K₃Fe(CN)₆: Pressure-Induced Polymerization and Enhanced Conductivity

Abstract

Recent theoretical studies indicate that applying high pressure (up to tens of gigapascals) to simple compounds with triple bonds can convert the triple bonds to conjugated double bonds, which results in these compounds becoming electrically conductive or even superconductive. This might indicate a new route for the synthesis of inorganic/organic conductors of various compositions and properties and could greatly expand the field of conductive polymers. Here, we present a study of the phase behavior and electrical properties of K3Fe(CN)6 up to ∼15 GPa using Raman spectroscopy, synchrotron X-ray diffraction, and impedance spectroscopy at room temperature. In this pressure range, two new crystalline phases were identified, and their unit cells and space groups were determined. The cyanide ions react to form conjugated CN bonds in two steps, and the electronic conductivity is enhanced by 3 orders of magnitude, from 10–7 to 10–4 S·cm–1. Because this material is also an ionic conductor, these studies might “shed light” on the development of new cathode materials for alkali metal batteries. Enhancing the electrical conductivity by applying high pressure to compounds containing triple bonds could provide a potential route for synthesizing multifunctional conductive materials.

Pressure-Induced Polymerization and Disproportionation of Li2C2 Accompanied with Irreversible Conductivity Enhancement

“…High pressure provides a straightforward method to build up conductive conjugated double-bond backbones, through addition polymerization of triple-bond monomers. This is also referred as pressure-induced polymerization (PIP), and was reported for many molecular compounds like acetylene, − ethylene, − and hydrogen cyanide , as well as ionic compounds like metal cyanide − and acetylide. − Under high pressure, a rich diversity of carbon structures in Li–C compounds including the nanoribbon, sheets, and frameworks were predicted, most of which are metallic . Li 2 C 2 was also predicted to polymerize to form Li polyacetylide and Li graphenide under external pressure, , and its phase transitions were investigated experimentally .…”

From Molecules to Carbon Materials—High Pressure Induced Polymerization and Bonding Mechanisms of Unsaturated Compounds

“…K3Fe(CN)6 is another important cyanide compound, which is well-studied in coordination chemistry and electrochemistry. It polymerized at a much lower pressure, around 4 GPa [69]. That is to say, the reaction pressures of transition metal cyanides are much lower than that of alkali cyanides.…”

“…We investigated the PIP of K 3 Fe(CN) 6 using in situ Raman (Figure 6a), XRD ( Figure 6b) and AC impedance (Figure 6c) techniques. K 3 Fe(CN) 6 undergoes reversible phase transition (O1-O2) at 2-4 GPa with an increase of conductivity [69]. At 7-8 GPa, an irreversible phase transition O2-O3 occurs with a decrease in conductivity.…”

High-Pressure Study of the Vibrational Properties and Chemical Reaction of N-Vinyl-2-pyrrolidinone by Raman Spectroscopy

“…In recent years, pressurization has provided an innovative and environmentally friendly approach to synthesize new materials. − The application of pressure can significantly tune intermolecular interactions, facilitate close packing, rearrange molecules, and change the properties of chemical bonding. − Therefore, monomers may undergo structural phase transitions in a smaller volume that is more favorable for polymerization. − In most polymerization reactions of high carbon-to-hydrogen ratio under high pressure, the approaching of each unsaturated bond leads to the formation of saturated bonds. − Thus, pressure-induced polymerization provides a promising alternative to synthetic chemical routes pertaining to green chemistry for no need of any initiator or solvent and less impurities in the reactions. In addition, the products of pressure-induced polymerization through new reaction pathways contain distinct structures and properties from those produced by conventional synthetic methods. − For instance, sp 3 -hybridized nanothreads have shown extraordinary strength and stiffness, which perform better than conventional sp 2 carbon-based materials and polymers …”