Nonlinear atomic vibrations and structural phase transitions in strained carbon chains

We consider longitudinal nonlinear atomic vibrations in uniformly strained carbon chains with the cumulene structure (= C = C =)(n). With the aid of ab initio simulations, based on the density functional theory, we reveal the phenomenon of pi- mode softening in certain range of its amplitude for the strain above the critical value n(c) approximate to 11%. Condensation of this soft mode induces the Peierls phase transition connected with doubling of the unit cell. We introduce a simple classical model without any adjustable parameters that allows one to describe quite well the properties of the p-mode softening. Besides the pmode, two other symmetry-determined nonlinear normal modes can exist in monoatomic chains with arbitrary interparticle interactions. They correspond to tripling or quadrupling of the vibrational unit cell. Softening of these modes allows one to suppose that two new types of carbon chains (besides cumulene and polyyne) can exist with bond lengths alternation different from that of polyyne. Since the phase transitions in the strained cumulene can significantly change its electronic properties, these phenomena can be used for constructing various nanodevices. (C) 2017 Elsevier B. V. All rights reserved.