Novel Rhenium Nitrides

We report the synthesis, structure, and properties of novel bulk rhenium nitrides, hexagonal ${\mathrm{Re}}_2\mathrm{N}$, and ${\mathrm{Re}}_3\mathrm{N}$. Both phases have very high bulk moduli of $>400\mathrm{GPa}$, similar to the most incompressible binary transition-metal (TM) carbides and nitrides found to date. However, in contrast to other incompressible TM carbides and nitrides, ${\mathrm{Re}}_3\mathrm{N}$ is better placed for potential technological applications, as it can be formed at relatively moderate pressures (13–16 GPa) and temperatures (1600–2400 K).

Figure 1

Powder x-ray diffraction patterns ($\lambda =0.4958\AA$) showing, from top to bottom, unreacted rhenium foil at 13.1(1) GPa, the presence of ${\mathrm{Re}}_3\mathrm{N}$ after laser heating at 1700(200) K at 13(1) GPa, and the presence of ${\mathrm{Re}}_2\mathrm{N}$ at 31(1) GPa after reaction during laser heating at 20(2) GPa and pressure increase. Symbols ($+$) represent experimental values. The result of the Le Bail fit is shown by the continuous line through the data points. Vertical bars show the positions of the allowed Bragg reflections of the different phases. The difference curve is plotted below each fit.

Figure 2

Comparison between the crystal structures of Re, ${\mathrm{Re}}_3\mathrm{N}$, and ${\mathrm{Re}}_2\mathrm{N}$.

Figure 3

Typical results from white beam x-ray microdiffraction showing a map of the phase distribution (${\mathrm{Re}}_3\mathrm{N}$ and Re in black and gray (blue) areas, respectively, top left), the distribution of the grains and their sizes ($\approx 3–8\mu \mathrm{m}$) derived from the coherent regions of $c$-axis orientations relative to the sample normal of ${\mathrm{Re}}_3\mathrm{N}$ (top right), and an image indexed with the unit cell of ${\mathrm{Re}}_3\mathrm{N}$ (bottom). The indices of a few reflections are shown as examples. The dashed curves (red) indicate extensions of the indexed Laue zones.