On stability of nonlinear atomic vibrations in strained carbon chains

G.M. Chechin, O.A. Usoltsev, D.A. Sizintsev

Abstract

Monoatomic carbon chain (carbyne) can exist in two different modifications: cumulene with double chemical bonds between its atoms, and polyyne with alternation of single and triple bonds. In our previous paper [Letters on materials 6 (2), 146-151 (2016)], we have discussed new physical phenomenon which was revealed in cumulene chains under uniform strain. It was found that above 11.2% strain softening of π-mode atomic vibrations takes place in some range of its amplitude. Condensation ("freezing") of this soft mode leads to structural phase transition of displacement type. It is the Peierls phase transition which was found earlier in [Nano Lett. 14, 4224-4229 (2014)] with the aid of a different approach. The above phenomenon occurs due to the fact that old atomic equilibrium positions (EQPs), near which atoms vibrate in the case of small strain, lose their stability and two new EQPs appear near each of them. The π-mode softening corresponds to vibrations in the vicinity of these new EQPs. In the present paper we discuss the problem of stability of the new EQPs, as well as a possibility of condensation in cumulene of two other symmetry-determined nonlinear normal modes, different from π-mode. Inferences of this paper may be useful for comparison of the results, obtained by methods of molecular dynamics, with those obtained with the aid of ab initio simulations based on the density functional theory in studying different physical phenomena.

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