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

Abstract

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).