Comment on "Charge Transfer-Triggered Bi³⁺ Near-Infrared Emission in Y₂Ti₂O₇ for Dual-Mode Temperature Sensing"

Undoped Y₂Ti₂O₇ exhibits impurity emission bands at low temperatures due to Mn⁴⁺ and Cr³⁺, as established by codoping with these ions. Contrary to a recent report by Wang et al., ACS Appl. Mater. Interfaces 2022, 14, 36834-36844, we do not observe Bi³⁺ emission in this codoped host, as also is the case for Fe³⁺. The emission reported in that paper as being due to Bi³⁺ in fact corresponds to Cr³⁺ emission. The Cr³⁺ and Mn⁴⁺ emissions are quenched with increasing temperature, so that Mn⁴⁺ emission is scarcely observed above 80 K. We present variable temperature optical data for Y₂Ti₂O₇ and this host codoped with Mn, Cr, Fe, and Bi, as well as a theoretical justification of our results.