Comment on "Charge Transfer-Triggered Bi 3+ Near-Infrared Emission in Y 2 Ti 2 O 7 for Dual-Mode Temperature Sensing".

Undoped Y 2 Ti 2 O 7 exhibits impurity emission bands at low temperatures due to Mn 4+ and Cr 3+ , 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 Bi 3+ emission in this codoped host, as also is the case for Fe 3+ . The emission reported in that paper as being due to Bi 3+ in fact corresponds to Cr 3+ emission. The Cr 3+ and Mn 4+ emissions are quenched with increasing temperature, so that Mn 4+ emission is scarcely observed above 80 K. We present variable temperature optical data for Y 2 Ti 2 O 7 and this host codoped with Mn, Cr, Fe, and Bi, as well as a theoretical justification of our results.