Predicted crystal structures of boron nitrides. (a–g) 3D BNNT polymers.^[85,86] The light blue shadows emphasize the components of parent BNNTs. For the 3D polymers in (a–c), they can be formed by compressing corresponding BNNTs at relative low pressures, and more dense forms of 3D polymers shown in (d–f) can be obtained at higher pressures. Distinct from the semiconducting 3D polymers in (a–f), the boron nitrides in (g–i) are conductive and considered rare.^[85,87,89] The gray regions indicate the conductive units in structures, composed of sp² or sp²/sp³-hybridized bonding. The B and N atoms are painted in green and white colors, respectively. The hardness calculations indicate that these boron nitrides are all superhard except for M-BN (see detail in Table 3).