Chemical unit co-substitution for a new far-red-emitting phosphor Ca_3-6y(NaLu)_3yLiSbO₆:Mn⁴⁺ to achieve high quantum efficiency and superb thermal stability

The discovery of Mn⁴⁺-doped red phosphors with a high quantum efficiency (QE) and good thermal stability is of great interest for plant cultivation lighting. Here, for the first time, a new highly efficient and thermally stable far-red-emitting phosphor Ca_3-6y(NaLu)_3yLiSbO₆:Mn⁴⁺ (CNULS:Mn⁴⁺) is reported. When a far-red-emitting phosphor Ca₃LiSbO₆:Mn⁴⁺ (CLS:Mn⁴⁺) was prepared via the solid-state reaction method, its maximum internal and external QEs are only 39.5% and 23.1%, respectively. Through the chemical unit co-substitution of [Na⁺−Lu³⁺] for [Ca²⁺−Ca²⁺] in CLS:Mn⁴⁺, its luminescence properties can be improved tremendously and the internal and external QEs for the substitution amount of y = 3% are as high as 80.1% and 47.4%, respectively. The large luminescence enhancement is attributed to the increase of structural rigidity induced by lattice contraction. The CNULS:Mn⁴⁺ phosphor exhibits excellent thermal stability with the emission intensity at 423 K remaining 87% of the value at 298 K. The LED device fabricated with the CNULS:Mn⁴⁺ (y = 3%) phosphor can produce bright and stable far-red emission, which matches well with the absorption spectrum of phytochrome P_FR. These results suggest that the CNULS:Mn⁴⁺ phosphor could be a promising far-red-emitting converter for plant growth as well as solid-state lighting.