Knowledge Bank

Results of near-ir absorption spectra for $Ir+4$in $A2MX6(A=K,Rb,NH4,M=Sn,Pt,X=Cl,Br)$ type host at $2\circ K$ are discussed and compared to earlier work. Our data indicate that the effect of reduced host symmetry is quite small in this $d5$ system and that the spectra of $IrCl6\mathrm{<mrow\; data-mjx-texclass="ORD">2-</mrow>}$ can be interpreted in a manner paralleling previous studies on $Ir+4$ in cubic hosts. The $IrBr6\mathrm{<mrow\; data-mjx-texclass="ORD">2-</mrow>}$ appears to have a somewhat different vibronic pattern. Shifts of zero-phonon origin and the nature of a vibronic progression were analyzed to correlate with host lattice counter-ions. By comparison to luminescence results of Flint and Paulusz, the latter data are proposed to be indicative of a strongly Jahn-Teller active mode.