Role of M-site cation in the negative linear compressibility of rutile-like MII(dca)2

Abstract

We investigate the pressure-induced structural deformations in a series of rutile-like network materials, M(dca)₂ (M = Mn, Fe, Ni), using synchrotron high-pressure powder X-ray diffraction (HP-PXRD). These materials adopt an orthorhombic structure (space group Pnnm) at ambient conditions. All compounds exhibit negative linear compressibility (NLC) along the c-axis in their orthorhombic phase (M(dca)₂-I), with magnitudes varying across the series: Mn(dca)₂ displays the largest NLC of −10(3) TPa⁻¹ (0.04–0.3 GPa), while Ni(dca)₂ and Fe(dca)₂ show −2.5(8) TPa⁻¹ (0.07–1.06 GPa) and −2.8(4) TPa⁻¹ (0.03–0.95 GPa), respectively. At higher pressures, these compounds undergo second-order phase transitions to a monoclinic structure (P2₁/n), with transition pressures dependent on the metal cation.