White light emitting diode (LED) is widely used as a high-quality lighting source, and its performance depends critically on the selected light-emitting materials. Early white LEDs were mainly composed of a combination of blue light chips and yellow light emitting phosphors, but suffered from low color rendering index, high color temperature, and blue light hazards. To solve these problems, researchers have turned to improving the color rendering performance of white LEDs by using red phosphors that can be excited by blue light.

Professor Zhiguo Xia's group at the State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, has proposed a new material design principle for Eu²⁺-activated oxide phosphors that are difficult to obtain red light emission: Eu²⁺ occupying a low coordination number of polyhedra can cause a large crystal field cleavage, which can in turn realize red light or even near-infrared emission. A novel Sr₃Ga₄O₉ phosphor matrix with multiple low-coordination cation lattice sites was designed and synthesized, and broadband red light emission under 450 nm blue light excitation was successfully achieved by doping Eu²⁺, with the emission peak located at 618 nm.

Sr₃Ga₄O₉∶Eu²⁺ and Sr₃Ga₄O₉∶Eu²⁺, 0.05Zn²⁺ phosphors were prepared by high-temperature solid-phase method. According to the proportional content of the different raw materials in the chemical formula, SrCO₃ (A.R.: 99.5% analytically pure), Ga₂O₃ (A.R.), H₃BO₃ (A.R.) and Eu₂O₃ (99.99%) were weighed and ground in an agate mortar for half an hour. The mixture was transferred to an alumina crucible and fired at 900 ℃ in an air atmosphere for 6 h, then cooled naturally to room temperature. The mixture was ground again to powder form and refilled into the crucible, which was placed in a tube furnace with a continuous flow of reducing gas ([V(N₂)∶V(H₂)] = 80%∶20%). The samples were burned at 1 200 ℃ for 10 h, cooled to room temperature naturally, and then ground carefully to obtain the desired phosphor samples.

A white light illumination device was fabricated using a blue LED chip stacked with green and red phosphors. The green light was partly provided by Lu₃Al₅O₁₂∶Ce³⁺ (LuAG∶Ce³⁺) commercial green phosphor, while the red light was contributed by the experimentally prepared Sr₃Ga₄O₉∶Eu²⁺,0.05Zn²⁺ phosphor.

The Sr₃Ga₄O₉ cell structure belongs to the P-1 space group of the triclinic crystal system. The results of XRD characterization tests indicate that the prepared samples are in pure phase. A heterovalent substitution strategy was used to introduce Zn²⁺ into Sr₃Ga₄O₉, and the XRD results showed that this co-doping did not change the structure of the matrix material. Further analysis revealed that Zn²⁺ ions replaced Ga³⁺ sites, while Eu²⁺ may occupy two 6-coordinated Sr²⁺ lattice sites.

Under the excitation of 450 nm, the Sr₃Ga₄O₉∶xEu²⁺ phosphor emits red light at 618 nm, and the emission intensity reaches the maximum at x = 0.04. The low-temperature emission spectra at 78 K can be decomposed into two Gaussian peaks, which indicates that the Eu²⁺ occupy two different luminescent centers and emit the red light, respectively. The fluorescence lifetime decay curves of the Sr₃Ga₄O₉∶0.04Eu²⁺ sample at low temperature (78 K) can be well fitted by the double-exponential function, which again demonstrates that the Eu²⁺ ions occupy two different lattice sites.

After Zn doping, the luminescence intensity of Sr₃Ga₄O₉∶Eu²⁺,0.05Zn²⁺ phosphor was enhanced and the thermal stability was also improved significantly. The enhancement of thermal stability is from the presence of defects in the crystal, which compensates for the heat loss. The average decay lifetime of the co-doped Zn²⁺ is significantly higher than that of the sample without Zn²⁺, and a clear bulge of new pyroelectric peaks at 210 ℃ and 325 ℃ is clearly seen. Thus the addition of Zn²⁺ can increase the concentration of traps and contribute to the formation of deeper defect energy levels.

For Sr₃Ga₄O₉∶0.04Eu²⁺, the thermal stability is poor due to its shallow trap depth, which causes the electrons to escape from the shallow traps immediately at room temperature. Whereas, Sr₃Ga₄O₉∶Eu²⁺,0.05Zn²⁺ has deeper trap 2 and trap 3 in it, and thus maintains a good thermal stability even at high ambient temperatures.

The white LED device was fabricated to cover the entire visible region of the emission spectrum with a color rendering index of Ra = 87, a color temperature of CCT = 3 500 K, and CIE color coordinates of (0.410, 0.380).

Novel Sr₃Ga₄O₉∶Eu²⁺ red phosphors that can be excited by blue light were designed and prepared, and their crystal structures and spectral properties were investigated. It is found that the Sr₃Ga₄O₉∶Eu²⁺ red light emission comes from two low-coordination polyhedra Sr1O₆ and Sr3O₆ occupied by Eu²⁺ with 5d-4f leaps. Zn²⁺/Ga³⁺ heterovalent substitution occurs by the introduction of Zn²⁺ into Sr₃Ga₄O₉, which is proved to introduce a defective energy level through the fluorescence decay lifetime and thermoluminescence to improve the thermal stability of the phosphor. A white LED device prepared using a commercial blue LED chip, Sr₃Ga₄O₉∶Eu²⁺,0.05Zn²⁺ red phosphor, and a commercial green phosphor, LuAG∶Ce³⁺, has a high color-rendering index, Ra = 87, and a low correlated color temperature, CCT = 3 500 K, which suggests that it is suitable for warm-white-light lighting.