Polymorphic phases of sp³-hybridized superhard CN

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, CALYPSO code, see http://nlshm-lab.jlu.edu.cn/~calypso.html.

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At 10 GPa, lattice parameters of P-3m1, P4₂/mnm, Pbam, P4/mnc, P6/m, Aea2 structures are listed below: P-3m1 structure: a = 2.3736 Å and c = 5.3481 Å, γ = 120, C atoms occupying 2c (0, 0, 0.64616) and N atoms occupying 2d (0.6667, 0.3333, 0.25962); P4₂/mnm structure: a = 6.577 Å and c = 2.3458 Å, C atoms occupying 8i (0.1282, 0.6969, 0) and N atoms occupying 8i (0.00788, 0.72488, 0.5); Pbam structure: a = 7.5784 Å, b = 2.7176, and c = 2.3554 Å, C atoms occupying 4h (0.31026, 0.04217, 0.5) and N atoms occupying 4g (0.9143, 0.38896, 0); P4/mnc structure: a = 4.74 Å and c = 4.6686 Å, C atoms occupying 8h (0.22133, 0.07635, 0.5) and N atoms occupying 8g (0.10383, 0.60383, 0.25); P6/m structure: a = 6.0878 Å and c = 2.3342 Å, γ = 120, C atoms occupying 6k (0.96498, 0.71798, 0.5) and N atoms occupying 6j (0.36301, 0.92847, 0); Aea2 structure: a = 4.4067 Å, b = 5.5978 Å, and c = 3.974 Å, C atoms occupying 8b (0.07564, 0.14342, 0.04917) and N atoms occupying 8b (0.86476, 0.31069, 0.20082).

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Program Phonopy available at http://phonopy.sourceforge.net/, the force constants matrix is determined by VASP. Convergence test gave the use of a 2 × 2 × 2 supercell and a 5 × 5 × 5 k mesh for both structures.

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