Negative and Zero Linear Compressibility in Copper Dicyanamide and Tricyanomethanide

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 behavior 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) in 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 Jahn-Teller distortion of metal coordination in both materials, in controlling their compression behaviour.