Synthesis and Compressibility of Novel Nickel Carbide at Pressures of Earth’s Outer Core

: We report the high ‐ pressure synthesis and the equation of state (EOS) of a novel nickel carbide (Ni 3 C). It was synthesized in a diamond anvil cell at 184(5) GPa through a direct reaction of a nickel powder with carbon from the diamond anvils upon heating at 3500 (200) K. Ni 3 C has the cementite ‐ type structure (Pnma space group, a = 4.519(2) Å, b = 5.801(2) Å, c = 4.009(3) Å), which was solved and refined based on in ‐ situ synchrotron single ‐ crystal X ‐ ray diffraction. The pressure ‐ volume data of Ni 3 C was obtained on decompression at room temperature and fitted to the 3rd order Burch ‐ Murnaghan equation of state with the following parameters: V 0 = 147.7(8) Å 3 , K 0 = 157(10) GPa, and K 0 ʹ = 7.8(6). Our results contribute to the understanding of the phase composition and properties of Earth’s outer core. interconnected CNi trigonal prisms. Compared to the predicted compressional behavior


Introduction
Nickel is known as the second most abundant element in Earth's core after iron [1,2]. Cosmochemical models and studies of meteorites suggest that Earth's core apart from Fe contains also about 5 wt.% of Ni [3,4] and, in the inner core, up to 10 wt.% of light elements [5][6][7]. Which elements exactly and their amount is a subject of active discussions [3]. A large amount of carbon in iron meteorites [8], its high solubility in liquid Fe at high pressure [5,9], and high abundance in the solar system [5] suggest carbon to be one of the most important light elements in Earth's core. Recent estimations of the inner core composition indicate up to 2.0 wt.% of carbon [3]. All these facts resulted in numerous high-pressure studies of the Fe-C system in recent decades. The intermediate Fe-C compounds Fe3C and Fe7C3 were suggested to be the most likely candidates to the carbonbearing phases in Earth's core, as they were found at relevant pressures and temperatures [2,5,[10][11][12]. Although at room temperature Fe3C was shown to be stable up to 187 GPa, it decomposes into a mixture of solid Fe7C3 and hcp-Fe at above 145 GPa upon laser heating and transforms into Fe-C liquid and solid Fe7C3 at temperatures of above 3400 K [13]. Moreover, the high Poisson's ratio of Fe7C3 at high pressures [2] indicates that the presence of carbon may significantly affect the elastic properties of iron. This corroborates the Preliminary Reference Earth Model (PREM) [14], which suggests the material of Earth's inner core also has a high Poisson's ratio.
Contrary to the binary iron-carbon system, the Fe -Ni -C, and Ni -C systems at high PT conditions are still poorly understood. Nickel can strongly modify the physical properties of pure Fe at elevated pressures and temperatures. Recent studies have shown that Ni alloying on Fe does not affect the melting temperature of Fe up to 100 GPa; however, it modifies its phase boundary by shifting the hcp/fcc/liquid triple point to the higher pressure-temperature region [6]. For example, for Fe-20 wt.% Ni alloy the triple point was found to be at 170 (20) GPa and 4000(400) K [6] as compared to 100(10) GPa and 3500(200) K for pure Fe [15]. Pressure-induced Invar effect in Fe-Ni alloys was reported by Dubrovinsky et al. [16]. The thermal expansion of the alloys Fe0.55Ni0.45 and Fe0.20Ni0.80 was found to be extremely low in the temperature interval of 291 K to 500 K at pressures of 7.7 and 12.6 GPa, correspondingly [16]. It was also proven that alloys of Fe with Ni have significantly higher strength in comparison with pure Fe [17]. The mineral cohenite, (Fe, Ni)3C, which is isostructural to Fe3C, was found in iron meteorites [18] and predicted to be stable at high pressures [19]. However, a pure-Ni cementite-type phase (Ni3C) has never been reported before.
Here, we report the synthesis and EOS of a novel high-pressure phase of nickel carbide (Ni3C) in a laser-heated diamond anvil cell (LHDAC) at 184(5) GPa and 3500(200) K which was solved and refined using in-situ synchrotron single-crystal X-ray diffraction.

Materials and Methods
In our experiments, we used the BX90-type large X-ray aperture Diamond Anvil Cell (DAC) equipped with Boehler-Almax type diamonds with 80 μm culet diameter. To form the sample chamber, a rhenium gasket was preindented to ~ 20 μm thickness and a hole of 40 μm in diameter was drilled at the center of the indentation. A nickel powder was clamped between two thin layers of LiF inside the DAC's sample chamber. LiF played a role of a pressure transmitting and thermal insulating medium in order to decrease temperature gradients in the sample during laser heating [20]. The pressure was determined using the equations of states (EOSes) of Ni [21] and monitored additionally using Raman signal from the diamond anvils [22].
The laser-heating (LH) of the samples was performed using in house laser heating setup [23]. The double-sided LH system is equipped with two YAG lasers (1064 nm central wavelength) and the IsoPlane SCT 320 spectrometer with a 1024x2560 PI-MAX 4 camera for the collection of thermal emission spectra from the heated spot. Temperatures were determined by fitting of thermal emission spectra of the sample to the grey body approximation of Planck's radiation function in a given wavelength range (570-830 nm).
To analyze the SCXRD data we used the CrysAlisPro software [25]. The analysis procedure includes the peak search, finding reflections belonging to a unique singlecrystal domain, indexing, and data integration. The crystal structures were solved using ShelXT [26] structure solution program and refined with the JANA 2006 software [27].
Powder diffraction measurements were performed either without or upon continuous sample rotation about the ω axis of a diffractometer in the range of ±20°. The images were integrated into powder patterns with Dioptas software [28] and analyzed with Le Bail fitting technique using TOPAS 4.2. The parameters of the equation of state were obtained by fitting the pressure-volume data using EoSFit7-GUI software [29].

