Crystal-Field Analysis and Temperature-Dependent Luminescence Mechanism of Broadband Near-Infrared Na$_3$Al$_2$Li$_3$F$_{12}$:Cr$^{3+}$ Phosphors

Garnet-structured fluoride phosphors are attractive for phosphor-converted near-infrared light-emitting diodes (NIR pc-LEDs) owing to their broadband emission and thermal stability. Herein, pure-phase Na$_3$Al$_{2-x}$Cr$_x$Li$_3$F$_{12}$ (x = 0--0.16; NALF:Cr$^{3+}$) phosphors were synthesized hydrothermally, and their structure, morphology, crystal field, and luminescence were systematically investigated. Crystal-field analysis yielded a Dq/B of $\sim$2.18, confirming a weak crystal-field environment for Cr$^{3+}$. Under 430 nm excitation, all samples show a broad NIR band over 650--900 nm with a FWHM of $\sim$101 nm, ascribed to the $^4T_{2g}\rightarrow{}^4A_{2g}$ transition. Fluorescence decay and time-resolved emission spectroscopy indicate that Cr$^{3+}$ solely replaces octahedral Al$^{3+}$, forming a single luminescent center, optimal at x = 0.08. The phosphor retains over 67% of its room-temperature intensity at 423 K, and temperature-dependent photoluminescence reveals the zero-phonon line (ZPL) of the $^4T_{2g}\rightarrow{}^4A_{2g}$ transition at 80 K, offering deeper insight into the Cr$^{3+}$ luminescence mechanism in fluoride garnets. NALF:Cr$^{3+}$ is thus a promising broadband NIR phosphor for blue-LED-pumped pc-LEDs.