Novel high-concentration Eu3+-doped CaLaNbWO8 red phosphors with high brightness for w-LEDs and temperature sensing applications

Abstract

Increasing demand for phosphor-converted white light-emitting diodes (w-LEDs) with high color rendering index (CRI) and low correlated color temperature (CCT) is driving the development of high-performance phosphors. In this work, a series of novel Eu³⁺-doped CaLaNbWO₈ red phosphors was prepared via a high-temperature solid-state reaction approach. The pure phase was confirmed by XRD analysis. Eu³⁺ content was optimized to be 70%, which was higher than that of most Eu³⁺-related systems. A detailed investigation about the luminescence properties of the optimal CaLa_0.3NbWO₈:0.7Eu³⁺ red phosphor was conducted, including excitation, emission and temperature-dependence spectra. The excitation spectrum demonstrated a broad range extending from 200 to 500 nm, encompassing a substantial broad band originating from W⁶⁺–O²⁻ charge transfer and numerous narrow bands attributable to Eu³⁺ characteristic transitions. Photoluminescence (PL) studies demonstrated the occurrence of Eu³⁺ characteristic transitions, with the main peak observed at 614 nm under ultraviolet irradiation. Moreover, an increase in the concentration of Eu³⁺ ions was shown to result in a decline in the fluorescence time, which can be attributed to the enhancement of energy transfer among Eu³⁺ ions. Thermal stability analysis indicated that the CaLa_0.3NbWO₈:0.7Eu³⁺ phosphor maintained 80% (433 K) of its initial intensity at 298 K, while the different emission peaks of Eu³⁺ presented diverse decreasing trends with increasing temperature. Consequently, the temperature sensing properties were also investigated, attaining a maximal absolute sensitivity (S_a) of 10.32% K⁻¹ and relative sensitivity (S_r) of 1.75% K⁻¹ by employing FIR_614/655, thereby signifying its promising potential in non-contact thermometers. Furthermore, the phosphor employed in the fabrication of a w-LED exhibited a correlated color temperature (CCT) of 5893 K and a color rendering index (CRI) of 87.1, in conjunction with a near-ultraviolet (NUV) 395 nm chip. These results indicated the promising potential of w-LEDs and fluorescence thermometers in practical applications.