Results and Discussion
Sample of Ni powder was pressurized in LiF pressure-transmitting medium up to 184 (5) GPa and laser-heated up to 3500 (200) K by scanning of the Ni sample with a laser beam. A direct reaction between Ni and carbon from the diamond anvil resulted in the synthesis of a new compound indexed as orthorhombic (Figure 1). In order to localize the point of interest, high-resolution two-dimensional X-ray diffraction mapping (40x40 steps of 1 μm each) through the whole sample was realized at the ID11 beamline at the ESRF (Figure 2). The reaction products consist of numerous single-crystalline grains that were identified using synchrotron single-crystal XRD. For one of such grains (one crystallite domain), we were able to collect 182 independent reflections and reduce the data with Rint = 7.3% at 184 (5) GPa. The structure solution and refinement (final R1 = 6.4%, see Table 1) revealed the cementite-type orthorhombic structure (space group Pnma, #62; a = 4.520(3) Å, b = 5.8014(17) Å, c = 4.009(4) Å at 184 (5) GPa) and the Ni3C chemical composition (Table  1, Supplementary Material, Crystallographic Information File: Ni3C_184GPa.cif). 3.09, -3.51 The structure can be described as built of distorted trigonal prisms formed by six nickel atoms coordinating a C atom (Figure 3). The Ni-C distances in the prism vary from 1.760 (19) to 1.830(16) Å at 184 (5) GPa. The trigonal prisms, interconnected through sharing edges and corners, form layers parallel to the ac plane stacking along the b direction. The previously observed trigonal Ni3C (R-3c space group), which is a product of the thermal decomposition of Ni succinate [30] is built of CNi6 octahedra with an average Ni-C distance of 1.86 Å. Thus, the average Ni-C distance depends on the coordination of C atoms. Our data suggest that at ambient pressure the average Ni-C distance in CNi6 trigonal prisms should be significantly lager compared to that inCNi6 octahedra. The Ni3C sample was studied on a stepwise decompression. SCXRD data were collected at seven pressure points down to 84(2) GPa. Below 84(2) GPa no diffraction pattern from Ni3C was observed; however, the reason remained unclear. That means the question as to if the quality of the sample deteriorated or the phase decomposed or amorphized stays open. The pressure-volume data ( Table 2) of Ni3C was fitted to the 3rd order Birch-Murnaghan (BM3) EOS and gave the following parameters: V0 = 147.7(8) Å 3 ; K0 = 157(10) GPa, K´ = 7.8(6) (Figure 4).   (3) 108.9 (4) 160 (4) 107.4 (4) 170 (4) 106.3 (3) 184 (5) 105.1 (2) Figure 5 demonstrates experimental data on Ni3C axial compression. The structure is most compressible along the b axis, the direction of stacking of the layers of interconnected CNi6 trigonal prisms. Compared to the predicted compressional behavior ofFe3C in the same pressure region [34], Ni3C is highly anisotropic. Based on obtained data, we calculated the bulk sound velocity for Ni3C as a function of pressure at 293 K and compared it with those known for Fe, Ni, and possible carbonbearing components of Earth's core (Fe3C and Fe7C3). Figure 6 shows that within the errors Ni3C exhibits similar bulk velocities as Fe3C and Fe7C3 at pressures up to 400 GPa. Figure 6. Calculated bulk sound velocity as a function of pressure for Ni3C (this study, black solid line with circles); Fe3C (green line with diamonds [33]) and Fe7C3 (blue line with squares [2]); Ni (red line with triangles [21]); Fe (purple line with pentagons [35]) at 293 K.
Thereby, the presence of Ni in the alloy likely should not affect the elastic properties of the Fe-Ni-C system at high pressure but potentially can change the carbon distribution. Due to the stability of Ni3C at conditions of Earth's outer core, it may be considered as one of the likely candidates to carbon-bearing phases in the core along with Fe7C3.

Conclusion
In the presented work, we have synthesized a nickel carbide yet unknown at ambient conditions. It was shown that Ni reacts with carbon at high-pressure and hightemperature conditions that result in the formation of an orthorhombic Ni3C compound (space group Pnma, a = 4.520(3) Å, b = 5.8014(17) Å, c = 4.009(4) Å at 84(5) GPa) with the cementite-type structure revealed using synchrotron single-crystal X-ray diffraction. The Ni3C was studied on decompression down to 84(2) GPa. We have shown that in the pressure range 84(2)-185(5) GPa, Ni3C is less compressible than cementite (Fe3C); the calculated bulk sound velocities are similar to those known for Fe3C and Fe7C3 at pressures up to 400 GPa and 297 K. Ni3C remains stable at pressure-temperature conditions relevant to Earth's core and thus can be considered as one of the likely candidates to carbonbearing phases in the core along with Fe7C3.

Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Data Availability Statement:
The data that support the findings of this study are available from the corresponding author upon reasonable request.