Mechanical anomalies in mercury oxalate and the deformation of the mercury cube coordination environment under pressure

In a recent work, the extremely anomalous mechanical behavior of the anhydrous zinc and cadmium oxalates was discovered. In this paper, the mechanical behavior of the anhydrous mercury oxalate containing the remaining transition metal element of the IIB group of periodic table is investigated. The crystal structure and elastic properties of $${\text{HgC}}_{2} {\text{O}}_{4}$$ are determined using first-principles solid-state methods. Although the structure of mercury oxalate and the coordination environment of mercury in this compound are radically different to those of the zinc and cadmium oxalates, a great degree of similarity in their mechanical behavior is unexpectedly found. The mechanical behavior of mercury oxalate is anomalous and exhibits the negative Poisson’s ratio (NPR) and the negative linear compressibility (NLC) phenomena. Under isotropic pressure, the compressibility along the [1,0,0] crystallographic direction is negative in the pressure range from − 2.50 to 0.25 GPa. For external pressures applied in the direction of minimum compressibility, which coincides with [1,0,0], mercury oxalate exhibits negative volumetric compressibilities from 0.22 to 1.40 GPa. The increase in volume in this material is significant for pressures larger than 1.0 GPa, and the compressibility becomes − 95.6 $${\text{TPa}}^{ - 1}$$ at P = 1.36 GPa. As for the zinc and cadmium oxalates, the increase in volume is so drastic that mercury oxalate becomes unstable for pressures larger than 1.4 GPa. The NLC effect in mercury oxalate is due to the highly deformable character of the cube coordination polyhedron of mercury. The coordination structure becomes octahedral near P = 1.3 GPa, just before its structural instability.