High-pressure synthesis, crystal growth, and compression behavior of hexagonal ${\mathrm{CrN}}_2$ having one-dimensionally aligned nitrogen dimer

A hexagonal chromium pernitride, ${\mathrm{CrN}}_2$, has been successfully synthesized $via$ a direct chemical reaction between chromium and molecular nitrogen at a pressure of approximately 70 GPa. The advanced synchrotron x-ray diffraction (XRD) measurements after recovery and structural refinement at 0.1 MPa demonstrated that ${\mathrm{CrN}}_2$ crystallizes in the anti-NiAs-type structure with lattice parameters of $a=2.7465\left(3\right)\AA$ and $c=7.3670\left(4\right)\AA$. Micron-sized hexagonal habit compounds were found in the synthesized products, which indicated that the single crystal of hexagonal ${\mathrm{CrN}}_2$ was grown in the supercritical nitrogen at high pressure. High-pressure in situ XRD measurements were performed to determine the zero-pressure bulk modulus, $K_0$, value of 286(6) GPa $\left[{K^{\prime }}_0=2.5\left(7\right)\right]$ and remarkable incompressibility of the $c$ axis. ${\mathrm{CrN}}_2$ is a unique example of the recoverable transition-metal pernitride which presents one-dimensionally aligned N-N dimers in the structure. The one-dimensionally aligned N-N dimers play an important role in controlling the mechanical and electronic properties of ${\mathrm{CrN}}_2$ as predicted by theoretical calculations.

Figure 1

Background-subtracted XRD profiles of the sample recovered from the high-pressure experiments (14.2, 50.6, and 67.2 GPa). The horizontal axis is defined as $1/d\left({\AA }^{-1}\right)$, as the XRD profile of the sample recovered from the experiment at 14.2 GPa was measured using $\mathrm{Cu}K\alpha$ radiation, whereas the other two samples were measured using synchrotron radiation. NC corresponds to sodium chloride, which was used as a thermal insulator and was attached to the sample recovered at 14.2 GPa. The asterisk * represents the diffraction peaks, which cannot be observed in the profile of the sample recovered at 50.6 GPa.

Figure 2

SEM images of the ambient recovered sample from the experiments above the pressure of 50 GPa. The hexagonal-plate-shaped products are found on the surface of the samples recovered from the experiments above pressures of approximately 67 GPa [Figs. 2], while no plate products were found from the sample recovered at approximately 50 GPa [Fig. 2].

Figure 3

(a) XRD profile of the sample recovered from the experiment at 67.2 GPa. The diffraction peaks assigned with the miller indices correspond to the anti-NiAs-type ${\mathrm{CrN}}_2$. The simulated intensities of the anti-NiAs-type ${\mathrm{CrN}}_2$ are also shown. (b) Schematic illustration of the crystal structure in which the large and small balls correspond to the chromium and nitrogen atoms, respectively. The solid lines represent the hexagonal unit cell. (c) Enlarged SEM image of the hexagonal plate crystals together with the atomic arrangement viewed from the $c$ axis.

Figure 4

Raman spectrum of the anti-NiAs-type ${\mathrm{CrN}}_2$ recovered from the experiments above a pressure of 67 GPa (top). The Raman spectra of pyrite-type ${\mathrm{PtN}}_2$ and marcasite-type ${\mathrm{RhN}}_2$ are also shown for comparison (middle and bottom). The spectrum of marcasite-type ${\mathrm{RhN}}_2$ measured at 43.2 GPa indicates a low stability at 0.1 MPa [8]. The ${\mathrm{PtN}}_2$ was synthesized in this study for comparison, and the Raman spectrum is consistent with that of previous studies [6, 10].

Figure 5

Pressure dependence of the normalized lattice parameters and unit-cell volume of ${\mathrm{CrN}}_2$ together with those of ${\mathrm{ReN}}_2$ [42] for comparison. The solid line in the $P\text{−}V$ relation for ${\mathrm{CrN}}_2$ represents the result of the third-order Birch-Murnaghan equation of state (EOS) [43, 44]. The normalized lattice constants are fitted to the modified third-order Birch-Murnaghan EOS. The $K_0=173\mathrm{GPa}$, ${\beta }_{a\text{-axis}}=1.1\times {10}^{-3}\mathrm{GP}{\mathrm{a}}^{-1}$, and ${\beta }_{c\text{-axis}}=3.4\times {10}^{-3}\mathrm{GP}{\mathrm{a}}^{-1}$ of ${\mathrm{ReN}}_2$ were reported by Kawamura et al. [42] to illustrate the pressure dependence of these parameters. The crystal structure of ${\mathrm{CrN}}_2$ and ${\mathrm{ReN}}_2$ are schematic views from the $a$ axis, and the solid line represents the unit cell.