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 sp³-hybridized boron nitride (BN) in capturing the polynitrogen structure synthesized under high-pressure and high-temperature (HPHT) conditions. The encapsulation of sodium azide (NaN₃) within BN nanotubes, followed by laser heating, leads to the formation of cyclo-N₅⁻ anions stabilized within the converted sp³-BN. The cyclo-N₅⁻ anion is a widely studied high-energy-density structure that can be synthesized under HPHT conditions but has long been regarded as inaccessible under ambient conditions. When embedded within the robust covalent network of sp³-BN, this metastable phase can be preserved after being 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 stabilizing high-pressure phases.