Cage-Based 3D Tetrahexagonal Boron Nitride Crystal\nwith Excellent Terahertz Light Absorption

In recent findings,\na new classification of 1D and 2D tetrahexagonal\nboron nitrides (<i>th</i>-BN) consisting of square and hexagonal\nrings has been documented. These materials exhibit impressive properties\nsuch as the tunable band gap, strong optical absorption, suitable\nsign-tunable Poisson’s ratio, and high ideal strength, making\nthem promising for applications in nano- and opto-electronic industries.\nStimulated by these studies, we have designed a cage-based three-dimensional\ntetrahexagonal boron nitride (3D <i>th</i>-B<sub>6</sub>N<sub>6</sub>) structure, which demonstrates excellent thermal, dynamic,\nand mechanical stability, including exceptional cohesive and formation\nenergies of 6.66 and −0.93 eV per atom. Unlike direct band\ngap 1D and 2D tetrahexagonal boron nitride semiconductors, the proposed\n3D tetrahexagonal boron nitride exhibits an insulating nature, with\na wide indirect band gap of 6.175 eV at the HSE06 level. Moreover,\nin contrast to the unequal chemical bonding and ultraviolet optical\nabsorption observed in the 2D <i>th</i>-BN sheet, all B\nand N atoms form a fully sp<sup>3</sup>-hybridized bonded 3D <i>th</i>-B<sub>6</sub>N<sub>6</sub> structure, with excellent\nterahertz light absorption in the range of 0.3–10 THz. Notably,\nit also exhibits a Debye temperature of 1304.55 K and substantial\nphonon inelastic scattering. Our study introduces the BN family with\nnovel properties and potential applications.