Vapor complexation in the ACl–CrCl3 (A = Li, Cs) system up to 1400 K and the tetrahedral ligand field states of chromium (III)

Electronic absorption spectroscopy is used in the temperature range 800–1400 K, to study the vapor species, over molten CrCl3–ACl (A = Li, Cs) mixtures and solid CrCl3. The observed Vis/near IR bands are assigned to d ← d transitions of Cr3+ in distorted “tetrahedral” coordination ( CrCl 4 - ) . Spectra of “octahedral” CrCl 6 3 - in molten alkali chlorides were also measured and used to estimate the spectroscopic constants (Dq, B, C) of Cr3+ in all chloride tetrahedral and octahedral environments. Composition and temperature-dependent measurements suggest that the predominant vapor species is the 1:1 monomer ACrCl4 and that an equilibrium is established: ACrCl4 (g) ⇆ CrCl3 (g) + ACl (g)Due to vapor complexation the apparent vapor pressure of CrCl3 increases. The volatility enhancement is higher for the LiCl–CrCl3 than the CsCl–CrCl3 system reaching values near 60, at ∼950 K. Based on the preferential octahedral ligand field stabilization energy of Cr(III) it is argued that dimeric and/or trimeric 1:1 species may be also present as minor components in the vapor phase. Finally, the vapor complexation and volatility enhancement for the MX3–AX (M = rare earth, Cr; X = halide) systems are discussed and correlated to the melt structure of the corresponding binary melts.