Prediction of a Very Hard Triclinic form of Diamond
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ABSTRACT In a theoretical search for new hypotetical sp3-bonded carbon structure containing five-fold rings as a possible result of fullerene transformation under pressure, we have found a triclinic form of diamond with 16 atoms per unit cell which we called tcl-16. We have calculated the ground state structure, the cohesive energy, the bulk modulus and the electronic density of states by means of tight binding molecular dynamics (TBMD). Finally we have compared the phonon spectra at F to existing Raman data for a non-cubic- phase of diamond.
INTRODUCTION The progress made in growing diamond films and diamond-like films has brought new interest in the identification of new possible allotropes of carbon in order to improve our knowledge of its phase diagram and, in turn, favours the identification of new phases and allotropes which can appear in the synthesis of diamond-like films. Since the calculation of Fahy and Louie [1], it became clear that these allotropes should be found only as three-fold or four-fold coordinated structures, any higher coordination having too low stability. As a consequence these studies have been concentrated only onto these two cases. The present study has been stimulated by the conjecture that the presence of five-fold rings in an
sp3 network may increase the stiffness of diamond-like materials. Moreover it may give an alternative and more favorable way of reconverting the fullerite structure under moderate pressure. We have predicted a possible sp 3 triclinic form of diamond with 16 atoms per unit cell, which we have called tcl-16. This new phase is characterized by periodic arrays of five-fold, seven-fold and six-fold rings Fig. 1 right panel. We have calculated the structural and electronic properties of this new phase by means of tight binding molecular dynamics (TBMD), based on very flexible potential that we have previously developed [2]. We have found that tc0-16 has a high stability and a large bulk modulus, comparable to body-centerd (BC8) and simple tetragonal (ST 12) phases [1], which makes it an interesting and competitive new hypothetical material.
RESULTS The lcl-16 phase The cl-16 phase can be generated through the rebonding of the bct-4 phase. The bct-4 phase is an all sp2 phase proposed by Hoffinan et al. [3] and calculated by Liu and Cohen [4], who found it relatively stable (0.3 eV above diamond). The bct-4 crystal consists of buckled layers of carbon chains joined by bonds parallel to the c axis (see Fig. I left panel) and displays a body centered tetragonal unit cell with 4 atoms per unit cell. It has been predicted to be mechanically stable with 351
Mat. Res. Soc. Symp. Proc. Vol. 408 0 1996 Materials Research Society
respect to the transformation into diamond. A similar phase of hexagonal symmetry, called H-6 [5], was instead found to be mechanically unstable with respect to the same transformation.The basal planes of H-6 and bct-4 are very similar to the (11) and the (100) diamond surface, respectively. As a consequence it was suggested that it would be poss
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