Reflection spectra of (alkali metal, thallium (I), silver (I)) iridium (IV) hexachlorides

Abstract
While the effect of rubidium and caesium ions in solid salts of IrCl6 -- is a shift to lower wave-numbers, comparable to that occurring in organic solvents relative to water, the dark blue silver (I) and thallium (I) salts have a new, broad band in the red. Mixed crystals of Cs2IrCl6 and Tl2IrCl6 are prepared, and the absorption spectra are discussed in terms of a tendency to ‘ferroelectric rattling’ and strong covalent bonding of Ag and Tl. Analogous cases are pointed out in solid salts of other 5d-hexahalides.