Preservation of High-Pressure Synthesized Pentazolate Anions Cyclo-N5⁻ Confined in Sp3-hybridized Boron Nitride

Abstract

High pressure offers a powerful approach for synthesizing novel functional materials, yet the preservation of such metastable phases under ambient conditions remains a major challenge. This work highlights the pivotal role of sp 3hybridized boron nitride (BN) in capturing the polynitrogen structure synthesized at high-pressure and high-temperature (HPHT) conditions. Through the encapsulation of sodium azide (NaN3) within BN nanotubes followed by laser heating, leading to the formation of N5 -anions stabilized within converted sp 3 -BN. The N5 -anion is a widely studied high-energydensity structure that can be synthesized under HPHT conditions but has been long regarded as inaccessible at ambient conditions. When embedded within the robust covalent network of the sp 3 -BN, this metastable phase can be preserved after released to ambient pressure. The exceptional chemical and thermal stability of BN establishes it as an ideal encapsulating template for HPHT experiments, thereby providing a general strategy for the stabilization of high-pressure phases.