# Negative and zero linear compressibility in copper dicyanamide and tricyanomethanide

## TL;DR

This paper studies how copper compounds with specific structures can expand or maintain size under pressure, revealing new insights into material behavior.

## Contribution

The study identifies distinct compression behaviors in copper dicyanamide and tricyanomethanide due to structural differences and linker flexibility.

## Key findings

- Cu(dca)2 shows negative linear compressibility of −6.5 TPa−1 along the c-axis.
- Cu(tcm)2 exhibits zero linear compressibility along the a-axis due to rigid linkers.
- Structural transitions occur at different pressures for each compound.

## Abstract

Rutile-structured materials can exhibit negative linear compressibility (NLC) following ferroelastic phase transitions, expanding in one direction under uniform compression. We investigate this phenomenon in structural analogues—transition metal dicyanamides (dca) and tricyanomethanides (tcm) with single and double rutile-like structures, respectively. The pressure-induced structural behaviour of Cu(tcm)2 and Cu(dca)2 are studied using high-pressure diffraction. Both systems undergo anisotropic deformation upon compression, with Cu(dca)2 exhibiting NLC of −6.5(10) TPa−1 along the c-axis, while Cu(tcm)2 shows zero linear compressibility (ZLC) along the a-axis. This difference is attributed to the single rutile-like network with flexible dca− linkers in Cu(dca)2, in contrast to the more constrained doubly interpenetrating structure of Cu(tcm)2 with rigid tcm− linkers. We also study the interplay between structural features and electronic effects arising from the Jahn–Teller distortion in both materials, in controlling their compression behaviour.

Single vs. double networks: Cu(dca)2 shows NLC = −6.5 TPa−1via flexible dca− then transitions to monoclinic at 1.11 GPa; Cu(tcm)2 exhibits ZLC through rigid tcm− maintaining structure to 3.75 GPa.