Thermal and pressure response of KHg(CN)₂(SCN)

When aqueous solutions of KSCN and Hg(CN)₂ were mixed, we observed the formation of long, needle-shaped single crystals with an unexpected structure and a formula of KHg(CN)₂(SCN). Single-crystal X-ray diffraction revealed that the CN ligands remain confined to the two apical positions of Hg(II) without forming an extended network. Instead, the sulphur atoms create a layered structure by forming four S-Hg bonds (3.144 Å), linking the equatorial positions of the Hg(II) octahedra. To investigate the response of this material, we conducted temperature-dependent (80-400 K) and in situ high-pressure (0.2-16.0 GPa) studies using Raman spectroscopy. Both measurements revealed unusual variations in the vibrational modes associated with SCN bending and its overtone, prompting further investigation into the material's potential as a negative thermal expansion (NTE) and negative linear compression (NLC) material through X-ray diffraction. In situ high-pressure X-ray diffraction studies indicated a structural phase transition from orthorhombic (Cmcm) to monoclinic (P2₁/c) below 4.6 GPa, with pressure-induced amorphization onset beyond 13 GPa. High-temperature X-ray diffraction up to 448 K revealed phase transitions around 340 K and 380 K. However, both X-ray diffraction studies displayed normal changes in lattice parameters and volume with temperature and under pressure, identifying KHg(CN)₂(SCN) as a positive thermal expansion (PTE) material above room temperature with positive linear compressibility (PLC).