Equivalent chemical substitution in double–double perovskite-type ALaLiTeO6:Mn4+ (A = Ba2+, Sr2+, Ca2+) phosphors enabling wide range crystal field strength regulation and efficient far-red emission

Mn4+-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 A2B'B''O6 with a tunable crystal structure and versatile octahedral sites have been extensively studied as good host matrixes for Mn4+-emitters to realize tunable far-red emissions. Herein, a series of double-double perovskite-type ALaLiTeO6:Mn4+ (A = Ba, Ba0.5Sr0.5, Sr, Sr0.5Ca0.5, 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 (I2/m → P21/n) were observed for ALaLiTeO6:Mn4+. In contrast, on the local scale, the degree of (Li/Te)O6-octahedral distortion is positively correlated with the ΔIR value, which is the ionic radius difference between A2+ and La3+. 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 (Dq/B = 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 ALaLiTeO6:Mn4+ phosphor series, CaLaLiTeO6:Mn4+ 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 CaLaLiTeO6:Mn4+ 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 Mn4+-activated double-double perovskite-type far-red phosphors.