Pressure-Stabilized High-Energy-Density Alkaline-Earth-Metal Pentazolate Salts

Abstract

Polynitrogen compounds especially pentazolate anion complexes recently have attracted substantial attention due to their promising potential as high-energy-density materials. Here, using a machine-learning-accelerated crystal structure search method and first-principles calculations, we predict a new hybrid compound by inserting a large fraction of nitrogen into alkaline-earth metals. It is a new stoichiometric type MN10 (M = Be, Mg), which possesses a metal-centering octahedral pentazolate framework with the space group Fdd2. This type of ionic-like molecular crystal is found to be energetically more favorable than the mixtures of M3N2 or MN4 compounds and pure nitrogen and is possibly synthesized at relatively low pressures (around 12 GPa for MgN10). The ab initio molecular dynamics simulations show that they are metastable and can be quenched to ambient conditions once synthesized at high pressure. Moreover, decomposition of this polymeric MN10 structure can release a large amount of energy and shows high performance in detonation. The detonation velocity and pressure of BeN10 are about twice and 4 times that of trinitrotoluene, respectively.

Synthesis of magnesium-nitrogen salts of polynitrogen anions

“…1), which are arranged in exotic planar infinite zigzag N-N chains parallel to the a -axis, akin to those found in FeN 4 and ReN 8 · x N 2 near 110 GPa 13,14 . While the MgN 4 compound’s crystal chemistry matches the theoretical calculations 17–19 , the measured lattice parameters do not, as the c parameter is double the predicted value and the structure adopts an I -type unit cell, opposite to the calculated C -type (see Supplementary Discussion). As shown in Supplementary Fig.…”

“…Atomistic model calculations highlight the outstanding potential of high pressure experiments, as a plethora of polynitrogen entities have been predicted to be stable 17–19 . Among them, the compression of magnesium and nitrogen was predicted to produce the MgN 3 , MgN 4 , and MgN 10 salts comprised of exotic anionic benzene-like N 6 rings, infinite 1D armchair chains and pentazolates, respectively 17–19 . To the best of our knowledge, these predictions have not been tested until now.…”

Solid Nitrogen and Nitrogen‐Rich Compounds as High‐Energy‐Density Materials

“…[ 104,105 ] Three possible bonding modes were proposed, i.e., monodentate, bidentate, and metallocene‐like coordination. Recent first‐principles calculations combined with modern structure search algorithms have predicted a rich variety of crystal structures, such as in MN 5 (M = H, Na, and Cu), [ 30,34,106 ] MN 10 (M = Be, Ca, Mg, Ba, and Zn), [ 33,107–109 ] MN 15 (M = Al, Ga, Sc, and Y), [ 110 ] and IrN x ( x = 4, 7), [ 111 ] and estimated the synthesis conditions and energetic performance for many. These studies are critical, as they often serve to guide the experimental exploration of new materials, such as in the case of LiN 5 and CsN 5 .…”

“…[ 28 ] The cyclo ‐N 5 − is a stable, aromatic, planar anion and has relatively large decomposition energy barrier of 117 kJ mol −1 , which shows the promise to obtain polynitrogen‐based HEDMs. In contrast to the extreme difficulty of isolating N 5 − at room conditions, studies have shown that N 5 − can be stabilized at high pressure in compounds such as LiN 5 , [ 29 ] NaN 5 , [ 30 ] CsN 5 , [ 31,32 ] MgN 10 , [ 33 ] and CuN 5 . [ 34 ] Moreover, N 5 3− is predicted to further aggregate and form higher oligomers or polymeric network in GaN 5 and GaN 6 .…”

Fully Active Nitrogen Energetic Chains Mg2(N5)2N2[Mg2(N5)2N2]n under Ambient Conditions

“…[ 14 ] Therefore, a chain structure is formed by combining Mg and cyclo‐N 5 − that exists stably under ambient conditions, which is different from previous magnesium nitrides. [ 39–41 ]…”