Experimental realization, decompression metastability, and energetic performance of the predicted polymeric nitrogen phases $P\bar{1}$-SrN6 and $C2/m$-BaN6

Zero-temperature crystal-structure prediction combined with density-functional theory predicts that the alkaline-earth azide compositions SrN6 and BaN6 host polymeric nitrogen at high pressure: a triclinic $P\bar{1}$-SrN6 phase with infinite zigzag nitrogen chains above 22 GPa and a monoclinic $C2/m$-BaN6 phase with a six-membered-ring nitrogen network above 110 GPa, both dynamically stable and semiconducting, while the analogous CaN6 chain phase is dynamically unstable. Whether these phases can actually be synthesized is unknown: room-temperature compression of the azide precursors gives amorphization (Sr(N3)2) or persistent azide frameworks (Ca(N3)2 to 54 GPa, Ba(N3)2 to 28 GPa) rather than polymerization, and the demonstrated polymeric-nitrogen synthesis routes (laser-heated Mg + N2 yielding MgN4 chains, laser-heated KN3 yielding K2N6 hexazine rings) have never been applied to strontium or barium. The problem is to convert the zero-kelvin enthalpy predictions into validated materials knowledge: a demonstrated pressure-temperature synthesis pathway, kinetic and decompression stability (do the chains and ring networks survive pressure release?), finite-temperature stability fields, and quantitative energy-release performance for polymeric SrN6 and/or BaN6 - or a demonstration that these phases cannot be realized, resting on evidence beyond the already-published statements that they are metastable rather than ground states (kinetic/pathway analysis, quantitative decomposition analysis against competing Sr-N/Ba-N phases, or experimental pressure-temperature evidence).

Background

Polymeric nitrogen - nitrogen bound into extended networks of N-N single and double bonds rather than N2 molecules - is a leading target of high-energy-density material (HEDM) research, because re-forming the strongly bound N≡N triple bond (954 kJ per mol of bonds) from single (160 kJ/mol) and double (418 kJ/mol) bonds releases large energies with clean N2 as the product. The archetype, cubic gauche nitrogen (cg-N), needs roughly 110 GPa and 2000 K to make and is not recoverable at ambient conditions, so a central strategy is to stabilize polymeric nitrogen motifs with electropositive metal atoms at far lower pressures. The alkaline-earth azides Sr(N3)2 and Ba(N3)2 are natural precursors because their metal-to-nitrogen ratio is already 1:6 - the formula XN6 is exactly the azide composition - and the open question is whether compression can fuse their discrete linear azide anions into extended nitrogen networks. The standard screening tool is crystal-structure prediction (here the swarm-intelligence CALYPSO method) combined with density-functional theory (DFT) enthalpy comparisons and phonon calculations: a candidate structure is called dynamically stable when its phonon dispersion, computed by the finite-displacement supercell method, contains no imaginary frequencies.

Liu et al. (RSC Advances, 2021) ran such a search for XN6 (X = Ca, Sr, Ba) at 10 GPa intervals over 0-200 GPa at zero temperature. They found that the ambient-pressure azide-type Fddd-SrN6 structure transforms into $P\bar{1}$-SrN6 at 22 GPa, whose nitrogen forms infinite zigzag chains with alternating N-N single and double bonds (formally Sr$^{2+}$(N6)$^{2-}$ repeat units), and that Fmmm-BaN6 transforms first into $P\bar{1}$(I)-BaN6 at 21 GPa (a phase previously found in Huang and Frapper's Ba-N study) and then into $C2/m$-BaN6 at 110 GPa, whose nitrogen forms a network of six-membered N6 rings linked by single bonds. Phonon spectra confirmed dynamical stability of $P\bar{1}$-SrN6 and $C2/m$-BaN6; both were predicted to be semiconductors; the analogous $P\bar{1}$-CaN6 showed imaginary phonon frequencies and was rejected. Notably, the authors' own introduction describes $P\bar{1}$-SrN6 as metastable under pressure and cites the earlier report that $C2/m$-BaN6 is a metastable phase - these are the lowest-enthalpy XN6 structures, not proven ground states against all competing stoichiometries - so realizing them is intrinsically a kinetic-trapping problem.

