Recoverable and Unrecoverable Bi3+-Related Photoemissions Induced by Thermal Expansion and Contraction in LuVO4:Bi3+ and ScVO4:Bi3+ Compounds

Most substances as thermodynamic law explicitly states will expand or contract upon heating or cooling, respectively, which sometimes may lead to changes in their crystallographic microstructures and therefore unexpected physicochemical and optoelectronic properties. Here, we report an efficient yellow photoemission from a compound of LuVO4:Bi3+, whose peak intensity and position after 11 rounds of yoyo experiments of heating and cooling can recover to their initial states. In sharp contrast, ScVO4:Bi3+, though crystallographically isomorphous to LuVO4, exhibits a completely different scenario, and it, submitted to the same thermal treatment, shows unrecoverable changes in both peak position and intensity of the red emission. In order to unravel why bismuth responds so differently upon the same thermal stimuli in the two isomorphous compounds, in situ high-temperature X-ray diffraction (HT-XRD), Rietveld refinement, static and dynamic high resolution photoluminescence, scanning electron microscopy, and single particle diagnosis techniques, as well as density functional theory (DFT) calculations have been employed to illustrate the microstructural changes along with environmental temperature. In situ HT-XRD measurements and consequent Rietveld refining analysis clearly illustrates that thermal expansion and contraction can induce permanent crystallographic microstructure changes, e.g., unrecoverable expansion of lattice cell in ScVO4:Bi3+ rather than LuVO4:Bi3+. Such expansion can be considered as an evidence for the removal of oxygen vacancy, which can be promoted by the accelerated oxygen diffusion rate as temperature increases. This, as DFT computation implies, can slightly increase the band gap of ScVO4:Bi3+, and it eventually leads to the unrecoverable blueshift and intensity loss of the red emission peak. The single particle diagnosis further reveals significant intensity reduction and peak shift for nearly half of the ScVO4:Bi3+ particles but not for all randomly selected LuVO4:Bi3+ particles. The diagnosis approach therefore provides a new strategy to distinguish and select the particles with desirable luminous intensity and color purity from a mass of powder mixture and in the meantime potentially gives new insights into unusual luminescence properties in phosphors.