Thermally driven radiative redistribution in Tb3+-activated K2B4O7 borate phosphors for optical thermometry: Effect of alkali co-doping

Abstract

The coupled influence of thermal excitation and alkali co-doping on radiative branching, concentration quenching, and emission redistribution in Tb3+-activated borate phosphors remains insufficiently understood. In this work, K2B4O7:Tb3+phosphors were synthesized via solid-state reaction and systematically investigated to establish a quantitative correlation between crystal structure, radiative branching behavior, and ratiometric thermometric performance, with particular emphasis on Li+/Na+co-doping effects. X-ray diffraction and Rietveld refinement confirm a single-phase tetraborate structure, while FTIR and Raman spectra verify preservation of the mixed BO3–BO4 framework upon doping. Under 377 nm excitation, the phosphors show intense green emission dominated by the Tb3+5D4 → 7F5 transition, with an optimal activator concentration x =0.03; concentration quenching analysis gives a critical transfer distance Rc ≈15.46 Å, indicating multipolar interaction–driven non-radiative transfer at higher Tb3+contents. Judd–Ofelt analysis quantitatively links local site asymmetry and covalency to radiative transition probabilities, providing a microscopic basis for both concentration quenching and temperature-induced redistribution of branching ratios. Temperature-dependent photoluminescence reveals an unusual anti-correlated response: the dominant green emission undergoes conventional thermal quenching, whereas a red band at ~670 nm increases by ~30-fold between 300 and 550 K, attributed to thermally driven radiative branching redistribution within the 5D4 manifold assisted by phonon-mediated symmetry relaxation. Arrhenius analysis yields an activation energy of ~0.28 eV for undoped K2B4O7:Tb3+, which increases with Li+ co-doping and decreases with Na+co-doping, reflecting opposite trends in lattice stiffening and phonon coupling. Time-resolved spectroscopy and radiative rate analysis demonstrate that alkali ions function as active crystal-field and phonon-coupling modulators rather than passive charge compensators, simultaneously tuning defect-assisted non-radiative decay, multipolar interaction dynamics, and intra-manifold branching behavior. Alkali co-doping markedly enhances emission intensity without significant chromaticity drift. Collectively, the combined structural, spectroscopic, thermal, and radiative analyses converge to a unified microscopic mechanism in which alkali-induced lattice perturbations simultaneously modulate multipolar interaction dynamics, radiative branching probabilities, and thermal quenching barriers within the same excited manifold. These findings provide a microscopic framework for alkali-controlled emission engineering and demonstrate that controlled lattice perturbation enables simultaneous tuning of radiative efficiency and thermally driven branching redistribution in a single-ion Tb3+system.