A cage-structured GaN allotrope with unconventional bonding breaks the ductility barrier of wide band gap semiconductors

Abstract

The exceptional properties of gallium nitride (GaN) have solidified its role in optoelectronics and power electronics. However, the practical application of its conventional phases (wurtzite, zinc blende) is constrained by their inherent brittleness and high hardness, limiting their use in flexible electronics and demanding mechanical environments. This creates a critical need for novel GaN allotropes that overcome these limitations while retaining a desirable wide band gap. Herein, we propose a novel three-dimensional tetrahexagonal Ga₆N₆ (3D th-Ga₆N₆) phase composed of 4- and 6-membered rings facilitated by rare sp³d² and sp²-hybridization. First-principles calculations confirm its dynamic, mechanical, and thermal stability. Remarkably, 3D th-Ga₆N₆ exhibits a direct WBG of 3.048 eV and a ductile character, with a low Vickers hardness of 6.842 GPa, addressing the core limitation of traditional GaN. It also demonstrates a high refractive index of 2.684 and strong optical absorption across the terahertz to ultraviolet spectrum. Furthermore, its exceptional thermal properties, including an ultra-low minimum thermal conductivity (0.957 W m⁻¹ K⁻¹) and fracture toughness (2.219 MPa m^1/2), rival those of the state-of-the-art thermal barrier coatings. This combination of electronic, mechanical, optical, and thermal properties suggests multifunctional potential for the next-generation flexible optoelectronics, THz devices, and advanced thermal management systems. Our work not only introduces a promising new allotrope but also opens a viable pathway for designing ductile, wide-band gap semiconductors.