We study the transformation mechanism for the phase transformation from rhombohedral boron nitride (rBN) to cubic boron nitride (cBN) using density functional theory methods within the local density approximation (LDA). The transformation mechanism is approximated by two independent parameters: one defining the compressional mode that brings layers in rBN closer together, the other describing the buckling mode, relating to pyramidalization of atoms and formation of new bonds. The complete mechanism is mapped to a two-dimensional energy profile, providing the transformation path together with activation energy and reaction enthalpy. For pure BN, we find 0.175 eV/atom for the activation energy of the collective transformation. Analysis of vibrational modes along the transition path reveals positive zone-center frequencies only, while the elastic constant c33 adopts negative values. Aliovalent substitution of 8.33 mol-% AlN in BN yields a lower activation energy (0.115 eV/atom) than for pure BN. The impact of substituting BN by C severely depends on the distribution of C atoms: while C2-pairs within BN layers increase the activation barrier, other substitution patterns lead to a decrease. High pressure decreases the activation energy for the rBN to cBN transition, for pure and alloyed systems alike. However, while pure BN still exhibits a noticeable activation barrier at 30 GPa, the AlN substituted system completely collapses to the tetrahedral structure at pressures above 20 GPa. The relevance of our study for the synthesis of cBN at high pressures as well as for the thermal stability of cBN compounds at high temperatures will be discussed.

Investigation of the rhombohedral-to-cubic transformation path in boron nitride

247th National Meeting (2014)

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