Synthesis and crystal structure of silicon pernitride SiN2 at 140 GPa

SiN2 was synthesized from the elements at 140 GPa in a laser-heated diamond anvil cell. Crystal-structure determination (single-crystal synchrotron X-ray data) revealed that the title compound crystallizes in the pyrite structure type (space group Pa ).

Silicon pernitride, SiN 2 , was synthesized from the elements at 140 GPa in a laserheated diamond anvil cell.Its crystal structure was solved and refined by means of synchrotron-based single-crystal X-ray diffraction data.The title compound crystallizes in the pyrite structure type (space group Pa3, No. 205).The Si atom occupies a site with multiplicity 4 (Wyckoff letter b, site symmetry .3.), while the N atom is located on a site with multiplicity 8 (Wyckoff letter c, site symmetry .3.).The crystal structure of SiN 2 is comprised of slightly distorted [SiN 6 ] octahedra interconnected with each other by sharing vertices.Crystal chemical analysis of bond lengths suggests that Si has a formal oxidation state of +IV, while nitrogen forms pernitride anions (N-N) 4-.

Chemical context
Nitrogen-rich materials have gained a lot of attention due to their diverse properties such as high hardness, incompressibility (Young et al., 2006;Bykov et al. 2019a) and high energy density (Bykov et al., 2021;Wang et al., 2022).Among these, binary high-pressure nitrides of group 14 elements are of particular interest, as they exhibit remarkable elastic and electronic properties compared to their ambient-pressure counterparts.In particular, cubic silicon nitride �-Si 3 N 4 , synthesized from the elements at about 15 GPa, is significantly more incompressible than the ambient-pressure �-and �-polymorphs (Zerr et al., 1999).Recently Niwa et al. (2017) have synthesized pernitrides of group 14 elements (SiN 2 , SnN 2 and GeN 2 ) by using laser-heated diamond anvil cells at pressures above 60 GPa.The crystal structures of GeN 2 and SnN 2 were solved and refined against powder X-ray diffraction data.However, the weak X-ray powder pattern of SiN 2 only allowed the suggestion that SiN 2 crystallizes in the pyrite structure type, while no structure refinement was performed.
In this work, we synthesized SiN 2 from the elements at pressures of 140 GPa and examined it by means of synchrotron single-crystal X-ray diffraction in order to solve and refine its crystal structure.

Structural commentary
SiN 2 crystallizes in fact in the pyrite structure type in the space group Pa3 (No. 205).The asymmetric unit comprises two atoms, a silicon atom (multiplicity 4, Wyckoff letter b, site symmetry .3.), and a nitrogen atom (8 c, .3.).The nitrogen atoms form N-N dimers, and consequently each of the N atoms is tetrahedrally coordinated by three Si atoms and one N atom.The centers of the N-N dimers form an fcc sublat-tice, which together with the interpenetrating fcc sublattice of Si atoms can be considered as a derivative of the rock salt structure type.Slightly distorted [SiN 6 ] octahedra [Si-N distance 6� 1.7517 (11) A ˚] interconnect with each other by sharing common vertices (Fig. 1).There is a linear correlation between the nitrogen-nitrogen distance in dimers and the formal ionic charge and bond order of the (N 2 ) xÀ anion (Laniel et al., 2022).In the case of SiN 2 , the refined nitrogennitrogen distance of 1.402 (8) A ˚indicates that the N-N bond has single-bond character.This distance is in a good agreement with N-N distances observed in other pernitrides that contain single-bonded (N-N) 4-units (Tasna ´di et al., 2021).However, it is longer compared to N-N bonds in diazenides (Laniel et al., 2022;Bykov et al., 2020) and in dinitrides of trivalent metals (Niwa et al., 2014;Bykov et al., 2019b).Based on the empirical formula suggested by Laniel et al. (2022) for dinitrides, FC = (BL À 1.104)/0.074,where FC is the absolute value of the formal charge on the dinitrogen unit and BL is the N-N bond length in A ˚, a clear assignment can be made.For SiN 2 , the value of FC was calculated as 4.04, which is in excellent agreement with the most common oxidation state of +IV for silicon.

Synthesis and crystallization
A piece of silicon (10�10�5 mm 3 ) was placed in the sample chamber of a BX90-type diamond anvil cell equipped with Boehler-Almax type diamonds using culets of 100 mm in diameter.The sample chamber was eventually created by laser-drilling a 50 mm hole in the Re gasket preindented to a thickness of 18 mm.Nitrogen, loaded using the high-pressure gas-loading system of the Bavarian Geoinstitute (Kurnosov et al., 2008), served both as a pressure-transmitting medium and as a reagent.Pressure was determined by the shift of the diamond Raman band (Akahama & Kawamura, 2006).Upon compression to the target pressure of 140 GPa, the sample was heated using a focused Nd:YAG laser (� = 1064 nm) to temperatures exceeding 2500 K.The heating duration was approximately 10 seconds.The reaction products consisted of multiple high-quality, single-crystalline domains.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1.The sample was studied by means of synchrotron single-crystal X-ray diffraction (SCXD) at the beamline ID11 (ESRF, Grenoble, France) with the following beamline setup: � = 0.28457 A ˚, beamsize �0.7�0.7 �m 2 , Eiger CdTe 2M detector.For the SCXD measurements, samples were rotated around a vertical !-axis in the range �30 � .The diffraction images were acquired at an angular step �! = 0.5 � and an exposure time of 5 s per frame.For analysis of the single-crystal diffraction data (indexing, data integration, frame scaling and absorption correction) we used the CrysAlis PRO software package (Rigaku OD, 2023).To calibrate an instrumental mode using CrysAlis PRO, i.e., the sample-todetector distance, detector origin, offsets of goniometer angles, and rotation of both X-ray beam and the detector around the instrument axis, we used a single crystal of orthoenstatite [(Mg 1.93 Fe 0.06 )(Si 1.93 ,Al 0.06 )O 6 , space group Pbca, a = 8.8117 (2), b = 5.18320 (10), and c = 18.239 (13) A ˚].
Data analysis followed several steps: 1.After collecting SCXD data sets (series of �120 frames), a 3D peak search procedure was performed as implemented CrysAlis PRO.This search identified reflections from all crystalline phases present in the collection spot, including reaction products, initial reagents, pressure-transmitting medium, diamonds and gasket material.
3. The reflection groups were assessed individually by indexing the reflections within the current group using built-in procedures in CrysAlis PRO.If indexing succeeded, the group was chosen for final data integration.4. Datasets were integrated, and data was reduced following standard procedures, taking into account the shadowing of the diamond anvil cell.

Special details
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.

Figure 1
Figure 1 Crystal structure of SiN 2 at 140 GPa in polyhedral representation with Si atoms in orange and N atoms in blue.Shown are [SiN 6 ] octahedra interconnected by N atoms.Displacement ellipsoids are represented at the 75% probability level.

Table 1
Experimental details.
and isotropic or equivalent isotropic displacement parameters(Å 2 )