Negative and zero linear compressibility in copper dicyanamide and tricyanomethanide

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 <i>c</i>-axis, while Cu(tcm)₂ shows zero linear compressibility (ZLC) along the <i>a</i>-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.