a–c) XRD patterns collected at ambient conditions with a copper radiation target. SEM images corresponding to (a,b) are shown in (d,e). The run product in (a,d) is phase–pure hexagonal δ–MoN synthesized at ~5 GPa and ~1300 °C for 20 min. (b,e) show mixed γ– and δ–MoN phases synthesized at 3.5 GPa by program–controlled heating for 3 hours (see Experimental Section). (c) Cubic γ–MoN0.86 obtained from re–sintering of phase–pure δ–MoN in (a,d) at ~5 GPa and ~2200 °C for 15 s. Insets show polyhedral views of crystal structures for δ and γ phases.
Nature Scientific Reports - The Hardest Superconducting Metal Nitride
TM mononitrides hold great promise for achieving the highest hardness and high–Tc superconductivity in the nitride systems.
In summary, stoichiometric hexagonal δ– and cubic γ–MoN were synthesized through an ion–exchange reaction route at high pressures. Based on single crystal measurements, δ– and γ–MoN exhibit high asymptotic hardness of ~30 and 23 GPa, respectively. Consistent with previous studies, the measured superconducting transition temperatures of 13.8 K for δ–MoN and 5.5 K for γ–MoN. δ–MoN is so far the hardest metal nitride with the second highest Tc, comparable to that of NbN (~16 K). The enhanced hardness in δ phase is attributed to three–dimensional, covalent Mo–N bonding network. In contrast, the Mo–N bonds in γ–MoN are linearly distributed and structurally unfavorable to achieve high hardness. Although δ–MoN is a low–density phase, it exhibits an anomalously higher elastic modulus than the high–density γ phase. Phase–pure δ–MoN can readily be synthesized at a moderate pressure of 3.5 GPa, making it practically feasible for massive and industrial–scale production.
SOURCES - Nature Scientific Reports