September 07, 2015

The Hardest Superconducting Metal Nitride

Transition–metal (TM) nitrides are a class of compounds with a wide range of properties and applications. Hard superconducting nitrides are of particular interest for electronic applications under working conditions such as coating and high stress (e.g., electromechanical systems). However, most of the known TM nitrides crystallize in the rock–salt structure, a structure that is unfavorable to resist shear strain, and they exhibit relatively low indentation hardness, typically in the range of 10–20 GPa. Here, we report high–pressure synthesis of hexagonal δ–MoN and cubic γ–MoN through an ion–exchange reaction at 3.5 GPa. The final products are in the bulk form with crystallite sizes of 50 – 80 μm. Based on indentation testing on single crystals, hexagonal δ–MoN exhibits excellent hardness of ~30 GPa, which is 30% higher than cubic γ–MoN (~23 GPa) and is so far the hardest among the known metal nitrides. The hardness enhancement in hexagonal phase is attributed to extended covalently bonded Mo–N network than that in cubic phase. The measured superconducting transition temperatures for δ–MoN and cubic γ–MoN are 13.8 and 5.5 K, respectively, in good agreement with previous measurements.

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

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