Equivalent chemical substitution in double-double perovskite-type ALaLiTeO 6 :Mn 4+ (A = Ba 2+ , Sr 2+ , Ca 2+ ) phosphors enabling wide range crystal field strength regulation and efficient far-red emission.

Mn-activated phosphors have shown wide prospective applications in phosphor-converted white light-emitting diodes (pc-WLEDs) and pc-LEDs used in illumination and indoor plant cultivation, respectively. Recently, double perovskites AB'B''O with a tunable crystal structure and versatile octahedral sites have been extensively studied as good host matrixes for Mn-emitters to realize tunable far-red emissions. Herein, a series of double-double perovskite-type ALaLiTeO:Mn (A = Ba, BaSr, Sr, SrCa, Ca) phosphors were synthesized and structurally characterized, and the correlations between their structure and luminescence were also studied systematically. With a decrease of the A-cation size, an increased distortion in the average structure and a structure symmetry lowering (2/ → 2/) were observed for ALaLiTeO:Mn. In contrast, on the local scale, the degree of (Li/Te)O-octahedral distortion is positively correlated with the Δ value, which is the ionic radius difference between A and La. The local structural changes were found to be irrelevant to the significant improvements in photoluminescence properties. In combination with careful spectroscopic analysis, we deciphered that a decreased A-cation is in fact helpful for the enhancements in crystal field strength (/ = 2.12-2.82) and Mn-O covalent bonding, thereby resulting in an improved quantum efficiency, a suppressed nonradiative transition, and a redshift in photoluminescence spectra. Amongst the ALaLiTeO:Mn phosphor series, CaLaLiTeO:Mn exhibits the highest external quantum efficiency of 70.1% and internal quantum efficiency of 96.4% and superior thermal stability (93.3%@423 K), making CaLaLiTeO:Mn very promising as far-red phosphors for pc-LEDs. The findings of this work will serve as a new guide for rational design of high-performance Mn-activated double-double perovskite-type far-red phosphors.