Adjacent experiments define what a credible answer looks like. Compressing Mg with N2 to about 50 GPa under laser heating in a diamond anvil cell (DAC, the standard device that squeezes micron-scale samples between two diamond anvils) has yielded MgN4, containing infinite nitrogen chains, and Mg2N4, whose four-nitrogen units are recoverable at ambient pressure. Laser heating of potassium azide KN3 above 45 GPa produced K2N6 containing planar six-nitrogen hexazine rings, metastable down to 20 GPa. High-pressure strontium and barium nitride chemistry exists experimentally only at the diazenide level (compounds of N2 dimers, such as Sr3(N2)4 and Ba(N2)3). Meanwhile, room-temperature compression of the actual precursors diverges from the zero-temperature phase diagrams: Sr(N3)2 amorphizes at relatively low pressure, Ca(N3)2 shows no phase transition up to 54 GPa, and Ba(N3)2 retains its azide framework to at least 28 GPa (with reversible azide-rotation transitions at 2.6 and about 11.8 GPa). The predicted polymerizations are therefore either kinetically hindered or require heating - exactly the unresolved territory between the prediction and any real material.

Problem Statement

The zero-kelvin predictions for polymeric SrN6 and BaN6 have never been confronted with experiment or with finite-temperature, kinetic, or performance theory. The problem is to convert them into validated materials-level knowledge, through three linked aspects of one question - do the predicted polymeric phases exist as realizable materials, and do they function as high-energy-density materials?

(1) Synthesis: determine whether pressure-temperature pathways exist that actually produce the polymeric phases at or above the predicted transition pressures (22 GPa for $P\bar{1}$-SrN6, 110 GPa for $C2/m$-BaN6) - for instance starting from the azides Sr(N3)2 or Ba(N3)2, or from Sr/Ba plus N2, with laser or resistive heating in a diamond anvil cell, the routes demonstrated for Mg + N2 and KN3 - and demonstrate their formation, e.g. by in-situ diffraction and spectroscopy; or else establish that they cannot be formed, on evidence going beyond the already-published statements that these phases are metastable rather than ground states: a formation/decomposition pathway or barrier analysis for the specific predicted structures (e.g. nudged elastic band or ab initio molecular dynamics), a quantitative decomposition analysis against competing Sr-N/Ba-N compositions (metal nitride, diazenide or polynitride plus N2) at the relevant pressures, or an experimental pressure-temperature study showing that competing products (amorphous phases, stable nitrides plus N2, diazenides) form instead under conditions spanning the predicted transition pressures.

(2) Stability and recovery: determine the kinetic barriers to formation, the finite-temperature stability fields (the source predictions are zero-kelvin enthalpies sampled every 10 GPa, with no entropy or temperature dependence), and the behavior of the polymerized nitrogen networks on decompression - whether the infinite chains of $P\bar{1}$-SrN6 and the N6 ring network of $C2/m$-BaN6 persist metastably (and down to what pressure) or decompose to N2 plus metal nitride.

(3) Performance: determine whether the polymeric phases deliver the high-energy-density behavior that the source asserts only qualitatively ('may be potential high energy-density materials') - energy-release quantities such as the decomposition enthalpy toward N2 plus the appropriate metal nitride products, mass and volumetric energy density, detonation velocity and pressure, and stability/sensitivity indicators, none of which the source study computed.

An answer may address one or both of the two compounds. The three aspects are linked facets of one question rather than independently sufficient deliverables: a synthesis claim without recovery or stability data does not settle whether the phase is a usable material, a performance number for a phase never shown to exist does not establish that it functions as a high-energy-density material, and a demonstration that a phase cannot be realized, meeting the evidential standard of component (1), resolves the question for that compound without components (2) and (3) applying to it. CaN6, predicted dynamically unstable, is not part of the question.

References

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