Luminescence of nanocrystalline BaFCl codoped with Eu^2+/3+ and Tb³

BaFCl:Eu,Tb nanocrystals were synthesized via hot-injection thermolysis of metal chlorodifluoroacetates and their luminescence response characterized between 80 and 430 K. Though unintended, partial reduction of Eu³⁺ to Eu²⁺ occurred during the synthesis stage, leading to luminescent nanocrystals coactivated by Eu²⁺, Eu³⁺, and Tb³⁺. Their emission was dominated by Eu²⁺, specifically by radiative transitions 4f⁷ (⁶P_7/2) → 4f⁷ (⁸S_7/2) and 4f⁶5d¹ (lowest level) → 4f⁷ (⁸S_7/2). Owing to the thermal coupling between 4f⁷ (⁶P_7/2) and 4f⁶5d¹ (lowest) levels, line-like emission from this manifold at 80 K evolved into band-like emission at 430 K. The change in the shape of the emission profile of Eu²⁺ was used to demonstrate that BaFCl:Eu,Tb may serve to realize bandshape luminescence thermometry as a more straightforward alternative to the usual ratiometric approach. The energy gap between 4f⁷ (⁶P_7/2) and 4f⁶5d¹ (lowest) levels and their radiative transition probabilities were estimated from time-resolved variable-temperature decays.