Emergence of Novel Polynitrogen Molecule-like Species, Covalent Chains, and Layers in Magnesium–Nitrogen Mg_xN_y Phases under High Pressure

Abstract

Stable structures and stoichiometries of binary Mg−N compounds are explored at pressures from ambient up to 300 GPa using ab initio evolutionary simulations. In addition to Mg 3 N 2 , we identified five nitrogen-rich compositions (MgN 4 , MgN 3 , MgN 2 , Mg 2 N 3 , and Mg 5 N 7 ) and three magnesium-rich ones (Mg 5 N 3 , Mg 4 N 3 and Mg 5 N 4 ), which have stability fields on the phase diagram. These compounds have peculiar structural features, such as N 2 dumbbells, bent N 3 units, planar SO 3 -like N(N) 3 units, N 6 six-membered rings, 1D polythiazyl S 2 N 2 -like nitrogen chains, and 2D polymeric nitrogen nets. The dimensionality of the nitrogen network decreases as magnesium content increases; magnesium atoms act as a scissor by transferring valence electrons to the antibonding states of nitrogen sublattice. In this context, pressure acts as a bonding glue in the nitrogen sublattice, enabling the emergence of polynitrogen molecule-like species and nets. In general, Zintl−Klemm concept and molecular orbital analysis proved useful for rationalizing the structural, bonding and electronic properties encountered in the covalent nitrogen-based units. Interestingly, covalent six-membered N 6 4− rings containing P−1 (I) MgN 3 phase is recoverable at atmospheric pressure. Moreover, ab initio molecular dynamics analysis reveals the polymeric covalent nitrogen network, poly-N 4 2− , encountered in the high-pressure Cmmm MgN 4 phase can be preserved at ambient conditions. Thus, quenchable MgN 4 , stable at pressures above 13 GPa, shows that high energy-density materials based on polymeric nitrogen can be achievable at reduced pressures. The highpressure phase P−1 (I) MgN 3 with covalent N 6 rings is the most promising HEDM candidate with an energy density of 2.87 kJ• g −1 , followed by P−1 MgN 4 (2.08 kJ•g −1 ).

High‐Pressure Synthesis of a Nitrogen‐Rich Inclusion Compound ReN8⋅x N2 with Conjugated Polymeric Nitrogen Chains

“…Chains of as imilar type were recently predicted in CaN 4 [33] (h 4 -coordination together with metallocyclic coordination), MgN 4 , [18,34] HfN 10 , [35] and in some hydrogen-nitrogen compounds. Chains of as imilar type were recently predicted in CaN 4 [33] (h 4 -coordination together with metallocyclic coordination), MgN 4 , [18,34] HfN 10 , [35] and in some hydrogen-nitrogen compounds.…”

“…To the best of our knowledge,R eN 8 ·x N 2 is the first synthetic compound that contains polydiazenediyl chains. Chains of as imilar type were recently predicted in CaN 4 [33] (h 4 -coordination together with metallocyclic coordination), MgN 4 , [18,34] HfN 10 , [35] and in some hydrogen-nitrogen compounds. [36] Interestingly,the structure of HfN 10 as predicted [35] is similar to the crystal structure of ReN 8 ·x N 2 .R ecently,w e reported an ovel compound FeN 4 containing polymeric nitrogen chains of ad ifferent type.T hese chains consist of sp 2 -a nd sp 3 -hybridized nitrogen atoms forming catena-poly-[tetraz-1-ene-1,4-diyl] chains.…”

“…Nitrogen-rich compounds in metalnitrogen systems are considered potential high-energy-density materials. [18][19][20][21] Theg reat interest in the Re À Ns ystem calls for more systematic investigations,w hich could provide unambiguous structural characterization of novel ReÀNp hases.A lthough there are several prominent examples of the synergy between powder XRD and ab initio structure prediction algorithms, [22,23] an independent self-consistent method is required to build ar eference basis for further theoretical and experimental works.I nc ontrast to most abundant highpressure powder XRD studies,s ingle-crystal XRD in diamond anvil cells allows not only an unequivocal structure solution but also provides information about the chemical composition of the products of the chemical reactions occurring in laser heated DACs.I nt his study,w eu sed this advantage.W es ynthesized an ovel nitrogen-rich compound, ReN 8 ·x N 2 ,inalaser-heated DACand characterized its crystal structure using high-pressure single-crystal X-ray diffraction. Our experiment is complemented with theoretical analysis of the discovered compounds in the framework of density functional theory.…”

Pressure‐Induced Stable Binary Compounds of Magnesium and Germanium

“…Moreover, a binary Mg–N compound, in which the Mg atoms played an important role in the stabilization of polymeric nitrogen networks, was explored up to 300 GPa. Two stoichiometries (i.e., MgN 3 and MgN 4 ) were proposed to be potential high‐energy‐density materials with energy densities of 2.87 and 2.08 kJ g −1 , respectively . Zhu et al.…”

“…Twos toichiometries( i.e.,M gN 3 andM gN 4 )w ere proposed to be potential high-energy-density materials with energyd ensities of 2.87 and 2.08 kJ g À1 ,r espectively. [10] Zhu et al reportedt hat, except for MgO, two extraordinary compounds of the Mg-O system,n amely,M gO 2 and Mg 3 O 2 ,w ere theoretically predictedt obestable at 116and 500 GPa, respectively. [11] Magnesium polyhydrides have been extensively studied, and the criticaltemperature (T c )v alues of MgH 2 ,MgH 4 ,a nd MgH 12 werer eported to be 16-23, approximately 10, and 47-60 Ka td ifferent pressures.…”