Melting and Solid–Solid Transition Temperatures at High Pressures for NH4SCN, RbSCN, T1SCN, and CsSCN

Abstract

Abstract Differential thermal analyses at high pressures have yielded the following trajectories for melting and solid–solid transitions in the several thiocyanates: (substance, transition, 1-bar transition temperature in °C, initial slope (dT/dp) in deg kbar−1, initial curvature (–d2T/dp2) in deg kbar−2, approximate maximum pressure in kbar) NH4SCN, melting, ≈146, ∼20 (tentative), –, ∼3.5; NH4SCN, (upper) solid–solid, 120±4, 12.6±1.5, –, 7.5; RbSCN, melting, 184±2, 15.7±0.3, 0.32±0.05, 36; RbSCN, solid–solid, 166±3, 13.5±1.0, 0.3±0.1, 30; TlSCN, melting, ≥234, ≥10 (tentative) >0, 34; TlSCN, solid-solid, 99±2, 9.6±1.0, ≈0.17, 34; CsSCN, melting, ≥206, ≈14±1.5, ∼0.2–0.4, 33; CsSCN, solid-solid, ≥197, ≈11.8±1.0, ∼0.2, 33. The solid–solid transitions investigated here for NH4SCN, RbSCN and T1SCN appear to be closely-related, structurally, to the well-investigated transition in KSCN. The thermodynamics of the KSCN transition at 1-bar is discussed anew so as to be consistent with high pressure data. The transitions in these other thiocyanates are probably thermodynamically similar to the KSCN transition but the necessary data to establish the similarities are lacking. Lattice parameters for NH4SCN and RbSCN show rapid variations with temperature below the solid–solid transition, as for KSCN. DSC and DTA results are presented for the 1-bar transition temperatures and entropy changes for NH4SCN, RbSCN, TlSCN, CsSCN and also AgSCN. The entropy changes apparently associated with the structurally similar solid–solid transitions decrease with increasing cation polarizability.