Graphite intercalation compounds under pressure

TL;DR

This study uses first-principles calculations to explore low enthalpy structures of C6Ca under pressure, revealing a new layered structure at high pressures with potential superconducting properties.

Contribution

The paper identifies a novel layered structure of C6Ca at high pressure and analyzes its electronic properties, advancing understanding of pressure-induced phase transitions in graphite intercalation compounds.

Findings

High-pressure C6Ca favors layered structures with five- and eight-membered rings.

A new stable high-pressure phase is predicted to be unstable towards superconductivity.

Multiple buckled structures are energetically competitive at intermediate pressures.

Abstract

Motivated by recent experimental work, we use first-principles density functional theory methods to conduct an extensive search for low enthalpy structures of C$_6$Ca under pressure. As well as a range of buckled structures, which are energetically competitive over an intermediate range of pressures, we show that the high pressure system ($\gtrsim 18$ GPa) is unstable towards the formation of a novel class of layered structures, with the most stable compound involving carbon sheets containing five- and eight-membered rings. As well as discussing the energetics of the different classes of low enthalpy structures, we comment on the electronic structure of the high pressure compound and its implications for superconductivity.