Linear carbon chains (LCCs) are a truly 1D sp-hybridized allotrope of carbon. These structures show unique mechanical, optical, and electronic properties that can be modified by altering the number of carbon atoms as well as the type of terminal groups. Due to its properties and versality, the LCCs are a promise in the next-generation electronic devices, such as molecular wires, and optical and energy storage devices. Despite its formidable properties, LCCs are extremely unstable and reactive at ambient conditions due the unsaturated bonds. Until recently, the longest polyyne synthesized and stabilized contained 44 carbon atoms. Carbon nanotubes (CNTs) and multiwall CNTs (MWCNTs) have the interesting capability of encapsulating different species, this feature allowed to use these structures as nanoreactor to synthesize long and stable LCCs at ambient conditions with > 6000 atoms as obtained by Shi et al. [1]. Since then, LCCs encapsulated MWCNT (LCC@MWCNT) have been the best alternative to study the properties of these materials. Recently, Sharma et al. [2] using Raman spectroscopy showed that LCC@MWCNT under hydrostatic pressure exhibit a reversible softening in polyynes vibrational mode associated with linear redshift, behavior in disagreement with the frequency hardening already reported for RBM, G- and 2D bands in sp2 materials. In this work we use ab initio calculation to study of the behavior of the vibrational properties of infinite and finite LCCs under pressure, which is still an open question and little explored in the literature.

[1] L. Shi et al., Nature Materials15, 634 (2016).

[2] K. Sharma et al., Phys. Rev. Lett.125, 105501 (2020).