (2012 ▸)	22.15 (5)	202 (7)	4.5 (2)	181.0(5.6)
Dubrovinsky et al. (2000 ▸)	22.35 (3)	155.6(3.5)	5.81 (6)	183.7(4.8)
Garai et al. (2011 ▸)	22.33 (3)	164 (2)	5.52 (5)	183.9(2.5)
Boehler et al. (2008 ▸)	22.46 (4)	160 (6)	5.6 (2)	187.5(8.2)

In order to locate the FeN4 grain in the sample chamber we used the following strategy: we collected 27 × 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 µm, allowing a 13 × 13 µm2 X-ray diffraction map of the sample chamber to be built up (Fig. 3 ▸). The images were then analyzed with XDI software (Hrubiak, 2017 ▸).

Figure 3

X-ray diffraction imaging of the sample chamber at 180 GPa. The colour intensity is proportional to the intensity of the following reflections: the (100) reflection of Re for the green region; the (101) reflection of Fe for the orange region; the sum of the (101), (11), (0 0 2), and ( 2) reflections of FeN4 for the blue region.

Refinement

Crystal data, data collection details and structure refinement details are summarized in Table 3 ▸. We have used the same non-reduced unit-cell setting and the structure model of FeN4 at 135 GPa (Bykov et al., 2018a ▸) for refinement of the crystal structure of FeN4 at 180 GPa. As a result of the limited angular range caused by the laser-heated DAC and the very small crystal size, the resolution of the data set was not sufficient to refine the atoms with anisotropic displacement parameters. Hence they were refined with isotropic displacement parameters only.

Table 3

Experimental details

Crystal data
Chemical formula	FeN4
M r	111.89
Crystal system, space group	Triclinic, P
Temperature (K)	293
a, b, c (Å)	2.4473 (10), 3.4688 (14), 3.5144 (13)
α, β, γ (°)	105.22 (4), 110.60 (4), 91.39 (3)
V (Å3)	26.72 (2)
Z	1
Radiation type	Synchrotron, λ = 0.30996 Å
μ (mm−1)	1.33
Crystal size (mm)	0.0005 × 0.0005 × 0.0005
Data collection
Diffractometer	ID11 @ ESRF
Absorption correction	Multi-scan (ABSPACK; Oxford Diffraction, 2005 ▸)
T min, T max	0.967, 1.000
No. of measured, independent and observed [I > 2σ(I)] reflections	117, 71, 70
R int	0.020
(sin θ/λ)max (Å−1)	0.901
Refinement
R[F 2 > 2σ(F 2)], wR(F 2), S	0.040, 0.082, 1.18
No. of reflections	71
No. of parameters	10
Δρmax, Δρmin (e Å−3)	0.76, −0.56

Computer programs: CrysAlis PRO (Rigaku OD, 2018 ▸), SHELXT (Sheldrick, 2015a ▸), SHELXL2014 (Sheldrick, 2015b ▸) and OLEX2 (Dolomanov et al., 2009 ▸).

Crystal structure: contains datablock(s) I. DOI: 10.1107/S2056989018012161/wm5459sup1.cif

Structure factors: contains datablock(s) I. DOI: 10.1107/S2056989018012161/wm5459Isup2.hkl

CCDC reference: 1864279

Additional supporting information: crystallographic information; 3D view; checkCIF report

FeN4	Z = 1
Mr = 111.89	F(000) = 54
Triclinic, P1	Dx = 6.953 Mg m−3
a = 2.4473 (10) Å	Synchrotron radiation, λ = 0.30996 Å
b = 3.4688 (14) Å	Cell parameters from 68 reflections
c = 3.5144 (13) Å	θ = 2.8–16.1°
α = 105.22 (4)°	µ = 1.33 mm−1
β = 110.60 (4)°	T = 293 K
γ = 91.39 (3)°	Irregular, black
V = 26.72 (2) Å3	0.001 × 0.001 × 0.001 mm

ID11 @ ESRF diffractometer	71 independent reflections
Radiation source: synchrotron	70 reflections with I > 2σ(I)
Synchrotron monochromator	Rint = 0.020
ω scans	θmax = 16.2°, θmin = 2.8°
Absorption correction: multi-scan (ABSPACK; Oxford Diffraction, 2005)	h = −3→3
Tmin = 0.967, Tmax = 1.000	k = −5→4
117 measured reflections	l = −6→5

Refinement on F2	0 restraints
Least-squares matrix: full	Primary atom site location: dual
R[F2 > 2σ(F2)] = 0.040	w = 1/[σ2(Fo2) + (0.0282P)2 + 0.3122P] where P = (Fo2 + 2Fc2)/3
wR(F2) = 0.082	(Δ/σ)max < 0.001
S = 1.18	Δρmax = 0.76 e Å−3
71 reflections	Δρmin = −0.56 e Å−3
10 parameters

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

	x	y	z	Uiso*/Ueq
Fe	0.5000	0.0000	0.0000	0.0072 (4)*
N1	0.163 (4)	−0.346 (4)	−0.485 (2)	0.0066 (10)*
N2	0.065 (3)	−0.309 (4)	−0.861 (2)	0.0068 (10)*

Fe—Fei	2.4473 (10)	Fe—N2vii	1.763 (6)
Fe—Feii	2.4473 (10)	N1—N1viii	1.277 (14)
Fe—N1	1.707 (10)	N1—N2	1.298 (8)
Fe—N1iii	1.707 (10)	N2—Feix	1.763 (6)
Fe—N2iv	1.783 (14)	N2—Fex	1.783 (14)
Fe—N2v	1.783 (14)	N2—N2xi	1.37 (3)
Fe—N2vi	1.763 (6)
Fei—Fe—Feii	180.0	N2vii—Fe—Feii	46.7 (5)
N1—Fe—Fei	96.6 (4)	N2iv—Fe—Feii	133.98 (18)
N1iii—Fe—Feii	96.6 (4)	N2iv—Fe—Fei	46.02 (18)
N1—Fe—Feii	83.4 (4)	N2vii—Fe—N2v	92.7 (5)
N1iii—Fe—Fei	83.4 (4)	N2vi—Fe—N2v	87.3 (5)
N1iii—Fe—N1	180.0	N2v—Fe—N2iv	180.0 (7)
N1—Fe—N2vii	81.3 (4)	N2vii—Fe—N2iv	87.3 (5)
N1iii—Fe—N2vii	98.7 (4)	N2vi—Fe—N2iv	92.7 (5)
N1iii—Fe—N2vi	81.3 (4)	N2vi—Fe—N2vii	180.0
N1—Fe—N2vi	98.7 (4)	N1viii—N1—Fe	118.9 (9)
N1—Fe—N2iv	90.5 (5)	N1viii—N1—N2	109.6 (9)
N1—Fe—N2v	89.5 (5)	N2—N1—Fe	129.4 (9)
N1iii—Fe—N2v	90.5 (5)	Feix—N2—Fex	87.3 (5)
N1iii—Fe—N2iv	89.5 (5)	N1—N2—Fex	110.9 (12)
N2vi—Fe—Fei	46.7 (5)	N1—N2—Feix	126.9 (5)
N2vi—Fe—Feii	133.3 (5)	N1—N2—N2xi	107.6 (12)
N2vii—Fe—Fei	133.3 (5)	N2xi—N2—Fex	113.1 (6)
N2v—Fe—Feii	46.02 (18)	N2xi—N2—Feix	109.9 (9)
N2v—Fe—Fei	133.98 (18)