First-principles calculations of boron-rich compounds of B₁₃N₂and B₁₂C₂X (X = Si, Ge)

Abstract

Recently, Kurakevych et al (2007 Acta Crystallogr. C 63 80) synthesized the boron-rich phase B 13 N 2 by a high-temperature and high-pressure technique. Here, we performed the non-spin-polarized and spin-polarized calculations for B 13 N 2 by mean of first-principles calculations based on density functional theory. Our calculations show that the magnetic states are more energetically stable than the nonmagnetic state. The elastic and electronic structure calculations demonstrate that the compound of B 13 N 2 is a stable magnetic half-metallic solid. The magnetic properties of B 13 N 2 mainly come from the partially filled p states of the boron atom and the considerable interstitial regions. The calculated hardness of B 13 N 2 is 40.8 GPa, indicating a potentially superhard material. Moreover, the similar boron-rich phase B 12 C 2 Ge is predicted to be of the highest hardness among all of the boron-rich phases.

Thermodynamic stability and properties of boron subnitrides from first principles

“…Table III shows the elastic constants and elastic moduli of the three αR-boron-like boron subnitrides, calculated at p = 0 GPa and T = 0 K, using the projection technique described in Sec. II D. As can be seen from Table III, the elastic constants as well as bulk and shear moduli of B 12 (NBN), calculated in this work, are in excellent agreement with the previous calculations, reported by Gou et al [61], thus confirming the reliability of our calculations. By inspecting the mechanical stability of the three subnitrides with the Born stability criteria for the rhombohedral structure, provided by Eq.…”

“…By performing spinpolarized calculations to the three subnitrides, we find that B 12 (NBN) favors a magnetic state with a magnetic moment of 1μ B , which is in accord with the previous calculations, done by Gou et al [61]. The two other compositions, B 6 N and B 38 N 6 , resulted in nonmagnetic solutions, even though the calculations were started with nonzero magnetic moments.…”

“…(8), we verify that all the criteria are fulfilled for the three compositions of boron subnitride, demonstrating that they are all mechanically stable. Recent theoretical works on B 12 (N-N) [73] and B 12 N(NBN) [61] have demonstrated elastic constants in line with potentially superhard materials. We thus expect [B 12 (N − N)] 0.33 [B 12 (NBN)] 0.67 , whose calculated elastic moduli are comparable to those calculated for the other two subnitrides, is potentially a superhard material.…”

Thermoelastic equation of state of boron subphosphide B12P2

“…O--O, contrary to the N-B-N and C-B-C chains in boron subnitride and carbide, respectively. Ab initio calculations [22,23] confirm the maximal bulk modulus for boron subnitride B 13 N 2 , although give overestimated B 0 values. …”

Microscopic models of hardness

“…The calculated hardness of the orthorhombic B 28 and B 13 N 2 by Mukhanov et al is 48.8 GPa and 40.3 GPa, respectively [47]. The calculated hardness of B 13 N 2 by Gou et al is 40.8 GPa [82]. These indicate that they are two new superhard materials.…”