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

Trivalent bismuth is a popular main group ion showing versatile luminescent behaviors in a broad spectral range from ultraviolet to visible, but barely in the near-infrared (NIR) region. In this study, we have observed unexpected NIR emission at ∼744 nm in a Bi 3+ -doped pyrochlore, Y 2 Ti 2 O 7 (YTOB). Our first-principles electronic structure calculation and analysis of the Bi local structure via extended X-ray absorption fine structure indicate that only Bi 3+ species appears in YTOB and it has a similar local environment to that of Y 3+ . The NIR emission is assigned to a Ti 4+ → Bi 3+ metal-to-metal charge transfer process. Moreover, we have demonstrated dual-mode luminescence thermometry based on the luminescence intensity ratio (LIR) and lifetime (τ) in 0.5% Bi 3+ and 0.5% Pr 3+ co-doped Y 2 Ti 2 O 7 (YTOB0.5P0.5). It exhibits high thermometric sensitivity simultaneously in the cryogenic temperature range from 78 to 298 K based on τ of the NIR emission of Bi 3+ at 748 nm and in the temperature range of 278-378 K based on the LIR of Bi 3+ to Pr 3+ emissions ( I 748 / I 615 ). As a novel LIR-τ dual-mode thermometric material over a wide temperature range, the maximum relative sensitivities of the YTOB0.5P0.5 reach 3.53% K -1 at 298 K from the τ mode and 3.52% K -1 at 318 K based on the LIR mode. The dual-mode luminescence thermometry with high responsivity from our Bi 3+ -based pyrochlore Y 2 Ti 2 O 7 phosphor opens a new avenue for more luminescent materials toward multi-mode thermometry applied in complex temperature-sensing conditions.